JPH08148954A - Gain variable amplifier - Google Patents

Abstract

PURPOSE: To provide a variable amplifier for obtaining an amplification gain proportional to a control voltage. CONSTITUTION: A first differential amplifier circuit 12 differentially amplifies a variable control voltage Vc and outputs two currents and a logarithmic compression circuit 11 outputs two signals for which the two currents from the first differential amplifier circuit 12 are logarithmically compressed. A second differential amplifier circuit 13 differentially amplifies the two signals from the logarithmic compression circuit 11 corresponding to the current of a current source I2 and outputs the two signals and an error amplifier 15 respectively controls the current sources Ie and I2 so as to let the error of the signal to which offsetting is applied in an offset circuit 14 and the other signal from the second differential amplifier circuit 13 be '0'. Thus, a setting current Ie inversely proportional to the control voltage Vc is supplied to an amplification system and the gain is set inversely proportionally to the setting current Ie in the amplification system. As a result, the gain proportional to the control voltage Vc is obtained in the amplification system.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable gain amplifier whose amplification gain is set to a desired value according to a control voltage, and more particularly to a variable gain amplifier used in electronic equipment such as video equipment.

[0002]

2. Description of the Related Art For example, in video equipment, more specifically, in imaging equipment such as electronic still cameras, in order to set the white balance of a signal obtained by imaging to a desired value,
Variable gain amplifiers that can change the amplification gain according to variations in camera sensitivity and the like are often used. Such a variable gain amplifier is formed so that the amplification gain can be adjusted within a predetermined range, and when it is incorporated into a device, it is controlled by a control voltage given by the system controller to obtain a desired amplification gain. Is set to.

As such a variable gain amplifier, there is known one in which differential amplifiers are connected in multiple stages and the amplification gain is set by controlling the current supply to each differential amplifier. . For example, including an input-side differential amplifier to which an input voltage is supplied and an output-side differential amplifier that further amplifies its output to obtain an output, and connects the output-side differential amplifier to a constant current source, It is known that an input side amplifier is connected to a voltage-current conversion circuit that controls a control voltage with a set current obtained by converting the control current. Specifically, in the differential amplifier on the input side, the input voltage is applied by being superimposed on the base voltage, and the set current from the voltage-current conversion circuit is supplied to the commonly connected emitter side via the emitter resistor.
A differential output is obtained by amplifying the input voltage from the collector according to the emitter current. Similarly, in the differential amplifier on the output side, the differential output from the differential amplifier on the input side is applied to each base voltage, the constant current source is connected to the commonly connected emitter side, and the load is applied to the collector. A differential output that is connected and amplifies the input voltage according to the emitter current is output as the output voltage applied to the load. In this case, the amplification gain (G _V ) is represented by the ratio of the output of the input side amplifier and the output of the output side amplifier. That is, the output of the input side amplifier is determined by the product of the current (I _E ) from the voltage / current source and the emitter resistance (R _E ), and the output of the output side amplifier is the current (I _O ) from the constant current source. It is determined by the product of load resistance (R _L ), so the amplification gain is G _V
= (I _O · R _L ) / (I _E · R _E ).

On the other hand, the voltage-current conversion circuit has a predetermined transconductance (gm) and converts it into a set current I _E which is proportional to the product of its value and a given control voltage (V _C ). Supply to the side amplifier. Specifically, the emitters of two transistors are connected by a predetermined resistor (R _C ) and a control voltage is applied to the base of one transistor. The other transistor has a base to which a reference voltage is applied and a collector connected to the power supply voltage. As a result, a current according to the value obtained by dividing the control voltage by the resistance flows through the collector of one of the transistors. That is, the control current (I _E ) is expressed in proportion to the product of the control voltage (V _C ) and the reciprocal value of resistance (1 / R _C ), that is, the transconductance gm = 1 / R _C. As a result, in the conventional variable gain amplifier, the amplification gain is set in inverse proportion to the control voltage.

[0005]

As described above, in the conventional technique, the gain G _{V of} the amplifier is set to be inversely proportional to the given control voltage V _C as shown in FIG. 9, for example. There is a problem that it is difficult to control the gain when setting it to a desired gain. In particular, when the control voltage V _C is linearly set by a digital circuit such as a microcomputer to control a desired gain G _V , there is a problem that it becomes difficult to set the control voltage V _C.

In order to solve this, the inventor of the present application has
The same applicant as the present invention, for example, Japanese Patent Laid-Open No. 4-1850
In the publication No. 07, etc., an output circuit further added after the output side amplifier is devised so that its gain is configured to be proportional to the control voltage. However, there has been no variable gain amplifier capable of obtaining a set current so that the gain is proportional to the control voltage on the input side. In other words, the variable gain amplifier that changes the amplification gain in proportion to the control voltage by devising the control input while maintaining the configuration of the multistage amplifier is
It has not been known yet, and the devise on the control system side was left as an issue.

The present invention solves the above-mentioned conventional drawbacks and freely sets the control input to the amplification system so that the gain changes in proportion to the control voltage while maintaining the configuration of the amplification system. It is an object of the present invention to provide a variable gain amplifier capable of

[0008]

In order to solve the above-mentioned problems, a variable gain amplifier according to the present invention has an amplification system for amplifying an input voltage with a desired amplification gain, and an amplification gain of the amplification system from an external control voltage. In a variable gain amplifier including a control system for setting based on, the amplification system includes an input side amplification means for amplifying an input voltage according to a set current based on a control voltage from the control system, and an input side amplification means. Output side amplifying means for further amplifying the amplified output based on a current value from a predetermined current source to obtain a desired output voltage, and the control system supplies the set current to the input amplifying means of the amplifying system. Current source,
First differential amplification means for differentially amplifying the control voltage to output two currents, logarithmic compression means for obtaining two signals logarithmically compressed from the two currents from the first differential amplification means, and logarithm Two signals received from the compression means and further differentially amplified 2
Second differential amplification means for obtaining two signals, a second current source for supplying a current for determining the gain to the second differential amplification means, and one output from the second differential amplification means The offset means for applying the offset signal, and the variable control signal so that the error between the signal to which the offset signal is added by the offset means and the other signal from the second differential amplifier means become "0" And an error detecting means for controlling the gain of the second differential amplifying means by supplying the current to the current source and controlling the set current from the first current source by the control signal.

In this case, both the first current source and the second current source generate a control current according to the variable control voltage, and the offset means offsets one signal from the second differential amplifier means. A voltage is applied to the first current source and the error detecting means so that the error between the voltage applied with the offset voltage from the offset means and the voltage of the other signal from the second differential amplifying means becomes "0". The second current source may be controlled.

Further, both the first current source and the second current source generate a current according to the variable control current, and the offset means applies the offset current to one signal from the second differential amplifier means. In addition, the error detecting means includes the first current source and the second current source so that the error between the current to which the offset current from the offset means is applied and the current of the other signal from the second differential amplifying means becomes "0". The current source may be controlled.

On the other hand, the variable gain amplifier of the present invention includes a gain system that amplifies an input voltage with a desired amplification gain and a control system that sets the amplification gain of the amplification system based on a control voltage from the outside. In the variable amplifier, the amplification system includes an input side amplification means for amplifying an input voltage according to a set current based on a control voltage from the control system, and an amplification output from the input side amplification means as a current value from a predetermined current source. Output side amplifying means for further amplifying based on the output voltage to obtain a desired output voltage, and the control system differentially amplifies the input voltage and outputs two currents; The first voltage-current conversion means for converting the voltage input to the first differential amplification means into the control current and supplying it to the input side amplification means of the amplification system, and the two currents from the first differential amplification means are logarithmic. Logarithmic compression means for obtaining two compressed outputs, A second differential amplifying means for receiving two signals from the compressing means and further obtaining two differentially amplified outputs, and a current for determining the gain of the second differential amplifying means for converting the control voltage into a current. Second voltage-current converting means to be supplied by the second differential amplifying means, offset means for adding an offset signal to one signal from the second differential amplifying means, and a signal to which the offset signal is added by the offset means and the second differential signal. A variable voltage is supplied to the first differential amplification means so that the error of the other signal from the amplification means becomes "0", and the same control signal is supplied to the first differential amplification means.
And an error detecting means for supplying the voltage to current converting means.

In this case, the offset means applies an offset voltage to one signal from the second differential amplification means, and the error detection means and the voltage to which the offset voltage is applied by the offset means and the second differential amplification means. The voltage supplied to the first voltage-current converting means and the first differential amplifying means may be controlled so that the error of the voltage of the other signal from the signal is zero.

The offset means applies an offset current to one signal from the second differential amplifying means, and the error detecting means uses the current to which the offset current is added by the offset means and the second differential amplifying means. The input voltage of the first voltage-current converting means and the first differential amplifying means may be controlled so that the error of the current of the other signal of the above becomes "0".

[0014]

According to the variable gain amplifier of the present invention, the applied control voltage is differentially amplified by the first differential amplifying means to output two currents, and the two currents are outputted to the logarithmic compression means. Logarithmically compressed and two signals are output. These two
The two signals are amplified by the second differential amplifying means in accordance with the second current source and supplied as two signals to the error detecting means via the offset means. In the error detection means,
One signal and the other signal offset by the offset means from the second differential amplifier are detected, and the first current source and the second current source are connected so that these signals become "0". It is controlled by a variable control signal. As a result, the difference between the two signals in the second differential amplification means becomes a signal proportional to the control voltage. As a result, the control current generated by the first current source is controlled in inverse proportion to the applied control voltage. In the amplification system, the input voltage is amplified in accordance with the control current given by the input side amplification means, and the output side amplification means further obtains the output voltage according to the predetermined current source. As a result, the amplification system amplifies the input voltage with a gain that is inversely proportional to the control current, so that the input voltage is amplified with a gain proportional to the control voltage given to the control system.

Further, in the present invention, the input voltage is differentially amplified by the first differential amplifying means to output two currents, and the two currents are logarithmically compressed by the logarithmic compressing means to obtain 2 currents. Two signals are output, and the variable control voltage is converted into a current by the second voltage-current conversion means. The difference between the two signals logarithmically compressed by the logarithmic compression means is supplied to the second differential amplification means in the same manner as described above, and is used as the voltage to be input to the first differential amplification means by the offset means and the error detection means. Control. This input voltage is converted into a control current by the first voltage-current conversion means and supplied to the amplification system. As a result, a control current inversely proportional to the control voltage is obtained, and the amplification system amplifies with a gain inversely proportional to the control current, so that the input voltage is amplified with a gain proportional to the given control voltage.

[0016]

Embodiments of the variable gain amplifier according to the present invention will be described below with reference to the drawings. 1 and 2 show an embodiment of a variable gain amplifier according to the present invention. As shown in FIG. 2, the variable gain amplifier of the present embodiment is an amplifier circuit 1 that amplifies an input voltage V _IN and outputs an output voltage V out.
And a control circuit 2 of a control system for controlling the amplification gain Gv of the amplifier circuit 1 based on the control voltage V _C. In particular, the control circuit 2 of the present embodiment has a control current Ie that is inversely proportional to the control voltage V _C and a gain that is inversely proportional to the control current Ie.
The amplifier circuit 1 for obtaining G _V is controlled, and as a result, the amplifier circuit 1 is obtained.
This is a circuit in which the amplification gain G _V of can be set in proportion to the control voltage V _C.

Specifically, as shown in FIG. 2, the amplifier circuit 1
Includes an input-side differential amplifier that is supplied with the input voltage V _IN superimposed on the base voltage V _B2 , and an output-side differential amplifier that outputs the output voltage Vout in response to the differential current. The differential amplifier on the input side includes transistors Q15 and Q16, and the input voltage V _IN and the base voltage V _B2 are superimposed and applied to the bases of the transistors Q15 and Q16, respectively. In this case, the polarities of the input voltage V _IN are applied to the bases of the respective transistors Q15 and Q16 with different polarities. The control circuit 2 is commonly connected to the emitters of the transistors Q15 and Q16 via resistors R _E and R _E having the same resistance value. The collectors of the transistors Q15 and Q16 are connected to the bases of the transistors Q11 and Q12 of the output side differential amplifier, and are connected to the power supply voltage Vcc.
It is connected to the emitter of 3, Q14 respectively.

Output side differential amplifier transistors Q11, Q1
The power supply voltage Vcc is applied to the collector of 2 through a resistor RL having the same resistance value. Transistors Q11 and Q12 supply current to these resistors RL in the same ratio as the output ratio of transistors Q15 and Q16. The emitters of the transistors Q11 and Q12 are commonly grounded via the bias current source I _O , and the output voltage Vout is obtained from between the collectors of the transistors Q11 and Q12 and the resistors RL. In this case, when the control current I _E changes according to the control voltage V _C , the gain G _V of the amplification system 1 is expressed by the following equation (1).

[0019]

[Equation 1] G _V = RL I _O / R _E I _E (Unit: V / V) (1) In this case, the gain G _V changes even if the bias current source I _O changes. Since the output DC voltage and the output DC range change, the bias current source I _O is usually fixed and the gain G _V is controlled only by the control current I _E.

The control circuit 2 of the present embodiment comprises a logarithmic compression circuit 11 and a first differential amplifier circuit 12 as shown in FIG.
And a second differential amplifier circuit 13, an offset circuit 14, an error amplifier 15, and a current source Ie. The current source Ie is
It is connected to each emitter side of the amplification system transistors Q15 and Q16 shown in FIG. 2, and the gain G _V of the amplification circuit 1 is the control current Ie.
It is controlled so that it is inversely proportional to (G _V ∝1 / Ie).

The logarithmic compression circuit 11 includes transistors Q21 and Q22.
And the emitter currents I11 and I12 of the transistors Q21 and Q22 are logarithmically compressed. This transistor Q21, Q22
The voltage Vcc commonly been applied to the collectors of, also, the voltage V _dc2 is commonly applied to each base. The emitters of the transistors Q21 _, Q22 are respectively the collectors of the transistors Q23, Q24 forming the first differential amplifier circuit 12, and the transistor Q26, forming the second differential amplifier circuit 13, respectively.
Connected to each base of Q25.

The first differential amplifier circuit 12 includes a transistor Q2
3, Q24, two resistors Ri, constant current source I1, control voltage source
V _C, and the variable control voltage V _C between the bases of the transistors Q23 and Q24 is differentially amplified to collect each collector current I1.
It is formed so that 1, I12 changes. Transistor
The Q23 base, the control voltage V _C and + voltage V _dc1 is applied, the voltage V _dc1 is applied to the base of the transistor Q24. The emitters of the transistors Q23 and Q24 are grounded via the resistors Ri having the same resistance and the constant current source I1.

The second differential amplifier circuit 13 includes a transistor Q2
6, Q25, two resistors R _o, and a current source I ₂ ,
The collector currents I ₂₁ and I _{22 of} the transistors Q26 and Q25 are formed so as to be differentially amplified and changed according to the variable current I ₂ . Each collector of the transistors Q26, Q25 is a voltage Vcc through respective resistors R _o of the same resistance value, each application, the emitter is grounded through a current source I ₂ in common. The current source I ₂ is controlled by the output voltage of the error amplifier 15.

The offset circuit 14 is a voltage source in this embodiment.
It is formed by V _off . The collector of the transistor Q25 is connected to the negative side of the voltage source V _{off, and} the positive side is the error amplifier.
Connected to 15 inverting input terminals. The collector of the transistor Q26 is connected to the non-inverting input terminal of the error amplifier 15, and the current source Ie and the current source I ₂ are controlled by the control voltage from the output terminal of the error amplifier 15. The output terminal of the error amplifier 15 is also the feedback loop phase compensation capacitor.
It is connected to C1. The transistor Q2 shown in FIG.
1 to Q26 are all NPN type.

In such a configuration, when the control voltage V _C is input to the first differential amplifier circuit 12, the output difference dv {= R _o due to the two resistors R _o of the second differential amplifier circuit 13. The relation of (I ₂₂ -I ₂₁ )} is expressed by the following equation (2).

[0026]

[Equation 2] dv = {(I ₂ × R _o ) / (I 1 × R i)} × V _C・ ・ ・ (2) In this case, one output R _o · I ₂₁ has a constant offset voltage V
_{If off} is added and supplied to the error amplifier 15, the error amplifier 15
So that the input voltage difference is 0V, that is, dv = V
_The current I ₂ (and Ie) is feedback-controlled so as to be _off .

For example, control is performed as shown in the following (a) and (b). (a) When V _off > dv = {(I ₂ × R _o ) / (I 1 × R i)} × V _C → → Control to increase the current I ₂ . (b) When V _off <dv = {(I ₂ × R _o ) / (I 1 × R i)} × V _C → → Control to reduce the current I ₂ .

In this way, even if the control voltage V _C changes, the error amplifier 15 controls the current I ₂ so that the following equation (3) is obtained.

[0029]

[Equation 3] V _off = dv = {(I ₂ × R _o ) / (I 1 × R i)} × V _C (3) Further, the error amplifier 15 includes both the current source Ie and the current source I ₂ . Since it is controlled, the following equation (4) is established.

[0030]

(4) I ₂ = Ie (4) The gain G _V of the amplification system 1 is controlled by such a control current Ie. At this time, the relationship between the control voltage V _C and the control current Ie is expressed by the equation
From (3) and (4), it is expressed as the following equation (5).

[0031]

[Equation 5] Ie = I ₂ = {(V _off × I 1 × Ri) / R _o } / V _C (5) where (V _off × I 1 × Ri) / R _o is a constant (= α ), The control current Ie can be expressed by the following equation (6).

[0032]

[Equation 6] Ie = α / Vc (6) Therefore, as is apparent from the above equation (6), the control current Ie is Ie∝1 / V _C with respect to the external control voltage V _C. So
The gain G _V is expressed by the following equation (7).

[0033]

[Equation 7] G _V ∝1 / Ie ∝ _{V C} (7) This realizes a variable gain amplifier in which the gain G _V of the amplification system 1 is proportional to the control voltage V _C given to the control circuit 2. You can

In order to clarify the effect of this embodiment, the conventional variable gain amplifier shown in FIG. 8 will be taken as an example and compared with this embodiment. The difference from the variable gain amplifier shown in FIG. 8 in this embodiment is that the control circuit for supplying the control current Ie to the amplification system 1
The point 2b is formed by a voltage-current conversion circuit. In the control circuit 2b, the voltage V _B3 is applied to the base of the transistor Q17, and the control voltage V _C is applied to the base of the transistor Q18. A power supply voltage Vcc is applied to the collector of the transistor Q17, and the emitters of the transistors Q15 and Q16 of the amplification system 1 are connected to the collector of the transistor Q18. The emitters of the transistors Q17 and Q18 are connected via a resistor RC and are also grounded via constant current sources I2 and I1, respectively.

In the variable gain amplifier having such a configuration, the control current I _E supplied from the control system 2 to the amplification system 1 is controlled by the control voltage.
Is proportional to _{V C (I E αV C)} , this result, the amplification system 1 gain G
_V is inversely proportional to the control voltage V _{C according} to the above equation (1) (G _V ∝1 / V
_C ) It will be. Therefore, as shown in FIG. 9, the setting of the control voltage V _C is obviously much more difficult than in the present embodiment.

Next, a second embodiment of the variable gain amplifier according to the present invention will be described with reference to FIG. Since the amplifier circuit 1 is similar to that shown in FIG. 2, only the control circuit 2d different from FIG. 1 will be described. In the example shown in FIG. 1, the error amplifier 15 controls the control current Ie by the voltage difference dv, but in the example shown in FIG. 3, the error amplifier 15a controls the control current Ie by the current difference dI (described later). . Logarithmic compression circuit 11
And the first differential amplifier circuit 12 and the second differential amplifier circuit 13,
The configuration is almost the same.

Explaining the error amplifier 15a, voltage
Vcc is applied to the emitters of PNP transistors Q27 and Q28 via resistors Rd of the same resistance value, to the emitter of PNP transistor Q29 via resistor Rt, and to the collector of NPN transistor Q31. Has been done.

The bases of the transistors Q27 and Q28 are directly connected to each other and connected to the collector of the transistor Q27. The collectors of the transistors Q27 and Q28 are the collectors of the transistors Q25 and Q26 forming the second differential amplifier circuit 13. It is connected to the. The current source I ₂ forming the second differential amplifier circuit 13 is shown as a transistor Q30 and a resistor Rs.

The collector of the transistor Q28 is connected to the base of the transistor Q29, grounded via the constant current source I _off forming the offset circuit 14a, and further connected to the phase compensation capacitor C1 of the feedback loop. There is. The collector of the transistor Q29 is connected to the base of the transistor Q31 and the collector of the NPN transistor Q32, and the emitter of the transistor Q31 is connected to the bases of the transistors Q30, Q32, Q33 and the resistor Rx, respectively. The emitter of the transistor Q32 is grounded via the resistor Rs, and the transistor Q33 and the resistor Rs form a current source Ie.

In such a configuration, the error amplifier 15a
Im is the collector current of the transistor Q27 that composes
When the collector current of the transistor Q28 is In and the current difference dI = Im-In when the control voltage V _C is input, the current difference dI is expressed by the following equation (8).

[0041]

[Equation 8] dI = {I ₂ / (I1 × Ri)} × V _C (8) This error amplifier 15a has a current dI and a constant offset current.
The current I ₂ (and the current Ie) is feedback-controlled so that the difference I _off becomes 0 (A), that is, I _off = dI.

For example, the current I ₂ is controlled as in the following (a) and (b). (a) I _off> dI = when _{{I 2 / (I1 × Ri} )} × V C, is controlled so as to increase the → current I _2. (b) when _{I off <dI = {I 2} / (I1 × Ri)} × V C, is controlled to reduce the → current I _2.

In this way, even if the control voltage V _C changes, the error amplifier 15a controls the current I ₂ so as to satisfy the following equation (9).

[0044]

[Equation 9] I _off = dI = {I ₂ / (I1 × Ri)} × V _C (9) Further, since the error amplifier 15a controls both the current source Ie and the current source I ₂ , Equation (10) holds.

[0045]

[Equation 10] I ₂ = Ie (10) The gain G _V of the amplification system 1 is controlled by this control current Ie.
At this time, the relationship between the control voltage V _C and the control current Ie is expressed by the above equations (9), (1
It is expressed by the following equation (11) from 0).

[0046]

[Equation 11] Ie = I ₂ = (I _off × I 1 × Ri) / V _C (11) where (I _off × I 1 × Ri) is a constant (= α),
The current Ie can be expressed by the following equation (12).

[0047]

[Equation 12] Ie = α / Vc (12) Therefore, as is apparent from the above equation (12), the control current Ie is Ie∝1 / V _C with respect to the external control voltage V _C. Therefore, the gain G _V is expressed by the following equation (13).

[0048]

[Formula 13] G _V ∝ _{V C} (13) Therefore, it is possible to realize a variable gain amplifier in which the gain G _V is proportional to the control voltage V _C.

FIG. 4 shows the control circuit 2 of the second embodiment.
2e, a constant current source Iz is added between the collector of the transistor Q23 and the ground side. According to such a configuration, by setting the constant current source Iz so that Iz> I1, it is possible to operate normally even if the control voltage V _C becomes negative.

Next, a third embodiment of the variable gain amplifier according to the present invention will be described with reference to FIG. Similar to the above, in this embodiment, the parts different from the above embodiments, the control circuit
Only 2f will be explained. The control circuit 2f of this embodiment is
Similar to the circuit 2 of the first embodiment, the offset voltage V _{off is used for} control, and the logarithmic compression circuit 11 has the same configuration as that of the first embodiment. Explaining only the difference from the first embodiment, the first voltage-current conversion circuit 16 makes the potential difference Vin between the bases of the transistors Q23 and Q24 forming the first differential amplifier circuit 12a proportional to this. The gain G _V of the amplifier circuit 1 is controlled by converting into the control current Ie.

Further, the second voltage-current conversion circuit 17 converts the control voltage V _C into a current I ₂ proportional thereto, and this current I ₂ flows into the second differential amplifier circuit 13a. Furthermore, an error amplifier is provided from the second differential amplifier circuit 13a through the offset circuit 14.
The voltage obtained at 15 is fed back to the base of the transistor Q23 of the first differential amplifier circuit 12a. The voltage-current conversion circuits 16 and 17, for example, can be formed by transistors Q41 and Q42, a resistor RG, and constant current sources I ₂₀ and I _{10 as} shown in FIG. The potential difference V _I and the current I _O of the bases of the transistors Q41 and Q42 are I _O = V _I / RG, and the transconductance Gm is Gm = 1 / RG.

In the circuit shown in FIG. 5, transistors Q23 and Q24 are used.
The relationship between the potential difference Vin of each base of the above and the output difference dv due to the two resistors R _o of the second differential amplifier circuit 13 is given by the following equation (14).

[0053]

Dv = {(I ₂ × R _o ) / (I 1 × R i)} × Vin (14) Then, as in the first embodiment, a constant offset voltage V
_{If off} is added and supplied to the error amplifier 15, the error amplifier 15
So that the input voltage difference is 0V, that is, dv = V _off
The voltage Vin is controlled so that

For example, as shown in (a) and (b) below, the voltage
Take control of Vin. (a) When V _off > dv = {(I ₂ × R _o ) / (I 1 × R i)} × Vin → Control to increase the voltage Vin. (b) When V _off <dv = {(I ₂ × R _o ) / (I 1 × R i)} × Vin → Control to decrease the voltage Vin.

This relationship holds even if the current I ₂ changes with the control voltage V _C. That is, the second voltage-current conversion circuit
When the transconductance of 17 is Gm ₁ , the current I ₂ is expressed by the following equation (15).

[0056]

[Equation 15] I ₂ = Gm ₁ × V _C (15) Even if the control voltage V _C changes, the error amplifier 15 outputs the voltage Vi
Control n as expressed by the following equation (16).

[0057]

## EQU16 ## V _off = dv = {(I ₂ × R _o ) / (I 1 × R i)} × Vin (16) Here, the transconductance of the first voltage-current conversion circuit 16 is G _m2 . Then, since Ie = G _m2 × Vin, the relation between the control voltage V _C and the control current Ie is expressed by the following equation (1) from the above equations (15) and (16).
It becomes like 7).

[0058]

Ie = G _m2 × Vin = G _m2 × V _off × I1 × Ri / (G _m1 / (G _m1 × V _C × R _o ) ... (17) where G _m2 × V _off × I1 × Ri / (G _m1 × R _o ) is a constant (=
α), the current Ie is expressed by the following equation (18).

[0059]

[Equation 18] Ie = α / V _C (18) Therefore, as is apparent from the above equation (18), it is possible to realize a variable gain amplifier in which the gain G _V is proportional to the control voltage V _C.

FIG. 7 shows a fourth variable gain amplifier according to the present invention.
3 shows a control circuit 2g in the embodiment of FIG. This embodiment is a combination of the second embodiment and the third embodiment, and a logarithmic compression circuit 1 similar to the third embodiment.
1, first differential amplifier circuit 12a, second differential amplifier circuit 12b
And the first and second voltage-current conversion circuits 16 and 17, the error amplifier 15b and the current offset circuit 14a. The error amplifier 15b consists of 6 transistors Q51 to Q56.
And six resistors Rd and Rz.

First, assuming that the collector current of the transistor Q51 forming the error amplifier 15b is Im, the collector current of the transistor Q52 is In, and the current difference dI input by the control voltage V _C is dI = Im-In, the current dI Is expressed by the following equation (19).

[0062]

[Equation 19] dI = {I ₂ / (I1 × Ri)} × Vin (19) This error amplifier 15b is configured so that the difference between this current dI and the constant offset current I _off is 0 (A). In other words, the voltage Vin is feedback-controlled so that I _off = dI.

For example, as shown in the following (a) and (b), the voltage Vin
Control. (a) When I _off > dI = {I ₂ / (I 1 × Ri)} × Vin → Control to increase voltage Vin. (b) When I _off <dI = {I ₂ / (I 1 × Ri)} × Vin → Control to decrease voltage Vin.

This relationship holds even if the current I ₂ changes with the control voltage V _C. That is, the second voltage-current conversion circuit
When the transconductance of 17 is G _m1 , the current I ₂ is expressed by the following equation (20).

[0065]

[Equation 20] I ₂ = G _m1 × V _C (20) Even if the control voltage V _C changes, the error amplifier 15 b
Vin is controlled as represented by the following equation (21).

[0066]

(21) I _off = dI = {I ₂ / (I1 × Ri)} × Vin (21) Here, if the transconductance of the first voltage-current conversion circuit 16 is G _m2 , then Ie = G _m2 Therefore, the relationship between the control voltage V _C and the control current Ie can be calculated from the above equations (20) and (21) by the following equation (2
It is represented by 2).

[0067]

(22) Ie = G _m2 × Vin = G _m2 × I _off × I1 × Ri / (G _m1 × V _C ) ... (22) where G _m2 × I _off × I1 × Ri / G _m1 is Since it is a constant (= α), the following equation (23) is established.

[0068]

[Equation 23] Ie = α / V _C (23) Therefore, as is apparent from the equation (23), it is possible to realize a variable gain amplifier in which the gain G _V is proportional to the control voltage V _C.

[0069]

As described above, according to the variable gain amplifier of the present invention, the applied control voltage is differentially amplified by the first differential amplifying means to output two currents. The current is logarithmically compressed by the logarithmic compression means and two signals are output. These two signals are amplified by the second differential amplifying means in accordance with the second current source and supplied as two signals to the error detecting means via the offset means. The error detecting means detects one signal and the other signal offset by the offset means from the second differential amplifier, and sets the first current source and the second signal so that these signals become “0”. And the current source are controlled by variable control signals. As a result, the difference between the two signals in the second differential amplification means becomes a signal proportional to the control voltage. As a result, the control current generated by the first current source is controlled in inverse proportion to the given control voltage, and the amplification system amplifies with a gain in inverse proportion to the control current from the control system. Therefore, a variable gain amplifier that amplifies the input voltage with a gain proportional to the control voltage can be realized.

In the present invention, the input voltage is differentially amplified by the first differential amplifying means to output two currents, and the two currents are logarithmically compressed by the logarithmic compression means to obtain two currents. A signal is output, and the variable control voltage is converted into a current by the second voltage-current conversion means. The difference between the two signals logarithmically compressed by the logarithmic compression means is supplied to the second differential amplification means in the same manner as described above, and is used as the voltage to be input to the first differential amplification means by the offset means and the error detection means. Control. This input voltage is converted into a control current by the first voltage-current conversion means and supplied to the amplification system. As a result, a control current inversely proportional to the control voltage is obtained, and the amplification system amplifies with a gain inversely proportional to the control current from the control system. Therefore, a variable gain amplifier that amplifies the input voltage with a gain proportional to the control voltage can be realized.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a control circuit of an embodiment of a variable gain amplifier according to the present invention.

FIG. 2 is a circuit diagram mainly showing an amplification system of a variable gain amplifier to which the embodiment of FIG. 1 is applied.

FIG. 3 is a circuit diagram showing a control circuit according to a second embodiment of the variable gain amplifier according to the present invention.

FIG. 4 is a circuit diagram showing a modified example of the control circuit of FIG.

FIG. 5 is a circuit diagram showing a control circuit according to a third embodiment of the variable gain amplifier according to the present invention.

6 is a circuit diagram showing an example of the voltage-current conversion circuit of FIG.

FIG. 7 is a circuit diagram showing a control circuit according to a fourth embodiment of the variable gain amplifier according to the present invention.

FIG. 8 is a circuit diagram showing a conventional variable gain amplifier.

FIG. 9 is an explanatory diagram showing a conventional gain characteristic.

[Explanation of symbols]

11 Logarithmic compression circuit 12,12a First differential amplification circuit 13,13a Second differential amplification circuit 14,14a Offset circuit 15,15a, 15b Error amplifier 16 First voltage / current conversion circuit 17 Second voltage / current Converter circuit Ie, I ₂ current source

Claims (6)

[Claims] 1. A variable gain amplifier including an amplification system for amplifying an input voltage with a desired amplification gain, and a control system for setting the amplification gain of the amplification system based on a control voltage from the outside. Is an input side amplification means for amplifying an input voltage according to a set current based on a control voltage from the control system,
Output side amplifying means for further amplifying an amplified output from the input side amplifying means based on a current from a predetermined current source to obtain a desired output voltage, and the control system controls the set current to the amplifying system. A first current source to be supplied to the input amplifying means, a first differential amplifying means that differentially amplifies the control voltage and outputs two currents, and two first amplifying means from the first differential amplifying means. Logarithmic compression means for obtaining two outputs which are logarithmically compressed current, and 2 from the logarithmic compression means
Second differential amplifying means for receiving two outputs and further obtaining two differentially amplified outputs; and a second current source for supplying the second differential amplifying means with a current for determining the gain of the means. An offset means for adding an offset signal to one output from the second differential amplifying means, a signal to which the offset signal is added by the offset means, and the other signal from the second differential amplifying means. A variable control signal is supplied to the second current source so that the error becomes "0" to control the gain of the second differential amplification means, and the control signal is used to control the first current source. A variable gain amplifier, comprising: an error detection means for controlling the set current output from the variable gain amplifier. 2. The variable gain amplifier according to claim 1, wherein both of the first current source and the second current source generate a current according to a variable control voltage, and the offset means includes the first current source and the second current source. An offset voltage is applied to one output from the second differential amplification means, and the error detection means outputs the voltage to which the offset voltage from the offset means is applied and the other output from the second differential amplification means. A variable gain amplifier characterized in that the first current source and the second current source are controlled by a variable control voltage so that a voltage error becomes "0". 3. The variable gain amplifier according to claim 1, wherein both of the first current source and the second current source generate a current according to a variable control current, and the offset means includes the first current source. An offset current is applied to one output from the second differential amplifying means, and the error detecting means detects the current to which the offset current from the offset means is applied and the other output from the second differential amplifying means. A variable gain amplifier characterized in that the first current source and the second current source are controlled by a variable control current so that a current error becomes "0". 4. A variable gain amplifier including an amplification system that amplifies an input voltage with a desired amplification gain, and a control system that sets the amplification gain of the amplification system based on a control voltage from the outside. Is an input side amplification means for amplifying an input voltage according to a set current based on a control voltage from the control system,
The control system includes an output side amplifying means for further amplifying an amplified output from the input side amplifying means on the basis of a current value from a predetermined current source to obtain a desired output voltage. A first differential amplifying means for amplifying and outputting two currents; and a first differential amplifying means for converting a voltage input to the first differential amplifying means into the set current and supplying the set current to the input side amplifying means of the amplifying system. 1 voltage-current conversion means, logarithmic compression means for obtaining two outputs by logarithmically compressing the two currents from the first differential amplification means, and two differential outputs receiving the two outputs from the logarithmic compression means. Second differential amplification means for amplifying to obtain two outputs, and second voltage-current conversion means for converting and supplying a current for determining the gain of the second differential amplification means from the control voltage. And an offset signal is added to one output from the second differential amplification means. And a variable voltage so that the error between the signal added with the offset signal by the offset means and the other signal from the second differential amplifying means becomes "0".
Variable gain amplifier for supplying the differential voltage to the differential amplifying means and for supplying the same voltage to the first voltage-current converting means. 5. The variable gain amplifier according to claim 4, wherein the offset means applies an offset voltage to one output from the second differential amplification means, and the error detection means offsets by the offset means. The first voltage-current converting means and the first differential amplifying means are arranged so that the error between the voltage applied and the voltage of the other signal from the second differential amplifying means becomes "0". A variable gain amplifier characterized by controlling a supply voltage. 6. The variable gain amplifier according to claim 4, wherein the offset means adds an offset current to one signal from the second differential amplification means, and the error detection means offsets by the offset means. The first voltage-current converting means and the first differential amplifying means are arranged so that the error between the current to which the current is applied and the current of the other signal from the second differential amplifying means becomes “0”. A variable gain amplifier characterized by controlling a supply voltage.