JPH05206783A - Tuner circuit - Google Patents

Abstract

PURPOSE:To always obtain a UHF IF signal of a prescribed level regardless of the type of a UHF intermediate frequency amplifying element by providing a gain selecting means to a UHF intermediate frequency amplifier circuit. CONSTITUTION:A switching transistor TR 43 is set in parallel to a series circuit of a TR 40 and a resistance 41 forming a UHF IF amplifier circuit 8U of an IF preamplifier circuit 8. In regard of a tuner using a silicone FET, no voltage is applied to an external terminal and the TR 43 is turned off as long as the TR 40 is turned on by a band switching circuit 34. Under such conditions, the gain of the circuit 8U is kept so that the signal level of an output terminal 42 is equal to the level set in a VHF reception state. If the tuner uses an AsGa FET, the TR 43 is turned on with the voltage applied to a terminal 44 and the TR 40 is inactivated. As a result, the gain of the circuit 8U is reduced and the total gain is equal to that secured with a silicon FET. Thus an IF signal of a fixed level is always obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a television receiver and a VTR.
The present invention relates to a tuner circuit used in, etc., and particularly to a tuner circuit having an intermediate frequency amplifier circuit.

[0002]

2. Description of the Related Art Conventionally, a tuner circuit of a television receiver has been
It is configured as shown in FIG. 4, and VH is received when VHF is received.
The VHF signal input from the F receiving antenna to the input terminal 1 is a bandpass filter 2 → high frequency (RF) amplifier circuit 3.
→ It is supplied to the VHF mixing circuit 5 through the band-pass filter 4, and is mixed with the local oscillation signal given from the local oscillation circuit 6 through the buffer amplification circuit 7 in the VHF mixing circuit 5 to obtain an intermediate frequency (hereinafter referred to as “IF”). ) Frequency converted to a signal. Then, this IF signal is led to an output terminal 10 through an IF preamplifier circuit 8 → SEPP (single-ended push-pull) type IF amplifier circuit 9, and is output from this output terminal 10 to a subsequent circuit. Is becoming

Further, during UHF reception, the UHF signal input from the UHF receiving antenna to the input terminal 11 is supplied to the UHF mixing circuit 15 through the bandpass filter 12, the high frequency amplifier circuit 13 and the bandpass filter 14,
The UHF mixing circuit 15 mixes with the local oscillation signal supplied from the local oscillation circuit 16 through the buffer amplification circuit 17 and frequency-converts it into an IF signal. Then, the IF signal is led to the output terminal 10 through the IF preamplifier circuit 8 → IF amplifier circuit 9, and is output from the output terminal 10 to the circuit in the subsequent stage.

Then, in an IC part surrounded by a dotted line in FIG. 4, VHF and UHF composed of a double balance type mixer are provided.
There is one mixing circuit for each, and a band switching circuit for performing band switching is separately provided.

FIG. 5 shows more specifically the structure within the dotted line frame in FIG. 4, and in particular, the IF preamplifier circuit 8 is shown here.
The circuit example of is clearly shown. The IF preamplifier circuit 8 is composed of a VHF IF amplifier circuit section 8V and a UHF IF amplifier circuit section 8U which are selectively operated by the band switching circuit 34, and their respective amplifier circuit sections. 8V and 8U are formed by a differential amplifier circuit. In FIG. 5, the VHF mixing circuit 5 and the UHF mixing circuit 15 are shown as blocks, but these mixing circuits 5 and 15 are shown.
Is also band-switched by the band-switching circuit in synchronism with the band switching of the amplifier circuit units 8V and 8U.

In FIG. 5, the VHF mixing circuit 5, UH
The differential outputs converted into the IF signals by the F mixing circuit 15 are once emitter-follower type transistors 21 and 22.
Are supplied to the bases of the transistors 27, 28, 35, and 36 via, respectively. In the differentially input signal, since the transistor 33 is turned on and the transistor 40 is turned off by the band switching circuit 34 at the time of receiving VHF, an amplified IF signal is output from the collector side of the transistor 28 and the output terminal is output. 42.

On the other hand, when UHF is received, the transistor 33
Is OFF and the transistor 40 is ON, so
The IF amplified output is led to the output terminal 42 from the collector side of the transistor 36. These amplified IF signals are supplied to the external IF amplifier circuit 9 via the output terminal 42.

[0008]

By the way, when a field effect transistor (hereinafter referred to as "FET") is used in a UHF high-frequency amplifier circuit of an electronic tuner, silicon is used as a material of the FET and low-performance type is used as a material of the FET. Gallium arsenide is used. As is well known, it is better to use gallium arsenide FET because it has a better noise figure, but since the gain of this gallium arsenide FET is quite large, there were the following problems.

That is, in the tuner circuit having the conventional configuration, if the gains after the mixing circuit 15 are the same, there is a difference in gain between the tuner using the gallium arsenide FET and the tuner using the silicon FET. Inconvenience occurred in the processing in the subsequent circuit. Further, in order to make the gains of both the same, it was necessary to develop an IC suitable for each. The present invention solves such a problem,
It is an object of the present invention to provide a tuner circuit that can always obtain a UHF / IF signal of a predetermined size regardless of the type of F high frequency amplification element.

[0010]

To achieve the above object, the present invention has a band switching function of VHF and UHF, and amplifies an IF signal from a VHF mixing circuit by a VHF IF amplification circuit when VHF is received. However, when receiving UHF, U
In the tuner circuit for amplifying the IF signal from the HF mixing circuit by the UHF IF amplifying circuit, gain selecting means is provided in the UHF IF amplifying circuit.

In this case, the UHF IF amplifier circuit is composed of a differential amplifier circuit, and the gain selection means is
It comprises a transistor connected in parallel to the constant current source of the differential amplifier circuit, and a terminal for applying a voltage to the control electrode of this transistor.

[0012]

According to this structure, when a silicon FET is used as a UHF high frequency amplifying element, the gain of the UHF IF amplifying circuit is set to a higher value by the gain selecting means and a gallium arsenide FET is used. By setting the gain low, it is possible to always output a UHF / IF signal of a constant magnitude regardless of the type of the high frequency amplification element used.

The gain selection means is composed of a transistor connected in parallel to the constant current source of the UHF IF amplification circuit composed of a differential amplification circuit, and a terminal for applying a voltage to the control electrode of this transistor. In this circuit, the gain of the UHF IF amplifier circuit is selected according to the voltage externally applied to the terminal.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. In FIG. 1 in which the present invention is implemented, the same parts as those in the conventional example of FIG. As shown in FIG. 1, in this embodiment, U of the IF preamplifier circuit 8 is used.
A switching transistor 43 is provided in parallel with a series circuit of a transistor 40 and a resistor 41 forming a constant current source of the HF IF amplification circuit unit 8U, and a terminal 44 for applying a control voltage is provided at the base of the switching transistor 43.

Next, the operation will be described. Now band switching circuit 3
4, the transistor 40 is turned on.
At this time, in the case of a tuner using a silicon FET in the high frequency amplifier circuit, no voltage is applied to the external terminal 44. Therefore, the switching transistor 43 is off
Therefore, the gain of the UHF IF amplifier circuit unit 8U is determined by the resistors 37, 38, 39 and 26. Specifically, 37, 3
Since the delta connection of 8 and 39 can be represented as a Y connection, the equivalent circuit of the UHF IF amplification circuit unit 8U is as shown in FIG. If the value of the resistor 26 is R0, the gain is R0 /
Although (R1 + R2), this value is determined so that the signal level at the output terminal 42 of the tuner using the silicon FET in the UHF high-frequency amplifier circuit at the time of receiving UHF is equal to that at the time of receiving VHF.

Next, gallium arsenide FE is added to the high frequency amplifier circuit.
In the case of a tuner using T, a voltage is applied to the external terminal 44 to turn on the switching transistor 43. This renders transistor 40 inoperative and U
An equivalent circuit of the HF IF amplification circuit unit 8U is as shown in FIG. If the value of the resistor 37 is R4, the gain is R0 / R4. Therefore, the values of the resistors 37, 38 and 39 are set to R0 /
It may be determined so that (R1 + R2) -R0 / R4 becomes equal to the gain difference between the gallium arsenide FET and the silicon FET.

[0017]

As described above, according to the tuner circuit of the present invention, the gain of the IF preamplifier circuit can be increased by the gain selecting means regardless of whether the high-frequency amplifier circuit for UHF is made of silicon or gallium arsenide. The overall gain can be made the same simply by setting the switching. As a result, the cost is lower than the case where different IF preamplifier circuits are designed and used for the case of using the silicon FET and the case of using gallium arsenide, and the IF preamplifier circuit is integrated into an IC. In that case, the versatility of the IC is maintained.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration example of a tuner circuit embodying the present invention.

FIG. 2 is an equivalent circuit diagram of a part thereof.

FIG. 3 is an equivalent circuit diagram of a part of the same in a different state.

FIG. 4 is a diagram showing a block configuration example of a conventional tuner.

FIG. 5 is a diagram showing a specific circuit configuration example thereof.

[Explanation of symbols]

 5 VHF mixing circuit 8 IF preamplification circuit 8V VHF IF amplification circuit section 8U UHF IF amplification circuit section 15 UHF mixing circuit 40 Transistor 43 constituting constant current source 43 Switching transistor 44 External terminal

Claims (2)

[Claims] 1. A band switching function of VHF and UHF,
A tuner circuit that amplifies the intermediate frequency signal from the VHF mixing circuit with the VHF intermediate frequency amplification circuit during VHF reception, and amplifies the intermediate frequency signal from the UHF mixing circuit with the UHF intermediate frequency amplification circuit during UHF reception. Tuner circuit in which a gain selecting means is provided in an intermediate frequency amplifier circuit for use. 2. The intermediate frequency amplifying circuit for UHF is composed of a differential amplifying circuit, and the gain selecting means includes a transistor connected in parallel with a constant current source of the differential amplifying circuit, and the transistor. A tuner circuit comprising a terminal for applying a voltage to a control electrode.