JPH0837425A - Low distortion mixer - Google Patents

Abstract

PURPOSE:To obtain the mixer circuit with an excellent intermodulation distortion characteristic even when an RF signal is leaked to a gate terminal through a parasitic capacitance by employing two FETs and supplying signals having a phase difference to the drains of the FETs. CONSTITUTION:A gate of a FET 4 and a gate of a FET 14 are connected to obtain a common gate, an LO signal is given from a terminal 1 and given to the common gate via a matching circuit. On the other hand, an RF signal is given from a terminal 2 and signals are fed to the drains of the FETs with a phase difference of 180omicron through a 180 deg. distribution circuit 9. IF signals are outputted with a phase difference of 180omicron via an IF filter from the drains of the FETs, and the IF signals are extracted from a 180omicron synthesis circuit 10 via a terminal 3. The RF signal from the drain of the FET 4 leaked to the common gate via a parasitic capacitance between the gate and the drain and the RF signal from the drain of the FET 14 to the common gate via the parasitic capacitance are opposite in phase to each other and they are cancelled out at the common gate terminal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mixer circuit used in a microwave circuit for communication.

[0002]

2. Description of the Related Art When a mixer is used as a receiver, distortion due to intermodulation between a plurality of radio frequency band (hereinafter also referred to as RF band) input signals has a great influence on reception characteristics. As a conventional example of a mixer made for the purpose of reducing such a distortion signal, there is a resistive FET mixer circuit shown in FIG.

This mixer utilizes the change of the channel resistance of FET (reference: Stephen A. Maa).
s, "A GaAs MESFET mixer wi
thvery low intermodulation
n, "IEEE Trans. Microwave T
history Tech. , Vol. MTT-35, p
p. 425-429, April 1987. reference).

In FIG. 4, the local oscillator output signal (hereinafter also referred to as LO signal) input from the input terminal 1 is input to the gate of the FET 4 via the matching circuit 5 and the RF input from the input terminal 2. The signal is input to the drain of the FET via the matching circuit 6. Then, an intermediate frequency band signal (hereinafter also referred to as an IF signal) having a difference frequency between the two signals is taken out from the output terminal 3 via the IF filter 3.

In order to explain the operating principle of this mixer,
FIG. 5 shows an example of the current-voltage characteristics of the FET. The characteristics differ from each other in the gate-source voltage (V _gs ).
In the mixer of FIG. 4, no DC voltage is applied to the drain, and only the RF signal is input to the drain.

Generally, the LO signal is a signal of large amplitude, R
Since the F signal is a small signal as compared with it, the drain-source voltage (V _ds ) changes at the RF frequency within the non-saturation region of the FET. Since the slope of the FET current-voltage characteristic in the non-saturated region represents the channel resistance of the FET, the channel resistance is always almost constant for the RF signal.

On the other hand, since the LO signal is input to the gate, the channel resistance is
It changes and operates as a mixer. In such a mixer, the nonlinearity to the RF signal is very weak, so that even if a plurality of RF signals are input, the intermodulation distortion between them is small.

[0008]

In the conventional mixer as described above, since a DC voltage is not applied between the drain and the source, the parasitic capacitance C _gd between the gate and the drain is relatively large. There is a problem that the intermodulation distortion characteristics are deteriorated due to the RF signal leaking into the gate terminal. This is the gate-source voltage V _gs
This is because the channel resistance of the FET greatly changes even with a slight change in

The present invention has been made in order to solve the above-mentioned conventional problems. Even if the RF signal leaks into the gate terminal due to the parasitic capacitance, the intermodulation distortion characteristic is excellent. An object is to provide a low distortion mixer circuit.

[0010]

According to the invention, the above mentioned objects are achieved by means of the patent claims.

That is, the first aspect of the invention is to provide a first field effect transistor (hereinafter referred to as FET) and a second FE.
T, and a gate of the first FET and a second FET
To form a common gate, the local oscillator output signal is input to the common gate, and the drain of the first FET,

Radio frequency band signals are input to the drain of the second FET so as to have a phase difference of 180 degrees from each other, and the source of the first FET and the second FE are input.
It is a low distortion mixer configured so that the source of T is grounded and an intermediate frequency band signal is taken out from the drain of the first FET and the drain of the second FET.

The invention of claim 2 has a first FET and a second FET, and a gate of the first FET and a second FET.
To form a common gate, a local oscillator output signal is input to the common gate, and an intermediate frequency band signal is applied to the drain of the first FET and the drain of the second FET. Input with a phase difference of 180 degrees,

The source of the first FET and the source of the second FET are grounded, and a radio frequency band signal is taken out from the drain of the first FET and the drain of the second FET. It is a low distortion mixer configured in.

According to a third aspect of the present invention, in the first and second aspects of the present invention, a circuit including a first FET and a second FET is provided with one source, one gate, and two drains. It is configured as an integrated FET having a.

[0016]

In the low distortion mixer of the present invention, two FETs are provided,
The local oscillator output signal is commonly input to the gates of the two FETs via a matching circuit. In addition, two FETs are provided for the RF signal or IF signal through the 180 degree distribution circuit and the matching circuit.
Input to the drain with a phase difference of 180 degrees. Then, the IF signal or the RF signal is combined by 180 degrees and taken out from the output terminal.

According to the above configuration, the RF signals leaking due to the parasitic capacitance cancel each other out at the gate terminal, and
The change in channel resistance due to the signal is eliminated, and the intermodulation distortion characteristic is improved. Hereinafter, the operation of the present invention will be described in more detail with reference to Examples.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing a first embodiment of the present invention, in which numeral symbols 4, 14 are FETs, 5 is an LO matching circuit,
6, 16 are RF matching circuits, 7, 17 are IF filters, 9
Is a 180 degree distribution circuit for RF signals, 10 is a 180 degree synthesis circuit for IF signals, 1 is an LO input terminal, 2 is R
An F input terminal, 3 is an IF output terminal, and 8 is a terminal for supplying a voltage to the gate.

In the same figure, the gate of the first FET 4 and the gate of the second FET 14 are connected to form a common gate, and the LO signal is input from the input terminal 1 and shared via the matching circuit 5. Input to the gate. On the other hand, the RF signal is inputted from the input terminal 2 and is inputted to the drains of the respective FETs with a phase difference of 180 degrees by the 180 degree distribution circuit 9.

The IF signal is I from the drain of each FET.
Since they are output with a phase difference of 180 degrees through the F filter, they are taken out through the 180 degree synthesizing circuit 10 and output from the output terminal 3.

According to such a configuration, the first FET 4
The RF signal leaking from the drain of the first gate to the common gate via the parasitic capacitance C _gd between the gate and the drain, and the second FET1.
R leaking from the drain of 4 into the common gate through C _gd
The F signal and the F signal have opposite phases to each other and cancel each other out at the common gate terminal.

Therefore, the channel resistance of the two FETs is
Since the change due to the RF signal disappears and the change occurs only with the LO signal, the distortion of the resistive FET mixer can be further reduced. Further, in this configuration, the LO input terminal 1 and the RF are compared to the conventional example.
There is also a side effect of increasing the isolation of the input terminal 2.

FIG. 2 shows a second embodiment of the present invention and shows an example of an up converter for converting an IF signal into an RF signal. The IF signal input of the up-converter used in the transmitter is generally one wave, but depending on the method, multiple IF signals may be input, in which case intermodulation distortion between IF signals may occur. It becomes a problem.

In FIG. 2, the LO signal is input via the input terminal 1 and the matching circuit 5 to the common gate terminal of the gate of the first FET 4 and the gate of the second FET 14. In addition, the IF signal is input from the input terminal 22
180 to the drain of each FET via the power distribution circuit 29
Input with a phase difference of degrees.

Since RF signals are output from the drains of the respective FETs through the matching circuits 6 and 16 with a phase difference of 180 degrees, these are taken out from the output terminal 23 through the 180 degree synthesizing circuit 30. According to such a configuration,
Among RF signals which are output signals, the signals leaking to the common gate via the parasitic capacitance C _gd cancel each other out. Therefore, even when a plurality of IF signals are input, the intermodulation distortion between those IF signals becomes small.

FIG. 3 is a diagram showing a third embodiment of the present invention, which is an example in which the two FET parts of the mixer are implemented by an integrated FET having a structure suitable for an IC circuit. First, the first FET 4 and the second FET 1 shown in FIG. 1 and FIG.
In No. 4, the source and the gate are connected, and only the drain is not connected.

Therefore, as shown in FIG. 3, the FET portion of the mixer according to the present invention has a source 32 and a source 32 indicated by numeral 31.
It can be configured as an integrated FET having one gate shown by and two drains shown by 34 and 35. FET
Then, n-GaAs formed on a semi-insulating GaAs substrate
Electrons flow from the source to the drain in the active layer,
This electron flow is controlled by the gate voltage.

Therefore, in this embodiment, as shown in FIG. 3, the first drain 33 and the second drain 34 are provided.
The n-GaAs active layer immediately below is divided into a first n-GaAs active layer 35 and a second n-GaAs active layer 36, so that an electron flow from the first drain 33 to the second drain 34. Is configured to not flow.

When the mixer of the present invention is implemented by using this structure as an IC circuit, the first drain 33 and the gate 32 are formed.
Since the parasitic capacitance between them and the parasitic capacitance between the second drain 34 and the gate 32 are exactly the same, R leaking into the gate 32
The effect of canceling the F signals with each other is further enhanced.

In the above embodiment, semi-insulating GaAs is used.
Although the case where the FET is formed by using the substrate and the n-GaAs active layer is shown, the present invention is not limited to this.
It goes without saying that any material may be used as long as it can form the FET.

[0031]

As described above, according to the present invention,
The RF signals leaking due to the parasitic capacitance cancel each other out at the gate terminal, so that there is no change in the channel resistance due to the RF signal, and a low distortion mixer with good intermodulation distortion characteristics can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a second embodiment of the present invention.

FIG. 3 is a diagram showing a third embodiment of the present invention.

FIG. 4 is a diagram showing an example of a conventional resistive FET mixer circuit.

FIG. 5 is a diagram showing an example of current-voltage characteristics of an FET.

[Explanation of symbols]

 1,2,22 Input terminal 3,23 Output terminal 4,14 FET 5,6,16 Matching circuit 7,17 IF filter 8 Power supply voltage terminal 9,29 180 degree distribution circuit 10,30 180 degree combining circuit 31 Source 32 Gate 33 first drain 34 second drain 35 first n-GaAs active layer 36 second n-GaAs active layer 37 semi-insulating GaAs substrate

Claims (3)

[Claims] 1. A first field effect transistor (hereinafter FE)
T)) and a second FET, the gate of the first FET and the gate of the second FET are connected to form a common gate, and the local oscillator output signal is supplied to the common gate. And inputting a radio frequency band signal to the drain of the first FET and the drain of the second FET so as to have a phase difference of 180 degrees from each other, and to the source of the first FET, A low distortion mixer, characterized in that the source of the second FET is grounded and an intermediate frequency band signal is extracted from the drain of the first FET and the drain of the second FET. 2. A first FET and a second FET, wherein the gate of the first FET and the gate of the second FET are connected to form a common gate, and the common gate is locally connected to the common gate. An oscillator output signal is input, an intermediate frequency band signal is input to the drain of the first FET and the drain of the second FET so as to have a phase difference of 180 degrees from each other, and A low frequency band signal is extracted from the drain of the first FET and the drain of the second FET by grounding the source and the source of the second FET. Distortion mixer. 3. A circuit composed of a first FET and a second FET is provided with one source, one gate and two drains.
3. The low distortion mixer according to claim 1 or 2, wherein the low distortion mixer is configured as an integral FET having two.