JPH07297656A - Variable gain amplifier circuit - Google Patents

Abstract

PURPOSE:To prevent the functions of input/output coupling circuits and a neutralization circuit from being impaired by changing and controlling the impedances of the input and output coupling circuits and the neutralization circuit accompanying the control of a power supply voltage to a transistor element. CONSTITUTION:When a switch SW1 is turned on or off and the power supply voltage VE 1 to the collector of a transistor TR1 is changed so as to control amplification gain, the collector capacitance of the transistor TR1 is changed. The switches SW2-SW4 are turned on or off in synchronism with the SW1 so as to compensate the change of the collector capacitance. Thus, the impedances of the respective varactor diodes D1-D3 of the input/output coupling circuits M1 and M2 and the neutralization circuit N1 are changed. Respective constants and supply voltage values VE2-VE4 are set beforehand so as to make the respective impedances of the input/output coupling circuits M1 and M2 and the neutralization circuit N1 be optimum values corresponding to the change of the impedances of the diodes D1-D3.

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 circuit, and more particularly to a variable gain amplifier circuit having an input / output coupling circuit at each of the input and output of an amplifying transistor element.

[0002]

2. Description of the Related Art FIG. 3 shows an example of this type of variable gain amplifier circuit. An input coupling circuit M1 for coupling with a preceding stage (not shown) is provided, which performs impedance matching with the preceding stage circuit and tuning for an input signal, and includes coils L1 to L3 and capacitors C1 and C3.

The amplifying transistor Tr1 for amplifying the output of the input coupling circuit is of a common base type and generally operates with a class B or C bias for improving efficiency.

An output coupling circuit M2 is provided to couple the amplified output to a subsequent stage (not shown), and the transistor amplified output contains many spurious components due to the class B or C operation of the transistor Tr1. Therefore, the spurious component is removed and only the necessary signal component is extracted and derived. This circuit M2 is composed of coils L4, L5 and capacitors C4 to C7.

The transistor Tr1 can control the gain by changing the power supply voltage applied to its collector. Therefore, the power supply voltage VE1 is switched by the parallel circuit of the switch SW1 and the resistor R1. Has become.

Incidentally, L7 is an AC blocking choke coil, and the capacitor C10 is for AC bypass.

[0007]

In such a variable gain amplifying circuit as described above, the collector voltage of the transistor Tr1 is controlled to control the gain, but the general relation between the collector voltage Vc of the transistor and the collector capacitance Cob is shown. Since Cob = 1 / Vc1 / ² (1), when the power supply voltage to the transistor Tr1 is changed, the collector capacitance changes accordingly. Therefore, the input impedance of the transistor Tr1 changes.

In this way, when the gain is switched, the input impedance of the transistor changes, so that each tuning frequency of the input / output coupling circuits M1 and M2 changes, causing impedance mismatch between the preceding and succeeding circuits. Further, the tuning frequency of the output coupling circuit also changes, and the output spurious of the transistor Tr1 cannot be cut off sufficiently.

Further, the influence of internal feedback from the collector to the emitter of the transistor cannot be ignored particularly at high frequencies. Therefore, a neutralization circuit is added between the input and output of the transistor to compensate for this internal feedback. In this case, the internal feedback voltage also changes due to the change in the collector capacitance of the transistor due to the gain switching, and the neutralization is not complete.

Therefore, in order to prevent the influence of the change in impedance due to the change in the collector capacitance of the transistor, a method of connecting an isolator to the input or output side of the variable gain amplifier circuit or matching with the preceding and following stage circuits is inserted. There is a method of stably operating the transistor by appropriately adjusting the output coupling circuit.

The method using an isolator has drawbacks such as that it is impossible to sufficiently prevent a reflected wave generated due to a mismatch with the preceding and following circuits outside the band of the isolator, and the scale is increased in terms of hardware.

In addition, the method of adjusting the input / output coupling circuit of the transistor is difficult to adjust, and when the switching width (gain width) of the applied power supply voltage of the transistor 1 is increased, matching is more and more difficult. Therefore, there is a drawback that the variable gain width becomes small.

In Japanese Patent Laid-Open No. 56-040309, a variable capacitance element is provided in a matching circuit in a high frequency transistor amplifier circuit, and the impedance of the variable capacitance element is controlled according to the amplitude of an input signal for matching. A technique for automatically controlling impedance matching of a circuit is disclosed.

However, in this technique, the matching of the matching circuit is merely automatically controlled according to the input signal amplitude, and the adjustment of the input / output coupling circuit due to the variation of the collector capacitance of the transistor due to the gain control is adjusted. The disadvantage is that the neutralization circuit cannot be adjusted.

An object of the present invention is to provide a variable gain amplifier circuit which automatically controls the impedance of an input / output coupling circuit in accordance with a change in the collector capacitance of a transistor which fluctuates as the gain is controlled.

Another object of the present invention is to provide a variable gain amplifier circuit capable of increasing the gain variable width.

Still another object of the present invention is to provide a variable gain amplifier circuit which automatically controls the neutralization characteristic of the neutralization circuit according to the change of the collector capacitance of the transistor due to the gain control. .

[0018]

According to the present invention, an input coupling circuit for coupling with the preceding stage, an amplifying transistor element for amplifying the output of this input coupling circuit, and an amplified output of this transistor element are used as inputs. A variable gain amplifier circuit, comprising: an output coupling circuit which is coupled to a subsequent stage; and a power supply voltage control means for controlling a change of a power supply voltage supplied to the transistor element so as to vary an amplification gain. A variable gain amplifier circuit is provided which has impedance control means for changing and controlling the impedance of the input and output coupling circuits according to the control of the power supply voltage by the means.

According to the present invention, the neutralization circuit provided between the input and output of the transistor element for compensating the internal feedback of the transistor element, and the neutralization according to the control of the power source voltage by the power source voltage control means. A variable gain amplifier circuit is obtained which further includes impedance control means for changing and controlling the impedance of the circuit.

[0020]

[Function] With the switching of the collector voltage for controlling the gain of the transistor, the impedance of the input / output coupling circuit is changed by the variable capacitance element, so that the impedance matching in the input / output coupling circuit and the tuning frequency of the output coupling circuit are always kept optimal. To do so.

Further, the impedance of the neutralization circuit is also variable by the variable capacitance element so that the neutralization due to the internal feedback of the transistor is always maintained well.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of the present invention,
The same parts as those in FIG. 3 are denoted by the same reference numerals. In the input coupling circuit M1, the variable capacitance diode D1 is provided in parallel with the capacitor C3, and the voltage VE2 applied to this diode D1 is switched by the parallel circuit of the switch SW2 and the resistor R2.

Further, in the output coupling circuit M2, the variable capacitance diode D2 is provided in parallel with the capacitor C7, and the voltage VE3 applied to this diode D2 is switched by the parallel circuit of the switch SW3 and the resistor R3.

The capacitors C2 and C8 are for DC blocking, the coils L6 and L8 are AC blocking choke coils, and the capacitors C9 and C11 are for AC bypassing.

Further, a neutralization circuit N1 is provided between the input and output (emitter and collector) of the transistor Tr1.
It is composed of capacitors C12 and C13 and a variable capacitance diode D3. In the neutralization circuit N1, the neutralization voltage VN having the same magnitude as the internal feedback voltage Vr due to the collector capacitance and having the opposite phase
Are returned to the input and Vr + VN = 0 are set to cancel each other.

The voltage VE4 applied to the diode D3 is switched by the parallel circuit of the switch SW4 and the resistor R4.

L9 is an AC blocking choke coil, C
14 is an AC bypass capacitor.

The other structure is the same as that of FIG. 3 and its explanation is omitted.

When the switch SW1 is turned on or off to control the amplification gain and the power supply voltage VE1 to the collector of the transistor Tr1 is changed, the collector capacitance of the transistor changes according to the relationship shown in the above equation (1). In order to compensate for this change in the collector capacitance, all the switches SW2 to SW4 are turned on or off in synchronization with the switch SW1.

By doing so, the input / output coupling circuit M
1, M2 and each variable capacitance diode D1 of the neutralization circuit N1
The impedance of ~ D3 also changes accordingly. Therefore, the constants and the supply voltage value V are set in advance so that the impedances of the input / output coupling circuit and the neutralization circuit become optimum values according to the changes in the impedances of the diodes D1 to D3.
By setting E2 to VE4, the matching function, tuning function, and neutralizing function of each circuit can always be optimally maintained.

FIG. 2 is a circuit diagram of another embodiment of the present invention, in which the same parts as in FIG. 1 are designated by the same reference numerals. In this example, the collector supply power supply voltage VE of the transistor Tr1
1 is switched in multiple stages to increase the gain variable width, and a digital control system using a computer 10 is adopted as the power supply control circuit 1.

The control digital signals output from the computer 10 are converted into analog signals by the D / A converters 11 to 14, and the voltage controllers 15 to 18 corresponding to the power supply voltages VE1 to VE4 in accordance with the levels of the analog signals. Is variably controlled and derived.

The output voltage of the voltage controller 16 is the collector power supply voltage of the transistor Tr1.
Output voltage of 5, 17, 18 is variable capacitance diode D
The control voltages are 1, D2 and D3.

By doing so, the impedances of the input / output coupling circuits M1 and M2 and the neutralization circuit N1 are also variably controlled in synchronization with the variable gain, so that even if the variable gain width is increased, the impedance of each circuit is automatically adjusted. The impedance is adjusted.

[0036]

As described above, according to the present invention, the tuning frequency fluctuation and impedance mismatch of the input / output coupling circuit, which occur when the collector applied voltage to the transistor is switched for variable gain, are eliminated, and spurious output is eliminated. The effect is that it can be suppressed. It also has the effect of canceling the internal feedback voltage of the transistor to complete neutralization.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention.

FIG. 2 is a circuit diagram of another embodiment of the present invention.

FIG. 3 is a diagram showing an example of a conventional variable gain amplifier circuit.

[Explanation of symbols]

 Tr1 transistor M1 input coupling circuit M2 output coupling circuit N1 neutralization circuit SW1 to SW4 gain changeover switch D1 to D3 variable capacitance diode

Claims (4)

[Claims] 1. An input coupling circuit for coupling with a preceding stage, an amplifying transistor element for amplifying an output of this input coupling circuit, and an output coupling circuit for coupling with a latter stage by using an amplified output of this transistor element as an input. And a power supply voltage control means for changing and controlling a power supply voltage to be supplied to the transistor element so as to change an amplification gain, wherein the power supply voltage control means controls the power supply voltage. A variable gain amplifier circuit having impedance control means for controlling the impedance of an input and output coupling circuit. 2. A neutralization circuit provided between the input and output of the transistor element for compensating internal feedback of the transistor element, and an impedance of the neutralization circuit according to control of the power source voltage by the power source voltage control means. The variable gain amplifier circuit according to claim 1, further comprising impedance control means for controlling change. 3. The impedance control means includes a variable capacitance element, and means for controlling the voltage applied to the variable capacitance element in accordance with the control of the power supply voltage by the power supply voltage control means. Item 2. A variable gain amplifier circuit according to item 1 or 2. 4. The power supply voltage control means is configured to switch and control the power supply voltage in multiple stages, and the impedance control means controls the voltage applied to the variable capacitance element in response to the multistage switching of the power supply voltage. 4. The variable gain amplifier circuit according to claim 3, wherein the variable gain amplifier circuit is configured to perform switching control in multiple stages.