JPS6228607B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a variable gain circuit using field effect transistors. Regarding variable gain circuits.

Generally, an AGC circuit is provided at the receiving output terminal of the carrier terminal device, etc., to automatically compensate for fluctuations in the received signal level due to temperature fluctuations in the transmission loss of the transmission line. This AGC circuit adjusts the gain by using a variable impedance element whose impedance value changes depending on the magnitude of a control voltage or control current, for example, as a feedback resistor of an amplifier.

A field effect transistor (hereinafter referred to as FET) can be used as one such variable impedance element. FIG. 1 shows an example of a circuit in which an FET is used as a variable impedance element. In the same figure, the impedance used as variable impedance is the drain D of FET F ₀
and the impedance R _DS between source S. This impedance value R _DS is the source S and gate G connected to the reference potential point (ground in Figure 1).
The voltage V _GS (external DC power supply voltage E in FIG. 1) between the two is given by the following equation.

R _DS =1/K (1/V _GS -V _P -V _DS /2)...
(1) Here, K is a constant specific to the FET, and V _P is the pinch-off voltage, which is also a value specific to the FET. Further, although V _DS in equation (1) is a DC voltage value between the drain and source, normally a minute AC signal V _DS is applied. Therefore, considering the minute alternating current impedance R _ds as the impedance between the drain and the source, the following equation can be obtained in the same way as equation (1).

R _ds = 1/K (1/V _GS -V _P -V _ds /2)...
(2) Equation (2) indicates that when the gate-source voltage V _GS changes, the drain-source impedance changes. Therefore, by using this drain-source impedance, for example, as a feedback resistance of the amplifier, the gain of the amplifier can be voltage-controlled.

By the way, when the minute current signal e _g is applied to the drain terminal of FIG. 1, the current I _ds flowing between the drain and source of the FET is given by the following equation.

I _ds = e _g /R _ds = Ke _g (V _GS −V _P −e _g /2) = K
(V _GS −V _P )e _g −Ke〓/2 ...(3) The second term on the right side of equation (3), Ke ² _g /2, is essential for the AC signal to which the current I _ds is applied. This indicates that second-order distortion is generated. Thus, for example, FDM, which multiplexes telephone signals in the frequency domain.
(Frequency Division Multiplex) type carrier terminal equipment, it is necessary to maintain good transmission quality.
When FETs are used in the AGC circuit of a device that strictly limits noise due to second-order distortion, it is necessary to improve the second-order distortion described above.

Conventionally, as a circuit for reducing distortion in a variable gain circuit using an FET as a variable impedance element, a circuit has been proposed that supplies the gate of the FET with an AC voltage that is half of the AC voltage applied between the drain and source of the FET. (See Special Publication No. 55-2769).

However, in the conventional type, a fairly complicated circuit is used to create an AC voltage that is half the AC voltage applied between the drain and source, and the channel and substrate (to which the source and drain electrodes of the FET are connected) are used. There was a problem in that distortion could not be sufficiently suppressed due to the influence of parasitic capacitance between the substrate and the substrate.

In view of the above-mentioned problems with the conventional type, an object of the present invention is to provide a control circuit output with sufficiently low impedance to the gate of the FET, which is a variable impedance element connected to a differential amplifier circuit, in a variable gain circuit using FETs. Connect and connect
Based on the idea of grounding the base of the FET using a capacitor in an alternating current manner, the aim is to sufficiently reduce the second-order distortion of the FET with an extremely simple circuit.

The present invention provides an amplifier circuit composed of a differential pair circuit, and a source between two points of the amplifier circuit to which AC signals having the same potential in DC and the same amplitude and opposite phases in AC are applied. and a field effect transistor to which the drain is connected, and a control circuit output having a sufficiently low impedance in terms of AC is connected to the gate of the field effect transistor, and the substrate of the field effect transistor is connected to the AC by using a capacitor. It is characterized by being grounded to.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 2 shows a variable gain circuit according to the present invention applied to a known modulation circuit. Transistor Q ₁
and T ₂ constitute a differential pair circuit in a differential amplifier circuit, and an input signal e _i such as an audio signal is applied between the bases of these transistors Q ₁ and Q ₂ , that is, between terminals 1 and 2. . The source and drain of FET _F1 are connected between the emitters of these transistors _Q1 and _Q2 , and the gain of the differential amplifier circuit is determined by the drain-source impedance _Rds of FET _F1 . Ru. transistor Q ₃
and Q ₄ constitute a constant current circuit provided so that the gain of the differential amplifier circuit is determined by the drain-source impedance R _ds of the FET F ₁ ,
A resistor is connected to the base of these transistors Q ₃ and Q ₄ .
A bias circuit consisting of R ₁ , R ₂ and diode D ₁ is connected. Resistors R ₃ and R ₄ connected between the emitters of transistors Q ₃ and Q ₄ and the negative voltage power supply -Vcc are resistors that determine the current flowing through the constant current circuit. Transistors Q ₅ or Q ₈ perform switching every half wave of carrier wave e _c applied between terminals 1 and 2, and together with resistor R ₆ and transformer T ₁ constitute a modulation circuit. In addition, in the above, the drain D and source S of FET F ₁ are connected to the emitters of transistors Q ₁ and Q ₂ , which are the points where signals having the same potential in DC and opposite phase and the same amplitude in AC are applied. ing. A control voltage source (not shown) having a sufficiently low AC output impedance is connected to the gate G of FET _F1 , that is, the control voltage terminal 7. Also, applicable
The base SUB of FET F ₁ is connected to the constant voltage power supply via resistor R ₅ .
It is connected to V ₀ and also to a reference potential point, for example, ground, via a capacitor C ₁ .

In the embodiment shown in FIG. 2, the input signal e _i input between terminals 1 and 2 is amplitude-modulated by the carrier wave e _c applied between terminals 3 and 4, and output from between terminals 5 and 6. However, since the operation of such a modulation circuit is well known, detailed explanation thereof will be omitted. However, the modulation circuit shown in FIG. 2 has a variable gain circuit using FET _F1 , and the magnitude of the modulated output voltage can be changed by the control voltage V _S applied to the control voltage terminal 7. There are characteristics.

Next, in the modulation circuit of Fig. 2, terminals 1-2
_The input signal e _i applied between _{FET F1}
Assuming that an AC signal of +e _i /2 is transmitted to the drain D and -e _i /2 to the source S, the drain-source voltage V _ds is expressed by the following equation. It can be done.

V _ds = e _i /2-(-e _i /2) = e _i ...(4) Also, as mentioned above, a control voltage is applied to the gate of FET F ₁ from a control voltage source whose drive impedance is sufficiently low in AC terms. Since V _S is applied, FET F ₁
The gate-source voltage V _GS of is given by the following equation.

V _GS = V _S + e _i /2 ...(5) Therefore, the alternating current signal I _ds flowing between the drain and source of FET F ₁ is calculated as follows by substituting equation (5) into equation (3). It is given by Eq.

I _ds = Ke _i (V _S +e _i /2−V _P −e _i /2) = Ke _i (V
_S −V _P ) ...(6) Since the second-order distortion term is eliminated in equation (6), the above-mentioned configuration eliminates the second-order distortion of the variable gain circuit using FETs. be able to.

However, as shown in Figure 3, FET
For example, a channel CH made of an N-type semiconductor is formed on a P-type semiconductor substrate SUB, and the channel
Drain electrode D, source electrode S and P on CH
The structure is such that a gate electrode G made of a semiconductor or the like is formed. FETs thus typically have a substrate SUB that is equal to the source or drain potential or is connected via a resistor to a lower potential point. In Figure 2, the base SUB
is connected to a power supply at a constant potential V ₀ via a resistor R ₅ . However, in reality, the channel CH and the substrate
Since there is a parasitic capacitance between SUB and SUB,
An alternating current signal is induced in the SUB, and due to the influence of this signal voltage, the second-order distortion cannot be completely suppressed as shown in equation (6). In the embodiment of FIG.
The capacitor C ₁ connected between the base body SUB of the FET F ₁ and the ground serves to prevent deterioration of secondary distortion due to induced voltage by connecting the base body to a reference potential point in an alternating current manner.

In addition, although the case where the variable impedance element using FET was used to change the gain of the modulation circuit was explained above, the variable gain circuit of the present invention is not limited to the above example, and can be used as a normal differential amplifier. It is clear that the method can also be applied to Further, in the above embodiment, a variable impedance element using an FET is connected between the emitters of the transistors forming the differential pair circuit, but it can also be connected between the collectors of the transistors. Furthermore, the above-mentioned control voltage source can be made to have low output impedance by, for example, configuring its output section with an operational amplifier, or by grounding its output section AC-wise using a capacitor. I can do it.

As described above, according to the present invention, an extremely simple circuit can be realized without using many additional parts.
The second-order distortion of the FET can be sufficiently reduced.

[Brief explanation of the drawing]

FIG. 1 is an electric circuit diagram showing a connection example when FET is used as a variable impedance element, FIG. 2 is an electric circuit diagram showing a variable gain circuit using FET according to an embodiment of the present invention, and FIG. 3 is a sectional view showing an example of an FET used in the circuit of FIG. 2. F ₀ ... Field effect transistor, D ... Drain, S ... Source, G ... Gate, E ... DC power supply, SUB ... Base, Q ₁ , Q ₂ , Q ₃ , Q ₄ , Q ₅ , Q ₆ ,
Q ₇ , Q ₈ ...transistor, R ₁ , R ₂ , R ₃ , R ₄ ,
R ₅ , R ₆ ...Resistor, D ₁ ...Diode, C ₁ ...Capacitor, T ₁ ...Transformer, F ₁ ...FET, 1,
2... Input signal terminal, 3, 4... Carrier wave input terminal, 5, 6... Modulation output terminal, 7... Control voltage terminal, 8... Constant voltage power supply terminal.

Claims (1)

[Claims] 1 An amplifier circuit composed of a differential pair circuit, and a source and a drain connected between two points of the amplifier circuit, to which AC signals of the same potential in direct current, opposite phases and the same amplitude in alternating current are applied. A control circuit output having a sufficiently low impedance in terms of alternating current is connected to the gate of the field effect transistor, and the substrate of the field effect transistor is grounded in terms of alternating current using a capacitor. A variable gain circuit using field effect transistors characterized by the following.