Automatic Gain Control Circuit

The invention relates to an automatic gain control (AGC) circuit, and more particularly, to an AGC circuit employing a field effect transistor (FET's).

AGC circuits employing FET are known, but involve the disadvantage that as the magnitude of an input signal increases, even harmonics increase to degrade the distortion factor.

By way of illustration, a conventional AGC circuit is shown in FIG. 1 wherein an N-channel FET Q.sub.1 of junction type has its drain electrode D connected with an input terminal 1 and its source electrode S connected with an input terminal of an amplifier 3. A negative feedback path including a rectifier 4, and a smoothing circuit comprising a capacitor 5 and a resistor 6 is connected between an output terminal 2 of the amplifier 3 and a gate electrode G of the FET Q.sub.1. FIG. 2 graphically shows the distortion characteristic (curve a) and input-output characteristic of the AGC circuit shown in FIG. 1. As will be evident, the distortion increases with an increase in the input level, due to increasing even harmonics. An increase of the distortion is attributable to a variation with time in the impedance across the drain and source of FET Q.sub.1 which results from a variation in the gate potential as it is fed by the rectified output, when the input level is high to cause a large variation of a signal level with time. This is illustrated in FIGS. 3 and 4. In FIG. 3, an input signal has a high amplitude, so that the impedance across the drain and source of the field effect transistor will be high at point A and low at point B, resulting in a distorted output signal as indicated by a curve C. However, the input signal shown in FIG. 4 has a reduced amplitude, so that there is little difference in the impedance between points Ao and Bo. Hence, the distortion in the output signal will be substantially reduced.

It is an object of the invention to provide an AGC circuit employing FET having a pair of gate electrodes which eliminates the disadvantage of the prior art.

In accordance with the invention, the superimposed application of an input signal to the gate permits the voltage across the gate and the drain to be maintained constant regardless of a variation in the magnitude of the input signal with time. Since the substitution of an FET having a pair of gate electrodes for a conventionally employed FET is all that is required, no substantial complication of arrangement and increase of cost results.

FIG. 1 is a circuit diagram of a conventional AGC circuit employing FET;

FIG. 2 graphically shows the distortion and the input-output characteristic of the AGC circuit shown in FIG. 1;

FIGS. 3 and 4 are graphical illustrations of the relationship between the input level, the gate level and the output signal;

FIG. 5 is a circuit diagram of an AGC circuit according to one embodiment of the invention;

FIG. 6 graphically shows the distortion and the input-output characteristic of the AGC circuit shown in FIG. 5; and

FIG. 7 is a graphical illustration of the relationship between the input level and the gate level.

Referring to FIG. 5, the AGC circuit according to the invention comprises an FET Q.sub.2 having a first and a second gate electrode G.sub.1, G.sub.2. In the example shown, FET Q.sub.2 is an N-channel MOS field effect transistor of depletion type, and has its second gate electrode G.sub.2 connected either directly or through a resistor 7 with its drain electrode D, which is in turn connected with the input terminal 1. A negative feedback path which includes the rectifier 4 and the smoothing circuit comprising the capacitor 5 and resistor 6 as before is connected between the output terminal 2 of the amplifier 3 and the first gate electrode G.sub.1, and the source electrode S is connected with the input terminal of the amplifier 3 as in the conventional arrangement.

When an input signal of sinusoidal waveform, for example, is applied to the input terminal 1, the arrangement of the invention maintains a constant voltage between the gate (G.sub.2) and drain, as illustrated at A.sub.1 and B.sub.1 in FIG. 7, so that the impedance across the drain and source is maintained uniform, yielding a satisfactory output waveform in both high and low regions of the waveform. As shown in FIG. 6 in which curves a and b are the replica of FIG. 2, the distortion characteristic represented by a curve c which is achieved with the AGC circuit of the invention minimizes distortions at higher input levels.