Simple voltage controlled amplifier using junction FETs

The basic amplification circuit uses an OP amplifier for out-of-phase amplification, and
controls the amplification factor by changing the voltage dividing ratio of the feedback circuit. If using 'Cvd8'
Rz, set. The resistance value of Rs can be changed, but when using FET, since one end is
grounded, only the resistance value of Rs can be changed. For example, the channel resistance of FET is r. ． ,
the amplification factor A of the dizzy circuit is;
 
ri. The usage range can be from 100 ohms to 10,000 ohms. Assume Ra=iok0,
R!=1kCl,-r,. The range is 50012-skQ. Then ^ can
vary between 2,66~7.6f times.
    Since the output characteristics of FET are asymmetric between positive and negative, when AC
voltage is applied to the drain, waveform distortion will occur. This circuit
achieves the purpose of reducing distortion through the local feedback composed of R, and C' .