FET circuit design

This circuit originates from the famous RS-4 INSPIRE VLF receiver. It is actually a modified version of it. The modification increased the gain a little bit. Care must be taken when connecting the FET to the circuit- be sure the leads are in thier proper places. The source lead connects to the grounded side of the circuit (R2). The drain lead connects to the V+ side (R3). The gate lead is of course the input lead. C1 blocks DC voltage and passes audio frequencies. C1 can be bypassed to increase low frequency response. R1 serves as a protection against stray DC voltage build-up, it is not advisable to change it. R3 and R2 sets the bias, gain, and impedance of the circuit. The modification involved changing these values, several days of tinkering was involved in the process. To change the circuit back to the original design, which had a lower output voltage and lower impedance level, change R3 to 680 ohms and R2 to 220 ohms. C2 has the effect of increasing the gain, change C2 to 10uF or higher for more gain. The circuit will function without C2, it was part of the modification. C3 prevents passing DC voltage to the next stage. To increase low frequency response increase C3's value to 10uF or better. If a passive filter network (low-pass filter) follows this stage you must insert a resistor in series with C3, preferably a 3.3k ohm resistor. If the resistor is not installed there could be oscillations at the output when your battery voltage drops below a certain point (about 8.0 volts). The circuit will draw around 2 mA of current. The circuit requires at least 6 volts to run. It seems to work best at 20 volts, however 9-12 volts will provide satisfactory results.