AGC Amplifier


This amplifier does not have AGC applied. Two 100K resistors are used to set the gate 2 bias at approximately six volts, bypassed with a .01 decoupling capacitor.

This amplifier may have a tendency to oscillate, if so, detune the 455 kHz IF can or add a resistor of 2.2K to 12K across the primary of the coil (between the drain and 12 Volts). A 12K resistor is shown in the schematic. Adjust S-meter responsiveness by tuning the output coil.

The LEDs in the IF strip will show the AGC action. Since the LEDs are in the drain circuit of the MOSFETs, changes in the drain current will show in the brightness of the LEDs.

For more gain from the IF strip to the Product Detector, take the 100K resistors off Gate 2, and connect the AGC line to Gate 2. Leave the .01 bypass cap on Gate 2.

Move the connection to the product detector from the Second amplifier to the output of this one. The output to the AGC circuit will be as shown in the schematic, without a coupling capacitor. Add a .01 cap at the output to connect to the 455kHz ceramic filter.

There will not be as much AGC action with these changes, but this is not a problem unless a transmitter is used with the receiver. The AGC will not decrease the audio output from strong signals as much with these modifications.

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Last Update: 12-18-2002
Web Author: David White, WN5Y