Third Amplifier of the IF strip, if it has a tendency to oscillate.

R1 is used to set the input impedance and the value depends on the preceding circuit. It is 100K following oscillators, 2.2K in the 455kHz IF strip, and 100 ohm following the First Mixer. The value of C1 also depends on the preceding circuit. C1 is 3.3 to 10 pf following the oscillators, and .01 in all the other circuits of this receiver.

Gate 2 shows a connection to it that is used for making a Single Balanced MOSFET mixer. Gate 2 of each MOSFET is connected together in the Single Balanced MOSFET Mixer.

Z1 is a ferrite bead. This bead is shown in a lot of circuits to prevent parasitic oscillations. The bead was used in earlier versions of this receiver, but I left them out of later versions because they did not seem to be needed. You can add them if you want to. They are usually hard to find and aren't in many junk boxes. I have seen 10 ohm to 100 ohm resistors used in place of a ferrite bead, and supposedly work better than a ferrite bead in most circuits. The two 100K resistors are used to set Gate 2 bias, and the .01 bypass capacitor is used with the resistors.

When used as an amplifier, only the 100K resistors and the bypass capacitor are used. The .01 capacitor going to the AGC connection is not used.

The .01 capacitor that connects to the AGC connection is used when the circuit is used as a mixer, when oscillator energy is fed through the .01 capacitor to Gate 2. The .01 bypass capacitor is removed, and the 100K bias resistors can be left in place or removed. In the circuits from the History of Development, it has been done both ways.

When the amplifier is AGC controlled, the .01 capacitor at the AGC connection is replaced with a wire, and the 100K resistors are removed. The .01 bypass capacitor is either left in place or removed, depending on AGC circuit requirements.

Two connections are shown at the 12 Volt connection, so that stages can be cascaded for power, rather than running a single line to each one.

The ground plane circles around the circuit, to provide easy grounding for the bypass capacitors, LED, and output transformer.

In order not to use a negative voltage in a MOSFET AGC controlled amplifier, the operating potential of the MOSFET has to be raised 1.5 volts. In a simple 455kHz IF receiver in the ARRL Handbook, the author used an LED as a 1.5 volt zener so that the AGC voltage could run from 0 to 6 volts (instead of -1.5 to 4 volts). I tried this circuit and found out that not only did the LED light, it's intensity varied with the drain/source current. It also proved to be very useful as an indicator of proper circuit function. Over the course of experimentation, the following diagnosis procedure evolved:

LED Does Not Light