Mosfet Circuit Details

The Universal Mosfet Amplifier

The picture above shows a circuit board layout giving every option that the MOSFET amplifier will use. This board could be built very easily using "Manhattan Construction" or "dead bug" style construction. I have also built it on mass produced prototype boards from Tanner Electronics. Radio Shack and others have similar prototype boards that can be used. Lay out the parts exactly like they are in the circuit diagram.

Build 13 of these and this receiver is almost built. Just add the oscillators (BFO, VFO, and 4MHz), the product detector, the AGC circuit, the audio amplifier, the three filters (TV & FM, Bandpass & crystal), and you are done. These circuits are just as easy or easier to build than the MOSFET circuit.

I have built the MOSFETs "dead bug" style on 2" by 2" copper squares with an assembly line like procedure, and finished all 13 in one evening. Trying to put them on a 1" by 1" copper square "dead bug" style has not been very successful, because the circuit didn't have enough room between the input and output and had a tendency to oscillate. The ones I have built spread over a 2" by 2" square worked perfect every time.

The output transformer can be a ferrite core to make a broadband transformer, an iron core to make a tuned transformer, or a commercial transformer for 10.7 MHz or 455kHz. Shown is a 455kHz transformer. The extra connections around the output transformer are used to connect a homemade ferrite or iron core transformer.

C2 may be needed for resonating a commercial IF can, if it doesn't have an internal capacitor, or as a variable trimmer to tune a homebrew transformer. R2 is used to help stabilize the amplifier, and can be any value from 2K to 12K. C2 and R2 are not used in the circuits of this receiver, except R2 in the Best online pharmacy for generic viagra 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 Generic arthrotec 75 mg, 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.

The Helpful LED

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

  • 12 Volts is not connected
  • Wrong value decoupling resistor - usually too high (at 12 Volts connection)
  • MOSFET installed backwards
  • Output coil open or installed improperly

    Dim LED - No Output

  • Forgot to install input impedance resistor - R1
  • Gate 2 bias voltage too low.

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