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 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.

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.

    Remove 12 Volts to Transformer, Power On

  • LED still lights up - bad MOSFET - Gate 2 shorted to drain
  • To check Gate 1 - short Gate 1 & 2, if LED lights up, bad Gate 1
  • LED does not light up, good MOSFET

    Disconnecting 12 Volts from the transformer as a way to check for a good MOSFET has turned out to be the only way to accurately test for a good MOSFET. I have tried to use a DVM to test them, in and out of circuits, and was never really sure about the readings.

    I came across this accidentally, of course, when I was just playing around with a MOSFET receiver that wasn't working up to its potential, and pulled a transformer out (they were 6 pin MCL T1-1T's) and the LED stayed on. The LED was supposed to go out! I pulled the other transformers, some had a dim LED, and in others, the LED turned off. In every case where the LED was dim or bright, I had a bad MOSFET, or a solder bridge. Both MOSFETs in the single balanced mixer can be tested by removing, or disconnecting the 12 Volts, to the output transformer (with power applied). I found one MOSFET in a mixer that was bad, two others in amplifiers, and one solder bridge between Gate 1 and the Source. The receiver worked great after this diagnosis.

    A bright LED indicates proper circuit function. The only time brightness varies is in the AGC controlled amplifiers. Since the LED's are at RF ground, the LED's can be mounted on the front panel to flash along with the action of the S-meter.

    When the receiver is finished, a dead amplifier can be quickly located and diagnosed. Because of the easy diagnosis, the LED's were left in all the amplifiers, regardless of whether they were AGC controlled amplifiers or not. Gate 2 bias voltage was raised to 6 volts using two 100k resistors.


    The decoupling resistor (from the 12 volt connection) and the source resistor are the same in this circuit. They set the drain current and the gain of the circuit. In circuits published in the early 1970's, the authors used decoupling and source resistors from 270 ohm to 470 ohm. (See History of Development) Higher value source resistors lower the gain of the amplifier.

    Bypass Capacitors

    The bypass capacitors need to be changed according to the frequency of operation. At audio frequencies, the decoupling capacitors should be 47pf to 470pf, at low IF frequencies (455 kHz), they should be .1 mfd, at medium IF frequencies they should be .01, and at high frequencies (10 MHz to 50 MHz) they should be .001.

    Gate 1

    The resistor on G1 of the MOSFET sets the input impedance of the amplifier. You will find that in this receiver it varies from 100 ohm to 100K. Lower values can help prevent unwanted oscillations. High values (100K) lightly load the preceding circuit and is used to promote stability in the oscillators. A 2K value is used at the 455 IF amplifiers to help stability and provide an ideal input match for the MOSFET.

    Gate 2

    What you do at G2 determines the function of the circuit. Bias G2 for 6 volts and connect a .01 bypass capacitor to ground, and you have an amplifier. This was done to make RF amplifiers.

    To use the circuit as a mixer, simply lift the bypass capacitor at G2 from ground and feed the oscillator frequency through it.

    To use the circuit as an AGC controlled amplifier, remove the bias resistors and bypass capacitor. Then connect the AGC control voltage directly to G2.

    To make a single balanced mixer, take two of the amplifiers and place them side-by-side. Take off the components going to G2 on one of the amplifiers. On the other amplifier, lift the .01 bypass capacitor at G2 from the ground connection, and connect it to the oscillator (leave one set of 100K resistors). Then connect the G2 of both amplifiers. Add the input and output toroids and you have a single balanced mixer. See the Original Single Balanced Mixer for details on published information. See Single Balanced Mixer for the example used in the LED MOSFET receiver using two of the MOSFET RF amplifiers.

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    Last Update: 02/1/2000
    Web Author: David White, WN5Y