I am currently tasked with building a class AB amplifier using double-gate MOSFETs.

Since there isn't much literature available on the MOSFET itself (regarding biasing and actually using the device in terms of modelling equations for the drain current etc.,) I have regretfully resorted to using a circuit from the the tubecad site, it can be found in the first figure attached and amending it to work as I would want it to (using a 12V supply) in Multisim.

The circuit works perfectly when simulated, connected to an 8ohm load, as I've attached in the Multisim image. I've breadboarded the circuit and have had no success in obtaining any kind of output signal from the amplifier.

A large problem in attempting the design of this amplifier, is that the double-gate MOSFET has been manufacteured as a P-Channel MOSFET.

Only N-Channel double-gate MOSFETs exist. The attached link suggests that both N-Channel MOSFETs be used, where one preserves the phase of the input signal, and the other inverts it.

After breadboarding the circuit, I injected a 200mV, 1kHz signal into the amplifier and my output is basically a straight line. I would expect an amplified signal of approximately 1.8, according to my simulation.

Any help would be appreciated in identifying what the probkem could be in the breadboarded circuit, and if it was a feasible solution in attempting the concept for the double-gate MOSFET.

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  • \$\begingroup\$ I see no schematics of an amplifier using a double gate MOSFET. Are you sure you are referring to a single MOSFET device that has two gates or, are you talking about a push-pull amplifier using two discrete MOSFETs? \$\endgroup\$ – Andy aka Sep 15 at 8:44
  • \$\begingroup\$ What is your goal? Using the additional gates as a HF modulation input? If yes, this won't work. The HF is AC at full swing, so it needed the push-pull stage. But your preamplifier is an LF one. \$\endgroup\$ – Janka Sep 15 at 9:00
  • \$\begingroup\$ what are your bypass capacitors? are these voltage (in red) from the actual breadboard, measured? \$\endgroup\$ – analogsystemsrf Sep 15 at 9:27
  • \$\begingroup\$ @Janka My goal is to create the class AB amplifier, for now, with whatever gain results from this circuit. I would like to establish a proof of concept, using the Double-Gate MOSFETs are used in amplifier design. \$\endgroup\$ – Suva23 Sep 15 at 9:38
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    \$\begingroup\$ So, what is the advantage of using a dual gate mosfet with both gates tied together? \$\endgroup\$ – Andy aka Sep 15 at 11:16

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