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Dual-gate MOSFETs

A dual-gate MOSFET consists of two MOSFETs in series:

Typical applications are:

  • An amplifier with gain control.

    The input signal is fed to G1. The voltage at G2 controls the gain, because it determines the thickness of the channel of the top MOSFET. The transfer characteristics below shows the drain current versus VG1 for different values of VG2.

    It clearly shows that yfs (=ID/VG1) depends on VG2. And, of course, this means that the gain depends on VG2.

    In an antenna amplifier, weak signals should be amplified enough to be processed by the next stage. But stong signals shouldn't overload the next stage. We only need a simple circuit that produces a voltage of, say, 5V for weak signals and 1V for strong signals and feed that voltage to G2. That's how AGC (Automatic Gain Control) implemented in many receivers.

  • An AM modulator.

    An AM modulator is a device that varies the amplitude of a high frequency signal with a low frequency signal. This is simply done by connecting G1 to the HF signal and G2 to the LF signal. Like above, the gain will vary with VG2 and thus with the LF signal.

  • HF amplifier.

    The amplifier in the previous section works fine for low frequency signals, but it's not suitable for amplifying antenna signals in a TV set. Due to the MOSFET's construction, the capacitance between Gate and Drain is relatively high, about 5pF for a small signal MOSFET. At 100MHz, this means an impedance of just 318Ω. But the capacitance between G1 and Drain of a dual-gate MOSFET can be as low as 20fF (1fF = 1 femto-Farad = 10-18F). At 100MHz, the impedance will be 79.6kΩ. G2 if usually connected to the possitive supply voltage, or to an AGC voltage (see above).

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