Transistor Bootstrapping Circuit Using FET

This bootstrapping amplifier circuit uses the 2N3819 FET in source follower configuration. If you are experimenting with radio amplifier circuits, then this is an elegant circuit design that uses one of the commonest and cheaply available FET transistors. The 2N3819 is a n-channel depletion JFET for VHF and UHF amplifier applications. It has a typical input capacitance of 3 pF.

Usually simple FET circuits have a large resistor that connects the gate junction to earth, which limits the source voltage swing to approximately 2 V. However, with the circuit shown above, the gate junction connects to a potential divider network providing a much stable operating point, and wider input swings. An additional bonus with this circuit is that a bootstrapping capacitor Cb, introduces positive feedback to point F, which thereby increases the input impedance to approximately 100 MΩ.

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