Common-gate circuit

Chapter Common-gate circuit

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • but the device could be an N-channel, depletion-mode MOSFET and the circuit diagramwould be the same. For an N-channel, enhancement-mode device, an extra resistorwould be necessary, running from the gate to the positive power supply terminal. ForP-channel devices, the supply would provide a negative, rather than a positive, voltage.426 The field-effect transistor23-11Common-source circuit configuration.This circuit is an almost exact replica of the grounded-emitter bipolar arrangement.The only difference is the lack of a voltage-dividing network for bias on the control elec-trode.Capacitor C1 and resistor R1 place the source at signal ground while elevating thiselectrode above ground for dc. The ac signal enters through C2; resistor R2 adjusts theinput impedance and provides bias for the gate. The ac signal passes out of the circuitthrough C3. Resistor R3 keeps the output signal from being shorted out through thepower supply.The circuit of Fig. 23-11 is the basis for amplifiers and oscillators, especially at ra-dio frequencies. The common-source arrangement provides the greatest gain of thethree FET circuit configurations. The output is 180 degrees out of phase with the input.Common-gate circuitThe common-gate circuit (Fig. 23-12) has the gate at signal ground. The input is ap-plied to the source. The illustration shows an N-channel JFET. For other types of FETs,the same considerations apply as described above for the common-source circuit.Enhancement-mode devices would require a resistor between the gate and the positivesupply terminal (or the negative terminal if the MOSFET is P-channel).