Common-drain circuit

Chapter Common-drain circuit

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • The dc bias for the common-gate circuit is basically the same as that for the com-mon-source arrangement. But the signal follows a different path. The ac input signal en-ters through C1. Resistor R1 keeps the input from being shorted to ground. Gate bias isprovided by R1 and R2; capacitor C2 places the gate at signal ground. In some com-mon-gate circuits, the gate electrode is directly grounded, and components R2 and C2are not used. The output leaves the circuit through C3. Resistor R3 keeps the outputsignal from being shorted through the power supply.The common-gate arrangement produces less gain than its common-source coun-terpart. But this is not all bad; a common-gate amplifier is very stable, and is not likelyto break into unwanted oscillation. The output is in phase with the input.Common-drain circuitA common-drain circuit is shown in Fig. 23-13. This circuit has the collector at signalground. It is sometimes called a source follower.The FET is biased in the same way as for the common-source and common-gatecircuits. In the illustration, an N-channel JFET is shown, but any other kind of FETcould be used, reversing the polarity for P-channel devices. Enhancement-mode MOS-FETs would need a resistor between the gate and the positive supply terminal (or thenegative terminal if the MOSFET is P-channel).The input signal passes through C2 to the gate. Resistors R1 and R2 provide gatebias. Resistor R3 limits the current. Capacitor C3 keeps the drain at signal ground. Fluc-tuating dc (the channel current) flows through R1 as a result of the input signal; thisCommon draine circuit42723-12Common-gate circuit configuration.