Common-collector circuit

Chapter Common-collector circuit

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

  • Instead of varying IB by injecting the signal at the base, it’s being done by injectingthe signal at the emitter. Therefore, in the common-base arrangement, the output sig-nal is in phase with the input, rather than out of phase.The signal enters through C1. Resistor R1 keeps the input signal from beingshorted to ground. Bias is provided by R2 and R3. Capacitor C2 keeps the base at signalground. Resistor R4 keeps the signal from being shorted out through the power supply.The output is through C3.The common-base circuit provides somewhat less gain than a common-emitter cir-cuit. But it is more stable than the common-emitter configuration in some applications,especially in radio-frequency power amplifiers.Common-collector circuitA common-collector circuit (Fig. 22-11) operates with the collector at signal ground.The input is applied at the base just as it is with the common-emitter circuit.The signal passes through C2 onto the base of the transistor. Resistors R2 and R3provide the correct bias for the base. Resistor R4 limits the current through the tran-sistor. Capacitor C3 keeps the collector at signal ground. A fluctuating direct currentflows through R1, and a fluctuating dc voltage therefore appears across it. The ac partof this voltage passes through C1 to the output. Because the output follows the emittercurrent, this circuit is sometimes called an emitter follower circuit.410 The bipolar transistor22-10Common-base circuit configuration.