Sometimes called the grounded emitter, since the emitter capacitor connects the emitter to ground at ac frequencies. Since, as far as ac is concerned, the emitter is joined to ground, both input and output are connected to the emitter. Current gain is Ic/Ib and can be quite high, typically 50. Voltage gain is high, typically 250 Input impedance is medium, say 5K. Output impedance is medium, say 20k. The output is inverted with respect to the input. Its most common application is as a voltage amplifier.
Common Collector Amplifier |
The positive power supply rail is joined to the zero volts rail by C3. As far as ac is concerned, both rails are joined together. Therefore they, and the collector, are common to both input and output. Since the emitter voltage follows the base voltage, it is also called the emitter follower. Current gain is Ie/Ib which is quite high, typically 50. Voltage gain is only 1 because of the undecoupled emitter. The input impedance is high, typically 500k, requiring only low power to drive it. The output impedance is low, typically 20 ohms. The output signal follows the input. There is no inversion. It is often used to match high impedances to low ones. It can be used to drive several high impedance loads.
Common Base Amplifier |
C3 connects the base to ground as far as ac is concerned. Therefore both input and output are connected to the base. (common base amplifier). Current gain is Ic/Ie which is less than 1. The voltage gain is high since it is Rc/Re. (Approximately the same current flows through them). It is typically 250. The input impedance is low, typically 20 ohms. The output impedance is high, typically 1Megohm. The output signal is not inverted with respect the input. It is often used to match low impedance devices to high impedance ones. It is commonly used at VHF. |
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