If a high impedance source is connected to low impedance amplifier then most of the signal is dropped across the internal impedance of the source. To avoid this problem common collector amplifier is used in between source and CE amplifier. It increases the input impedence of the CE amplifier without significant change in input voltage.
Fig. 1, shows a common collector (CC) amplifier. Since there is no resistance in collector circuit, therefore collector is ac grounded. It is also called grounded collector amplifier. When input source drives the base, output appears across emitter resistor. A CC amplifier is like a heavily swamped CE amplifier with a collector resistor shorted and output taken across emitter resistor.
vout = vin - vBE
Therefore, this circuit is also called emitter follower, because VBE is very small. As vin increases, vout increases.
If vin is 2V, vout = 1.3V
If vin is 3V, vout = 2.3V.
Since vout follows exactly the vin therefore, there is no phase inversion between input and output.
The output circuit voltage equation is given by
VCE = VCC – IE RE
Since IE » IC
\ IC = (VCC – VCE ) / RE
This is the equation of dc load line. The dc load line is shown in Fig. 1.
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