Common Collector Amplifier or the Emitter Follower: Figure 1(a) gives the basic circuit of a common collector amplifier, popularly called emitter follower. Its voltage gain is close to unity (one) and, therefore, any increment in the input voltage i.e. the base voltage appears as an equal increment in the output voltage across the load resistor in the emitter circuit.

Thus, the emitter may be said to follow the input signal and hence the name emitter follower. The input resistance R_{i} of a CC amplifier or emitter follower is very high, of the order of hundred of kilo-ohms while its output resistance is very low, of the order of tens of ohms. High R_{i} and low R_{0} permits us to use emitter follower as an impedance matching stage to match the high output impedance of one amplifier stage to the low input impedance of the next stage over a wide frequency range. With voltage gain very close to but slightly less than unity, the emitter follower serves the function of resistance transformation. Further emitter follower increases the power level of the signal since its current gain A_{I} is quite large.

Analysis: Figure 1(b) gives the small signal equivalent circuit of the emitter follower of figure 1(a) on

- Omitting dc voltage source
- Omitting the biasing resistor R
_{1}– R_{2}and the coupling capacitor C_{C}and - Replacing the transistor by its small signal two generator h-parameter model.

Here also we assume sinusoidal input signal. Hence in the equivalent circuit, we have used rms values of voltages and currents nearly I_{b}, V_{b}, I_{e}, V_{e}.

The equivalent circuit of figure 1(a) is similar to that of CE amplifier of Figure 2. Hence the analysis procedure is exactly the same. The expression of A_{I}, R_{i}, A_{V}, A_{IS}, A_{VS} and Y_{0} are, therefore, the same as of CE amplifier except that h-parameters of CC configuration are used. We may make use of Table 1 and simultaneously use conversion Table 2 to get the expression for A_{I}, R_{i}, A_{V} and R_{0}.

Table 1: Result of small single analysis of low frequency cc amplifier | |

Table 2 Approximate Conversion Formulas for h-parameter | |

Thus, from Table 1 and Table 2,

Current gain …..(1)

Input resistance …..(2)

Voltage gain …..(3)

But from equation (2)

Hence …..(4)

Overall voltage gain …..(5)

Overall current gain …..(6)

Output admittance ……(7)

Power gain …..(8)

**Example 1 A transistor amplifier in CC configuration is driven by a voltage source V _{s} of internal resistance R_{s} = 1000. The load impedance is a resistor R_{L} = 4000. The h-parameters are: , h_{fc} = -51 and . Calculate for this amplifier, the current gain A_{I}, input resistance R_{i}, voltage A_{V}, overall voltage gain A_{VS}, overall current gain A_{IS}, output resistance R_{0} and power gain A_{P}.**

Solution:

Current gain

Input Resistance

Voltage gain

Overall Voltage gain

Overall current gain

Output admittance

Output resistance

Power gain