Figure 1 is a practical example of a common-collector transistor amplifier. Note that the output is taken off of the emitter instead of the collector (as in the common-emitter configuration).

A common-collector amplifier is not capable of voltage gain. In fact, there is a very slight loss of voltage amplitude between input and output.

However, for all practical purposes, we can consider the voltage gain at unity. Common-collector amplifiers are noninverting, meaning the output signal is in phase with the input signal.

Essentially, the output signal is an exact duplicate of the input signal. For this reason, common-collector amplifiers are often called emitter-follower amplifiers, because the emitter voltage follows the base voltage.

Common-collector amplifiers are current amplifiers. The current gain for the circuit illustrated in Fig. 1 is the parallel resistance value of R1 and R2, divided by the resistance value of R3. R1 and R2 are both 20 Kohms in value, so their parallel resistance value is 10 Kohms.

This 10 Kohms divided by 1 Kohm (the value of R3) gives us a current gain of 10 for this circuit. Because the voltage gain is considered to be unity (1), the power gain for a common-collector amplifier is considered equal to the current gain (10, in this particular case).

The input impedance of common-collector amplifiers is typically higher than the other transistor configurations. It is the parallel resistive effect of R1, R2, and the product of the value of R3 times the beta value.

Because beta times the R3 value is usually much higher than that of R1 or R2, you can closely estimate the input impedance by simply considering it to be the parallel resistance of R1 and R2. In this case, the input impedance would be about 10 Kohms.

The traditional method of calculating the output impedance of common-collector amplifiers is to divide the value of R3 by the transistor’s beta value. Although this method is still appropriate, a closer estimate can probably be obtained by considering the output impedance of most transistors to be about 80 ohms.

This 80-ohm output impedance should be viewed as being in parallel with R3, giving us a calculated output impedance of about 74 ohms (80 ohms in parallel with 1000 ohms). Resistors R1 and R2 have the same function within a common-collector amplifier as previously discussed with common-emitter amplifiers.

The high negative feedback produced by R3 provides excellent temperature stability and immunity from transistor variables. The circuit illustrated in Fig. 1 can be a valuable building block toward future projects.

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