TRANSISTOR CONFIGURATIONS

Common Collector

The common-collector configuration (CC) shown in figure 2-16 view C is used mostly for impedance matching. It is also used as a current driver, because of its substantial current gain. It is particularly useful in switching circuitry, since it has the ability to pass signals in either direction (bilateral operation).

In the common-collector circuit, the input signal is applied to the base, the output is taken from the emitter, and the collector is the element common to both input and output. The common collector is equivalent to our old friend the electron-tube cathode follower. Both have high input and low output resistance. The input resistance for the common collector ranges from 2 kilohms to 500 kilohms, and the output resistance varies from 50 ohms to 1500 ohms. The current gain is higher than that in the common emitter, but it has a lower power gain than either the common base or common emitter. Like the common base, the output signal from the common collector is in phase with the input signal. The common collector is also referred to as an emitter-follower because the output developed on the emitter follows the input signal applied to the base.

Transistor action in the common collector is similar to the operation explained for the common base, except that the current gain is not based on the emitter-to-collector current ratio, alpha (). Instead, it is based on the emitter-to-base current ratio called GAMMA (), because the output is taken off the emitter. Since a small change in base current controls a large change in emitter current, it is still possible to obtain high current gain in the common collector. However, since the emitter current gain is offset by the low output resistance, the voltage gain is always less than 1 (unity), exactly as in the electron-tube cathode follower

The common-collector current gain, gamma (), is defined as

and is related to collector-to-base current gain, beta (), of the common-emitter circuit by the formula:

Since a given transistor may be connected in any of three basic configurations, there is a definite relationship, as pointed out earlier, between alpha (), beta (), and gamma (). These relationships are listed again for your convenience:

Take, for example, a transistor that is listed on a manufacturer's data sheet as having an alpha of 0.90. We wish to use it in a common emitter configuration. This means we must find beta. The calculations are:

Therefore, a change in base current in this transistor will produce a change in collector current that will be 9 times as large.

If we wish to use this same transistor in a common collector, we can find gamma () by:

To summarize the properties of the three transistor configurations, a comparison chart is provided in table 2-1 for your convenience.

Table 2-1. - Transistor Configuration Comparison Chart

Now that we have analyzed the basic transistor amplifier in terms of bias, class of operation, and circuit configuration, let's apply what has been covered to figure 2-12. A reproduction of figure 2-12 is shown below for your convenience.