TRANSISTOR CONFIGURATIONS

Common Emitter

The common-emitter configuration (CE) shown in figure 2-16 view A is the arrangement most frequently used in practical amplifier circuits, since it provides good voltage, current, and power gain. The common emitter also has a somewhat low input resistance (500 ohms-1500 ohms), because the input is applied to the forward-biased junction, and a moderately high output resistance (30 kilohms-50 kilohms or more), because the output is taken off the reverse-biased junction. Since the input signal is applied to the base-emitter circuit and the output is taken from the collector-emitter circuit, the emitter is the element common to both input and output.

Since you have already covered what you now know to be a common-emitter amplifier (fig. 2-12), let's take a few minutes and review its operation, using the PNP common-emitter configuration shown in figure 2-16 view A.

When a transistor is connected in a common-emitter configuration, the input signal is injected between the base and emitter, which is a low resistance, low-current circuit. As the input signal swings positive, it also causes the base to swing positive with respect to the emitter. This action decreases forward bias which reduces collector current (I_C) and increases collector voltage (making V_C more negative). During the negative alternation of the input signal, the base is driven more negative with respect to the emitter. This increases forward bias and allows more current carriers to be released from the emitter, which results in an increase in collector current and a decrease in collector voltage (making V_C less negative or swing in a positive direction). The collector current that flows through the high resistance reverse-biased junction also flows through a high resistance load (not shown), resulting in a high level of amplification.

Since the input signal to the common emitter goes positive when the output goes negative, the two signals (input and output) are 180 degrees out of phase. The common-emitter circuit is the only configuration that provides a phase reversal.

The common-emitter is the most popular of the three transistor configurations because it has the best combination of current and voltage gain. The term GAIN is used to describe the amplification capabilities of the amplifier. It is basically a ratio of output versus input. Each transistor configuration gives a different value of gain even though the same transistor is used. The transistor configuration used is a matter of design consideration. However, as a technician you will become interested in this output versus input ratio (gain) to determine whether or not the transistor is working properly in the circuit.

The current gain in the common-emitter circuit is called BETA (). Beta is the relationship of collector current (output current) to base current (input current). To calculate beta, use the following formula:

( is the Greek letter delta, it is used to indicate a small change)

For example, if the input current (I_B) in a common emitter changes from 75 A to 100 A and the output current (I_C) changes from 1.5 mA to 2.6 mA, the current gain (I) will be 44.

This simply means that a change in base current produces a change in collector current which is 44 times as large.

You may also see the term h_fe used in place of I. The terms h_fe and Iare equivalent and may be used interchangeably. This is because "h_fe" means: h = hybrid (meaning mixture)

f = forward current transfer ratio
e = common emitter configuration

The resistance gain of the common emitter can be found in a method similar to the one used for finding beta:

Once the resistance gain is known, the voltage gain is easy to calculate since it is equal to the current gain (I) multiplied by the resistance gain (E = IR). And, the power gain is equal to the voltage gain multiplied by the current gain I (P = IE).