content_image/574633.jpg content_image/574632.jpg
The three basic amplifier types, named for which terminal of the transistor is the common terminal to both transistor circuits, include: 

1. Common-emitter amplifier 

2. Common-collector amplifier 

3. Common-base amplifier 

## Common-Emitter Amplifier 

The common-emitter amplifier controls the amplitude of an electric signal and inverts the phase of the input signal. Figure 11-47  illustrates a common-emitter amplifier for AC using a NPN transistor and its output signal graph. Common emitter circuits are characterized by high current gain and a 180° voltage phase shift from input to output. It is for the amplification of a microphone signal to drive a speaker. As always, adequate voltage of the correct polarity to the base puts the transistor in the active mode or turns it on. Then, as the base input current fluctuates, the current through the transistor fluctuates proportionally. However, AC cycles through positive and negative polarity. Every 180°, the transistor shuts off because the polarity to the base-emitter junction of the transistor is not correct to forward bias the junction. To keep the transistor on, a DC biasing voltage of the correct polarity (shown as a 2.3 volts (V) battery) is placed in series with the input signal in the base circuit to hold the transistor in the active mode as the AC polarity changes. This way the transistor stays in the active mode to amplify an entire AC signal. 

<img src='content_image/574633.jpg'>

Transistors are rated by ratio of the collector current to the base current, or Beta ( β ). This is established during the manufacture of the unit and cannot be changed. A 100 β  transistor can handle 100 times more current through a collector-emitter circuit than the base input signal. This current in Figure 11-47  is provided from the 15V battery, V$_{1}$. So, the amplitude of amplification is a factor of the beta of the transistor and any in-line resistors used in the circuits. The fluctuations of the output signal, however, are entirely controlled by the fluctuations of current input to the transistor base. 

If measurements of input and output voltages are made, it is shown that as the input voltage increases, the output voltage decreases. This accounts for the inverted phase produced by a common-emitter circuit. [Figure 11-47] 

## Common-Collector Amplifier 

Another basic type of amplifier circuit is the common-collector amplifier. Common-collector circuits are characterized by high current gain, but virtually no voltage gain. The input circuit and the load circuit in this amplifier share the collector terminal of the transistor used. Because the load is in series with the emitter, both the input current and output current run through it. This causes a directly proportional relationship between the input and the output. The current gain in this circuit configuration is high. A small amount of input current can control a large amount of current to flow from the collector to the emitter. A common collector amplifier circuit is illustrated in Figure 11-48 . The base current needs to flow through the PN junction of the transistor, which has about a 0.7V threshold to be turned on. The output current of the amplifier is the beta value of the transistor plus 1. 

During AC amplification, the common-collector amplifier has the same problem that exists in the common-emitter 

<img src='content_image/574632.jpg'>

