The three basic amplifier types, named for which terminal  of the transistor is the common terminal to both transistor  circuits, include:  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] 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  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. Figure 11-48.  A basic common-collector amplifier circuit. Both  the input and output circuits share a path through the load and the  emitter. This causes a direct relationship of the output current to  the input current. Figure 11-47.  A common-emitter amplifier circuit for amplifying  an AC microphone signal to drive a speaker (top) and the graph  of the output signal showing a 180 degree shift in phase (bottom).  11-22 1.  Common-emitter amplifier 2.  Common-collector amplifier  3.  Common-base amplifier Common-Emitter Amplifier Common-Collector Amplifier  V$_{input}$ E R$_{load}$ C B V$_{output}$ Q1 R1 1kΩ AC V$_{1}$ V$_{bias}$ 2.3V v (1) I ( v (1)) I ( v (1)) V − 40 mA − 20 mA V$_{input}$ 1.5V 2 kHz Speaker 8Ω 15 V + − 4.0 2.0 0.0 0 0.0 500.0 1000.0 −2.0 −4.0 Units V Input Output Time uS