+ − + − R$_{load}$ v (1) v (3) −1.0 −2.0 −3.0 0.0 500.0 1000.0 −4.0 0.0 Time uS V$_{input}$ + − + − R$_{E }$50 Ω R$_{B }$20K Ω E$_{EB}$ E$_{CE}$ Input C Output Input signal Output signal V$_{1 }$15V R$_{load }$ 5 kΩ v (1) v (3) 4.0 3.0 2.0 0.0 500.0 1000.0 1.0 Time uS V$_{input}$ 1.5V 2 kHz V$_{bias}$ 2.3V 11-23 Figure 11-51.  This common-collector circuit has high input  impedance and low output impedance.  Figure 11-49.  A DC biasing current is used to keep the transistor of  a common-collector amplifier in the active mode when amplifying  AC (top). The output of this amplifier is in phase and directly  proportional to the input (bottom). The difference in amplitude  between the two is the 0.7V used to bias the PN junction of the  transistor in the input circuit.  Figure 11-50.  A common-collector amplifier circuit with a PNP  transistor has the same characteristics as that of a common- collector amplifier with a NPN transistor except for reversed voltage  polarities and current direction.  on the circuit output is direct, so the output voltage phase is  the same as the input signal but much greater in amplitude. As with the other amplifier circuits, when amplifying an  AC signal with a common-base amplifier circuit, the input  signal to the base must include a DC source to forward bias  the transistor’s base-emitter junction. This allows current  A third type of amplifier circuit using a bipolar transistor  is the common-base amplifier. In this circuit, the shared  transistor terminal is the base terminal.  [Figure 11-52]  This  causes a unique situation in which the base current is actually  larger than the collector or emitter current. As such, the  common-base amplifier does not boost current as the other  amplifiers do. It attenuates current but causes a high gain in  voltage. A very small fluctuation in base voltage in the input  circuit causes a large variation in output voltage. The effect  A common-collector amplifier can also be built with a PNP  transistor.  [Figure 11-50]  It has the same characteristics as the  NPN common-collector amplifier shown in  Figure 11-50 . When  arranged with a high resistance in the input circuit and a small  resistance in the load circuit, the common-collector amplifier can  be used to step down the impedance of a signal.  [Figure 11-51] amplifier. The transistor must stay on or in the active mode  regardless of input signal polarity. When the AC cycles  through zero, the transistor turns off because the minimum  amount of current to forward bias the transistor is not  available. The addition of a DC biasing source (battery)  in series with the AC signal in the input circuit keeps  the transistor in the active mode throughout the full AC  cycle.  [Figure 11-49] Common-Base Amplifier 