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| SEC. 11-1 CC AMPLIFIER A CC amplifier, better known as an emitter follower, has its collector at ac ground. The input signal drives the base and the output signal comes from the emitter. Because it is heavily swamped, an emitter follower has stable voltage gain, high input impedance, and low distortion. SEC. 11-2 OUTPUT IMPEDANCE The output impedance of an amplifier is the same as its Thevenin impedance. An emitter follower has a low output impedance. The current gain of a transistor transforms the source impedance driving the base to a much lower value when seen from the emitter. SEC. 11-3 CASCADING CE AND CC When a low resistance load is connected to the output of a CE amplifier, it may become overloaded resulting in a very small voltage gain. A CC amplifier placed between the CE output and load will significantly reduce this effect. In this way, the CC amplifier is acting as a buffer. SEC. 11-4 DARLINGTON CONNECTIONS Two transistors can be connected as a Darlington pair. The emitter of the first is connected to the base of the second. This produces an overall current gain equal to the product of the individual current gains. SEC. 11-5 VOLTAGE REGULATION By combining a zener diode and an emitter follower, we get a zener follower. This circuit produces regulated output voltage with large load currents. The advantage is that the zener current is much smaller than the load current. By adding a stage of voltage gain, a larger regulated output voltage can be produced. SEC. 11-6 COMMON-BASE AMPLIFIER The CB amplifier configuration has its base at ac ground. The input signal drives the emitter and the output signal comes from the collector. Even though this circuit has no current gain, it can produce a significant voltage gain. The CB amplifier has a low input impedance and high output impedance, and is used in high-frequency applications. |