![]() ![]() ![]() Many amplifiers are designed with emitter-follower output stages placed after voltage gain stages. The current gain and low output impedance of the emitter-follower amplifier make it ideal for driving low impedance loads, which may be DC- or AC-coupled. The bias point considerations for the Class A emitter-follower amplifier are the same as for the Class A CE amplifier. In many, if not most, applications, we can view the emitter follower as having unity voltage gain with the output voltage shifted down from the input voltage by one v BE drop. This same feedback loop helps regulate the operating bias point in the CE amplifier. This negative feedback action regulates the bias point and voltage gain of the emitter-follower, keeping the voltage gain close to unity. In the emitter-follower amplifier, the negative feedback occurs as follows: starting at the emitter, as the emitter voltage v E increases, the base-emitter voltage v BE decreases, which in turn reduces the base current i B, which reduces the collector current i C by the i C = βi B relationship recalling that for large β we can say that i E ≈ i C (the exact relationship between i E and i B) is i E = (1 + β)i B, and (1 + β)i B ≈ βi B for large β) we see that i E decreases as i C, thuis reducing v E. Reference to the schematic in the Procedure section of the lab will be helpful when reading the remainder of this section. Negative feedback systems are often used to regulate physical phenomena. Automotive cruise control is an example of an everyday negative feedback system in which the speed of an automobile is constantly monitored and adjusted in such a way as to minimize the error between the current speed and the speed set by the driver. Negative feedback systems are discussed in further detail in later labs, but can be briefly described as systems in which part or all of an output quantity is fed back upstream in the system in such a way as to reduce an error that exists between the existing condition and the desired condition. The operation of an EF amplifier constitutes a form of negative feedback. The collector is generally connected directly to the power supply and the emitter is connected to another supply voltage - often ground - through an emitter resistor, R E. This is the sense in which the output voltage at the emitter “follows” the input. Unlike the CE amplifier, the output voltage and input voltage of an EF amplifier are in-phase with each other and of nearly the same magnitude. As with the CE amplifier, we will study the EF amplifier using a single transistor biased in a Class A configuration. “Emitter-follower” is more descriptive than “common-collector,” and will be used henceforth for this reason. ![]() The name “emitter-follower” originates from the fact that the output signal, taken at the emitter, follows the input signal, applied at the base, with nearly unity gain. Emitter-follower amplifiers are commonly used as output stages that are capable of driving low impedance loads due to their current gains and low output resistances. The emitter-follower, EF, also called common-collector, CC, amplifier provides nearly unity voltage gain, and current gain, which can be large, and low output resistance. The common-emitter amplifier introduced in the “Class A NPN Common-Emitter Amplifier” lab provided voltage and current amplification, but suffered from a large output resistance that was equal to the equivalent collector resistance that was present for AC signals. ![]()
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