Follower

follower

Engineering a machine part that derives its motion by following the motion of another part

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follower [′fäl·ə·wər]

(engineering)

A drill used for making all but the first part of a hole, the first part being made with a drill of larger gage.

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Follower 

an electronic amplifier stage with a gain of almost unity; as a rule, the gain is between 0.95 and 0.99. Followers are widely used in various radio-engineering devices. They serve as a buffer stage (impedance transformer) that isolates the signal source from the load in cases where the resistances of the source and the load are markedly different. Followers can be classified as voltage and current types. They can also be classified as inverting and noninverting types, where inversion means that the polarity or the phase of the input-signal oscillations is changed

Figure 1. Followers: (a) cathode, (b) emitter, (c) source: (ET) electron tube; (T) transistor; (FT) field-effect transistor; (R) load resistor; (U_in) voltage of input signal; (U_out) voltage of output signal; (E_an), (E_co), and (E_dr) constant voltages fed to anode, collector, and drain, respectively

at the output of the stage. Amplification of power—the general property of an amplifier stage—is also exhibited by followers. Voltage followers therefore amplify the input current, and current followers amplify the input voltage.

Noninverting voltage followers are most widely used. In contrast to conventional amplifier stages, they typically exhibit an increased input impedance—in particular, a smaller input capacitance—and a decreased output impedance. Because of this characteristic, they can transmit signals from a high-impedance source—for example, a source with an impedance of 1 megohm—to a load with a low impedance, such as ~ 10 ohms, practically without attenuation. A typical example of a low-impedance load is a length of coaxial cable that connects individual units of electronic equipment.

Voltage followers are also classified by the type of electronic device used in their circuitry. Cathode followers use an electron tube (Figure 1,a), emitter followers a bipolar transistor (Figure 1,b), and source followers a field-effect transistor (Figure 1,c). All of these types exhibit a strong (“100-percent”) negative feedback. As a result, they have an increased linearity and stability of characteristics, a greater bandwidth, and a wider amplitude range of the input signal. Voltage followers in integrated circuits generally make use of a dual or a compound transistor.

REFERENCES

Erglis, K. E., and I. P. Stepanenko, Elektronnye usiliteli, 2nd ed. Moscow, 1964
Stepanenko, I. P. Osnovy teorii tranzistorov i tranzistomykh skhem, 3rd ed. Moscow, 1973.

I. P. STEPANENKO