Common collector

In electronics, a common collector amplifier (also known as an emitter follower) is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a voltage buffer.

In this circuit, the base terminal of the transistor serves as the input, the emitter is the output, and the collector is common to both (for example, it may be tied to ground reference or a power supply rail), hence its name.

The circuit can be explained by viewing the transistor as being under the control of negative feedback.

Thus the two voltages are subtracted according to Kirchhoff's voltage law (KVL) (the subtractor from the function block diagram is implemented just by the input loop), and their difference Vdiff = Vin − Vout is applied to the base–emitter junction.

The transistor continuously monitors Vdiff and adjusts its emitter voltage to equal Vin minus the mostly constant VBE (approximately one diode forward voltage drop) by passing the collector current through the emitter resistor RE.

Intuitively, this behavior can be also understood by realizing that VBE is very insensitive to bias changes, so any change in base voltage is transmitted (to good approximation) directly to the emitter.

It depends slightly on various disturbances (transistor tolerances, temperature variations, load resistance, a collector resistor if it is added, etc.

It never saturates even if the input voltage reaches the positive rail.

The common-collector circuit can be shown mathematically to have a voltage gain of almost unity: A small voltage change on the input terminal will be replicated at the output (depending slightly on the transistor's gain and the value of the load resistance; see gain formula below).

Typically, the emitter resistor is significantly larger and can be removed from the equation: The common collector amplifier's low output impedance allows a source with a large output impedance to drive a small load impedance without changing its voltage.

That resistance reduction makes the combination a more ideal voltage source.

This configuration is commonly used in the output stages of class-B and class-AB amplifiers.

The base circuit is modified to operate the transistor in class-B or AB mode.

In class-A mode, sometimes an active current source is used instead of RE (Fig.

[1] At low frequencies and using a simplified hybrid-pi model, the following small-signal characteristics can be derived.

Using Ohm's law, various currents have been determined, and these results are shown on the diagram.

Applying Kirchhoff's current law at the emitter one finds: Define the following resistance values: Then collecting terms the voltage gain is found: From this result, the gain approaches unity (as expected for a buffer amplifier) if the resistance ratio in the denominator is small.

Figure 1: Basic NPN common collector circuit (neglecting biasing details).
Figure 2: A negative-feedback amplifier
Figure 3: PNP version of the emitter-follower circuit, all polarities are reversed.
Figure 4: NPN voltage follower with current source biasing suitable for integrated circuits
Figure 5: Small-signal circuit corresponding to Figure 3 using the hybrid-pi model for the bipolar transistor at frequencies low enough to ignore bipolar device capacitances
Figure 6: Low-frequency small-signal circuit for bipolar voltage follower with test current at output for finding output resistance. Resistor .