Common Collector - Basic Circuit

Basic Circuit

The circuit can be explained by viewing the transistor as being under the control of negative feedback. From this viewpoint, a common-collector stage (Fig. 1) is an amplifier with full series negative feedback. In this configuration (Fig. 2 with β = 1), the entire output voltage VOUT is placed contrary and in series with the input voltage VIN. Thus the two voltages are subtracted according to 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 monitors continuously Vdiff and adjusts its emitter voltage almost equal (less VBEO) to the input voltage by passing the according collector current through the emitter resistor RE. As a result, the output voltage follows the input voltage variations from VBEO up to V+; hence the name, emitter follower. Intuitively, this behavior can be also understood by realizing that the base-emitter voltage in the bipolar transistor 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, collector resistor if it is added, etc.) since the transistor reacts to these disturbances and restores the equilibrium. 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_\mathrm{v}} = {v_\mathrm{out} \over v_\mathrm{in}} \approx 1

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). This circuit is useful because it has a large input impedance, so it will not load down the previous circuit:

r_\mathrm{in} \approx \beta_0 R_\mathrm{E}

and a small output impedance, so it can drive low-resistance loads:

r_\mathrm{out} \approx {R_\mathrm{E}} \| {R_\mathrm{source} \over \beta_0}

Typically, the emitter resistor is significantly larger and can be removed from the equation:

r_\mathrm{out} \approx {R_\mathrm{source} \over \beta_0}

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