Common collector

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Figure 1: Basic NPN common collector circuit (neglecting biasing details).
Figure 1: Basic NPN common collector circuit (neglecting biasing details).

In electronics, a common collector (also known as an emitter follower or voltage follower) amplifier 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 the output and the collector is common to both, hence its name. An analogous circuit called the common drain is constructed using field effect transistors.

One aspect of buffer action is transformation of impedances. For example, the Thévenin resistance of a combination of a voltage follower driven by a voltage source with high Thévenin resistance is reduced to only the output resistance of the voltage follower, a small resistance. That resistance reduction makes the combination a more ideal voltage source. Conversely, a voltage follower inserted between a small load resistance and a driving stage presents a large load to the driving stage, an advantage in coupling a voltage signal to a small load.

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[edit] Applications

Figure 2: PNP version of the emitter follower circuit, all polarities are reversed.
Figure 2: PNP version of the emitter follower circuit, all polarities are reversed.

The common collector circuit can be shown to have a voltage gain of almost unity by taking the output voltage and dividing it by the input voltage. Mathematically, gain looks like:

{A_\mathrm{v}} = {v_\mathrm{out} \over v_\mathrm{in}} \approx 1

Therefore 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:[1]

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}

This allows a source with a large output impedance to drive a small load impedance; it functions as a voltage buffer.

In other words, the circuit has current gain (which depends largely on the hFE of the transistor) instead of voltage gain. A small change to the input current results in much larger change in the output current supplied to the output load.

This configuration is commonly used in the output stages of class-B and class-AB amplifier — 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 to improve linearity and/or efficiency. See [2].

[edit] Characteristics

At low frequencies and using a simplified hybrid-pi model, the following small signal characteristics can be derived. (The parallel lines indicate components in parallel.)

Definition Expression Approximate expression Conditions
Current gain {A_\mathrm{i}} = {i_\mathrm{out} \over i_\mathrm{in}} \beta_0 + 1 \ \approx \beta_0 \beta_0 \gg 1
Voltage gain {A_\mathrm{v}} = {v_\mathrm{out} \over v_\mathrm{in}} {g_m R_\mathrm{E} \over g_m R_\mathrm{E} + 1} \approx 1 g_m R_\mathrm{E} \gg 1
Input resistance r_\mathrm{in} = \frac{v_{in}}{i_{in}} r_\pi + (\beta_0 + 1) R_\mathrm{E}\ \approx \beta_0 R_\mathrm{E} (g_m R_\mathrm{E} \gg 1) \wedge (\beta_0 \gg 1)
Output resistance r_\mathrm{out} = \frac{v_{out}}{i_{out}} R_\mathrm{E} || \left( {r_\pi + R_\mathrm{source} \over \beta_0 + 1} \right) \approx {1 \over g_m} + {R_\mathrm{source} \over \beta_0} (\beta_0 \gg 1) \wedge (r_\mathrm{in} \gg R_\mathrm{source})

Where R_\mathrm{source} \ is the Thévenin equivalent source resistance.

[edit] See also

v • d • e
Transistor amplifiers

[edit] External links