Common collector

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

In electronics, a common collector circuit is a basic bipolar transistor amplifier topology, commonly used as a voltage buffer (that is, the output circuit is isolated from the input circuit) or as a voltage follower (that is, the output voltage follows the input voltage, voltage gain is unity). The emitter node closely tracks ('follows') the voltage applied to the input, hence the common name emitter follower. The FET equivalent of the common collector is the common drain.

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.

In this circuit arrangement, as shown in Figure 1, the collector node of the transistor is connected to a power supply (a voltage source), the base node acts as the input and the emitter node is used as the output. A pnp version is shown in Figure 2.

1 Applications
2 Characteristics
3 See also
4 External links

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:

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

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 h_FE 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 R_E to improve linearity and/or efficiency. See [2].

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 thevenin equivalent source resistance.

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