Common collector

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Typical common collector or emitter follower circuit.

In electronics, a common collector circuit, also known as an emitter follower circuit, refers to one type of circuit arrangement in which a bipolar transistor drives a load circuit such as a resistor or the next stage in an electronic amplifier. In this circuit arrangement, the collector node of the transistor is tied to a power rail or a common node, the emitter node is connected to the output load to be driven, and the base node acts as an input. Owing to the physics of the bipolar transistor, the emitter node closely tracks ('follows') the voltage applied to the input node, which is useful in many applications.

The common collector circuit is found to have a voltage gain of almost unity, meaning AC signals appearing on the input will be nearly identically replicated on the output, assuming the output load is not too difficult to drive. The circuit has a typical current gain which depends largely on the h_FE of the transistor. A small change to the input current results in much larger change in the output current supplied to the output load. Thus a weakly driven input node can be used to drive a lower resistance at the output node. 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 [1].

[edit] Characteristics

(The parallel lines indicate components in parallel.)

Inherent voltage gain:

${V_\mathrm{out} \over V_\mathrm{in}} = {(1 + \beta_0)(R_\mathrm{E} \| R_\mathrm{load}) \over r_\pi + (1 + \beta_0) (R_\mathrm{E} \| R_\mathrm{load})}$

Input resistance:

$r_\mathrm{in} = R_1 \| R_2 \| (r_\pi + (1 + \beta_0) (R_\mathrm{E} \| R_\mathrm{load}))\,$

Current gain:

$A_\mathrm{vm} {r_\mathrm{in} \over R_\mathrm{load}}$

Output resistance:

$r_\mathrm{out} = R_\mathrm{E} \| {r_\pi + R_1 \| R_2 \| R_\mathrm{source} \over 1 + \beta_0}$

The variables not listed in the schematic are:

g_m is the transconductance in siemens, calculated by , where:
- $I_\mathrm{C} \,$ is the collector bias current
- $V_\mathrm{T} = kT / q \,$ is the thermal voltage, calculated from Boltzmann's constant, the charge on an electron, and the transistor temperature in kelvins. At room temperature this is about 25 mV (Google calculator).
$\beta_0 = I_\mathrm{C} / I_\mathrm{B} \,$ is the current gain at low frequencies (commonly called h_FE). This is a parameter specific to each transistor, and can be found on a datasheet.
$r_\pi = \beta_0 / g_m = V_\mathrm{T} / I_\mathrm{B} \,$

[edit] See also

Transistor amplifiers
Bipolar junction transistor: Common emitter • Common collector • Common base Field effect transistor: Common source • Common drain • Common gate Multiple transistors: Darlington transistor • Cascode • Parallel circuit