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In electronics, a 'common collector' circuit is a basic bipolar transistor amplifier topology, commonly used as a voltage buffer. In this circuit arrangement, 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. 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.

Contents

Applications

Characteristics

See also

External links

Applications

The common collector circuit can be shown to have a voltage gain of almost 1:
: $$
{A_mathrm{v}} = {v_mathrm{out} over v_mathrm{in}} pprox 1

This means that voltage signals appearing on the input will be nearly identically replicated on the output (minus a constant diode drop, and 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} pprox eta_0 R_mathrm{E}

and a small output impedance, so it can drive low-resistance loads:
: $$
r_mathrm{out} pprox R_mathrm{E} | {R_mathrm{source} over eta_0}

(Typically, the emitter resistor is significantly larger and can be removed from the equation):
: $$
r_mathrm{out} pprox {R_mathrm{source} over eta_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].

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.)
{| class="wikitable" style="background:white;text-align:left"
! !! Definition !! Expression !! Approximate expression !! Conditions
|-
! 'Current gain'
|$\{A\_mathrm\{i\}\}\; =\; \{i\_mathrm\{out\}\; over\; i\_mathrm\{in\}\}$
|
|
|
|-
! 'Voltage gain'
|$\{A\_mathrm\{v\}\}\; =\; \{v\_mathrm\{out\}\; over\; v\_mathrm\{in\}\}$
|$\{g\_m\; R\_mathrm\{E\}\; over\; g\_m\; R\_mathrm\{E\}\; +\; 1\}$
|
|$g\_m\; R\_mathrm\{E\}\; gg\; 1$
|-
! 'Input resistance'
|
|
|
|
|-
! 'Output resistance'
|
|
ight)
|
|
|}
The variables not listed in the schematic are:

★ ''g_m'' is the transconductance in siemens, calculated by $g\_m\; =\; I\_mathrm\{C\}\; /\; V\_mathrm\{T\}\; ,$, where:

★
★ $I\_mathrm\{C\}\; ,$ is the quiescent collector current (also called the collector bias or DC collector 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 (★ class=wikiexternal target=_blank>+k+%2F+elementary+charge+in+millivolts+%3D Google calculator).

★ 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.

★

★ $R\_mathrm\{source\}$ is the thevenin equivalent source resistance.

See also

External links

★ Basic BJT Amplifier Configurations

★ NPN Common Collector Amplifier — HyperPhysics

★ The Common Collecter Amplifier, Physics Lecture Notes, D.M. Gingrich, University of Alberta Department of Physics