emitter follower

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. 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.

Applications

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:

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]

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

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

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

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 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.)

	Definition	Expression	Approximate expression	Conditions
Current gain				Failed to parse (unknown function\gg): \beta_0 \gg 1
Voltage gain				Failed to parse (unknown function\gg): g_m R_\mathrm{E} \gg 1
Input resistance				Failed to parse (unknown function\gg): (g_m R_\mathrm{E} \gg 1) \wedge (\beta_0 \gg 1)
Output resistance				Failed to parse (unknown function\gg): (\beta_0 \gg 1) \wedge (r_\mathrm{in} \gg R_\mathrm{source})

Where is the thevenin equivalent source resistance.

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 pair • Sziklai pair • Cascode • Long-tailed pair

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

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