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. Common collector amplifier. ... This is a hub page for electronics. ... The schematic symbols for pnp_ and npn_type BJTs. ... Resistor symbols A pack of resistors A resistor is a two-terminal electrical or electronic component that resists the flow of current, producing a voltage drop between its terminals in accordance with Ohms law. ... An electronic amplifier is a device for increasing the current, voltage or power of a signal. ... Semiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium and gallium arsenide. ...

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]. city lights viewed in a motion blurred exposure. ... This topic has already been covered in electronic amplifier. ... Class A amplifiers tend to have very low distortion when used with small signals, and are very inefficient Class B amplifiers tend to be efficient but suffer from high distortion when used with small signals. ... This topic has already been covered in electronic amplifier. ... An ideal current source, I, driving a resistor, R, and creating a voltage V A current source is an electrical or electronic device that delivers or absorbs electric current. ...

Characteristics

(The parallel lines indicate components in parallel.) The term Parallel has a number of important meanings: Parallel (geometry) occurs in geometry. ... Left: Series / Right: Parallel Arrows indicate direction of current flow. ...

Inherent voltage gain: In electronics, gain is usually taken as the mean ratio of the signal output of a system to the signal input of the system. ...

Input resistance: The input impedance or load impedance of a circuit or electronic device is the impedance actually experienced by a signal which is connected to it. ...

Current gain:

Output resistance: The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to the flow of an alternating current (AC) of a particular frequency as a result of resistance, induction and capacitance. ...

The variables not listed in the schematic are:

• g_m is the transconductance in siemens, calculated by g_m = I_C / V_T, where:
  • I_C is the collector bias current
  • V_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).
• β₀ = I_C / I_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_π = β₀ / g_m = V_T / I_B

Transconductance, also known as mutual conductance, is a property of certain electronic components. ... The siemens (symbol: S) is the SI derived unit of electric conductance. ... The Boltzmann constant (k or kB) is the physical constant relating temperature to energy. ... The elementary charge (symbol e or sometimes q) is the electric charge carried by a single proton, or equivalently, the negative of the electric charge carried by a single electron. ... The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. ...

Transistor amplifiers

• Common emitter
• Common base
• Common source
• Common drain
• Common gate