common collector amplifiers
In electronics, a common collector push pull 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 common collector amplifier 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 hFE 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 RE to improve linearity and/or efficiency. See [1].
Characteristics
(The parallel lines indicate components in parallel.)
Inherent voltage gain:
Input resistance:
Current gain:
Output resistance:
The variables not listed in the schematic are:
- gm is the transconductance in siemens, calculated by gm = IC / VT, where:
- IC is the collector bias current
- VT = 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).
- IC is the collector bias current
- β0 = IC / IB is the current gain at low frequencies (commonly called hFE). This is a parameter specific to each transistor, and can be found on a datasheet.
- rπ = β0 / gm = VT / IB
Transistor amplifiers
- Common emitter
- Common collector
- Common base
- Common source
- Common drain
- Common gate
Electronics Topics
The field of electronics is the study and use of systems that operate by controlling the flow of electrons or other electrically charged particles in devices such as thermionic valves and semiconductors. The design and construction of electronic circuits to solve practical problems is part of the fields of electronic engineering, and the hardware design side of computer engineering. The study of new semiconductor devices and their technology is sometimes considered as a branch of physics.
# - A | B | Co - Cz | C - Cm | D
Em - F | E - El | G - H | I - K | L - Ma
