what is bjt amplifier


Common emitter - Wikipedia, the free encyclopedia

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** Common emitter **

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Figure 1: Basic NPN common-emitter circuit (neglecting biasing details).

In electronics, a *common emitter* amplifier is one of three basic
single-stage bipolar-junction-transistor (BJT) amplifier topologies,
typically used as a voltage amplifier.

In this circuit the base terminal of the transistor serves as the input,
the collector is the output, and the emitter is /common/ to both (for
example, it may be tied to ground reference or a power supply rail), hence
its name. The analogous field-effect transistor circuit is the common
source amplifier, and the analogous tube circuit is the common cathode
amplifier.

*Contents*

· 1 Emitter degeneration
· 2 Characteristics

· 2.1 Bandwidth

· 3 Applications

· 3.1 Low frequency voltage amplifier
· 3.2 Radio

· 4 See also
· 5 References
· 6 External links

*Emitter degeneration[edit]*

Figure 2: Adding an emitter resistor decreases gain, but increases
linearity and stability

Common emitter amplifiers give the amplifier an inverted output and can
have a very high gain that may vary widely from one transistor to the next.
The gain is a strong function of both temperature and bias current, and so
the actual gain is somewhat unpredictable. Stability is another problem
associated with such high gain circuits due to any unintentional positive
feedback that may be present.

Other problems associated with the circuit are the low input dynamic range
imposed by the small-signal limit; there is high distortion if this limit
is exceeded and the transistor ceases to behave like its small-signal
model. One common


Source: en.wikipedia.org/wiki/Common_emitter

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