The bipolar transistor usually has three terminals: the base, the emitter and the collector. Depending on the polarity, the transistor is either called NPN (plus pole on collector, minus pole on emitter) or PNP (minus pole on collector, plus pole on emitter).
In the following we show you the basic circuits on the basis of a NPN transistor (emitter arrow pointing away from the centre). The arrow of the bipolar transistor indicates the technical current direction (from plus to minus).
The basic circuitry of an elementary amplifier is named after which of the
transistor's terminals is common to input and output.
Since the three basic circuit arrangements have totally different properties, there are described below:
VU = RK / RE = 3.3 kOhm / 1 kOhm = 3.3
The common emitter circuit inverts the phase of input and output signal.
The circuit's power amplification results from the product of current and
voltage amplification; the resulting values vary from 10 to 1000. The resistor
at the emitter (negative feedback resistor) stabilises the operating point,
controls the voltage amplification and ensures less harmonic distortion at
low voltage amplification, as long as the amplifier is not overdriven.
The voltage amplification is < 1, Current amplification results in power amplification.
This circuit amplifies the voltage; possible applications are high-frequency amplifiers (above 10.000.000 Hz) or differential amplifiers resp. current mirrors.
Current amplification < 1, Voltage amplification results in power amplification.