Basics Basics, amplifier

The Transistor, Basics

Dr.-Ing. Peter Strassacker
The transistor is an electronic component that is used to amplify and switch currents and voltages. There are bipolar and field-effect transistors (FET). A bipolar transistor/field-effect transistor can be imagined as a controllable current source/controllable resistor that is controlled by an input current/input voltage .

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:

The common emitter circuit

The common emitter circuit is the most commonly used one since both current and voltage are amplified.
Picture above: common emitter circuit
Using both capacitors, one with 1 μF and one with 10 μF, left and right, the DC voltage is extracted (rendered ineffective). Both the left resistors with 33k and 150kΩ adjust the operating point of the transistor. The operating point has to be chosen correctly to ensure that a matching current flow is generated. The voltage amplification VU results from the ratio of the collector's total resistance to the emitter's the total resistance, whereby for a big load resistor >20 kΩ the following approximately applies.

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 common collector circuit

Picture above: the common collector circuit
The common collector circuit is the simplest one. Here the collector has a common reference potential; it's driven via the base opposite the collector. Since the emitter follows the collector (the voltage amplification is just below 1) it's also called emitter-follower. At the output, however, a higher current may be obtained than applied at the input; this circuit is therefore a current amplifier.

The voltage amplification is < 1,
Current amplification results in power amplification.

The common base circuit

Picture above: the common base circuit
The common base circuit is similar to the common emitter circuit since also here the input signal controls the base in relation to the emitter. The emitter current, however, has to be supplied at source. Therefore the current amplification is just below 1 (collector current = emitter current - base current).

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.