Since a bipolar transistor is a classic three-terminal device, there are three possible ways to incorporate it into an electronic circuit with one common output and output:
- with a common base (OB) - high voltage transmission coefficient;
- with a common emitter (OE) - an amplified signal both in current and in voltage;
- with a common collector (OK) - amplified current signal.
In each of the three varieties of the transistor switching circuit, it reacts differently to the input signal, since the static characteristics of its active elements depend on the particular solution.
A common base circuit is one of three typical configurations for turning on bipolar transistors. It is usually used as a current buffer or voltage amplifier. Such transistor switching circuits differ in that the emitter acts as an input circuit here, the output signal is removed from the collector, and the base is “grounded” to the common wire. A similar configuration is used for switching PT in amplifiers with a common gate.
Table 1 The main parameters of the amplifier stage according to the OB scheme.Parameter | Expression |
Current gain | I k / I in = I k / I e = α [α <1] |
Vh. resistance | R in = U in / I in = U be / Ie |
The switching circuits of OB transistors are characterized by stable temperature and frequency properties, which ensures a small dependence of their parameters (transmission coefficient by voltage, current, input resistance) from the temperature conditions of the working environment. The disadvantages of the circuit include a small R BX and the lack of current gain.
The common emitter circuit provides a very high gain and gives an inverted signal at the output, which can have a rather large spread. The transfer coefficient in this circuit is largely dependent on the temperature of the bias current, as a result of which the actual gain is somewhat unpredictable. These transistor switching circuits provide high R BX , current and voltage gain, inverting the input signal, and convenient switching. The disadvantages include problems associated with overpowering - the possibility of spontaneous positive feedback, the appearance of distortion with small signals due to the low input dynamic range.
Table 2. The main parameters of the amplifier stage according to the OE schemeParameter | Expression |
Coeff. current gain | I out / I in = I k / I b = I k / (I e -I k ) = α / (1-α) = β [β >> 1] |
Vh. resistance | R in = U in / I in = U be / I b |
A common collector circuit (also known as an emitter follower in electronics) is one of three varieties of a transistor switching circuit. In it, the input signal is supplied through the base circuit, and the output is removed from the resistor in the emitter circuit of the transistor. Such an amplifier stage configuration is typically used as a voltage buffer. Here, the base of the transistor functions as an input circuit, the emitter is the output, and the grounded collector serves as a common point, hence the name of the circuit. Analogs can serve as a circuit for switching on field-effect transistors with a common drain. The advantage of this method is the relatively high input impedance of the amplifier stage and the relatively low output.
Table 3. The main parameters of the amplifier stage according to the OK scheme.Parameter | Expression |
Coeff. current gain | I out / I in = I e / I b = I e / (I e -I k ) = 1 / (1-α) = β [β >> 1] |
Coff voltage gain | U out / U in = U Re / (U be + U Re ) <1 |
Vh. resistance | R in = U in / I in = U be / Ie |
All three typical transistor switching circuits are widely used in circuitry, depending on the purpose of the electronic device and the conditions for its use.