Biasing Transistors for Switching Circuits

Biasing Transistors for Switching Circuits

• Transistors are widely used in digital logic circuits and switching applications. In these applications, the voltage levels periodically alternate between a "Low" and a "High" voltage, such as 0 V and + 5 V.

• In switching applications, the transistor operates either in cut-off region or saturation region. It is connected in common emitter configuration as shown in the Fig. 2.10.1.

• In this circuit, when input is HIGH, base current flows and it is greater than I_C / βihence transistor is operated in saturation. In saturation condition, voltage between collector and emitter, VCE(gat^ is typically 0.2 V to 0.3 V and hence transistor acts as closed switch. This is illustrated in Fig. 2.10.2.

• When input is LOW, base current and collector current is zero and hence transistor is operated in cut-off. In cut-off VCE = Vcc and transistor acts as open switch. This is illustrated in Fig. 2.10.3.

• For switching operation we have to operate transistor into saturation and cut-off.

• In CB configuration, current gain IC j is less than 1

and hence it requires high input (Emitter) current to drive the transistor into saturation.

• In CC configuration, voltage gain is less than 1 and hence it requires high base voltage to forward bias BE-junction. Furthermore, as collector is tied to VCC' it is not possible to forward bias the CB-junction and hence to drive transistor in saturation.

• Due to these reasons, the CB and CC configurations are not preferred to operate as switch.

Review Questions

1. Explain the working of a transistor as a switch.

2. Why CC and CB configurations are not preferred for BJT as a switch ?