BJT Biasing

BJT biasing is the process of applying DC voltages to a BJT circuit to establish a fixed operating point for the transistor. This is necessary because BJTs are nonlinear devices, and their characteristics vary depending on the operating point.

There are three main types of BJT biasing circuits: fixed bias, voltage divider bias, and emitter bias.

Fixed bias

Fixed bias is the simplest type of BJT biasing circuit. It uses two DC voltage sources to establish the operating point of the transistor. One voltage source is applied between the base and emitter terminals (VBE), and the other voltage source is applied between the collector and base terminals (VBC).

The operating point of the transistor is determined by the values of the bias resistors, RB and RC. The base current is given by the equation IB = (VBB - VBE)/RB, and the collector current is given by the equation IC = βIB, where β is the current gain of the transistor.

Fixed bias is not a very stable biasing circuit, because the operating point of the transistor can be affected by changes in temperature and VBE.

Voltage divider bias

Voltage divider bias is a more stable biasing circuit than fixed bias. It uses a voltage divider circuit to establish the base voltage of the transistor. The voltage divider circuit consists of two resistors, R1 and R2, which are connected in series to the VCC voltage source.

The base voltage of the transistor is given by the equation VBE = VCC R2 / (R1 + R2). The operating point of the transistor is determined by the values of the bias resistors, R1, R2, and RC. The base current is given by the equation IB = (VCC R2 - VBE)/R1, and the collector current is given by the equation IC = βIB.

Emitter bias

Emitter bias is the most stable biasing circuit of the three. It uses a resistor in the emitter circuit to stabilize the operating point of the transistor. The emitter resistor, RE, is connected in series between the emitter terminal of the transistor and the ground terminal.

The voltage across the emitter resistor is given by the equation VE = RE * IE, where IE is the emitter current. The emitter current is given by the equation IE = IC + IB. The operating point of the transistor is determined by the values of the bias resistors, RE and RC. The collector current is given by the equation IC = (VCC - VE)/RC.

Emitter bias is the most popular biasing circuit for BJTs, because it is very stable and easy to design.