DC Biasing Circuits

Introduction – Biasing The analysis or design of a transistor amplifier requires knowledge of both the dc and ac response of the system. In fact, the amplifier increases the strength of a weak signal by transferring the energy from the applied DC source to the weak input ac signal. The analysis or design of any electronic amplifier therefore has two components: The dc portion and The ac portion During the design stage, the choice of parameters for the required dc levels will affect the ac response. What is biasing circuit? Biasing: The DC voltages applied to a transistor in order to turn it on so that it can amplify the AC signal.

VCB I E  IC  I B VCE V BE   IC Purpose of the DC biasing circuit To turn the device “ON” To place it in operation in the region of its characteristic where the device operates most linearly . Proper biasing circuit which it operate in linear region and circuit have centered Q-point or midpoint biased Improper biasing cause Improper biasing cause „Distortion in the output signal „Produce limited or clipped at output signal Important basic relationship I E  IC  I B   IC I B I E  ( 1)I B  IC VCB VCE V BE

Operating Point Active or Linear Region Operation Base – Emitter junction is forward biased Base – Collector junction is reverse biased Good operating point Saturation Region Operation Base – Emitter junction is forward biased Base – Collector junction is forward biased Cutoff Region Operation Base – Emitter junction is reverse biased The DC input establishes an operating or quiescent point called the Q-point.

No matter what type of configuration a transistor is used in, the basic relationships between the currents are always the same, and the base-to-emitter voltage is the threshold value if the transistor is in the “on” state

Voltage divider bias circuit Emitter-stabilized bias circuit BJT Analysis DC analysis AC analysis Calculate the DC Q-point Calculate gains of the amplifier solving input and output loops Graphical Method DC Biasing Circuits Fixed-bias circuit Voltage divider bias circuit Emitter-stabilized bias circuit Collector-emitter loop DC bias with voltage feedback

Base bias (fixed bias). The fixed-bias configuration is the simplest of transistor biasing arrangements, but it is also quite unstable For the dc analysis of a transistor network, all capacitors are replaced by an open-circuit equivalent

The dc equivalent circuit of the fixed bias circuit where the capacitor is replaced with an open-circuit

The Base-Emitter Loop From Kirchhoff’s voltage law: +VCC – IBRB – VBE = 0 Solving for base current:

Collector-Emitter Loop Collector current: From Kirchhoff’s voltage law:

Base bias characteristics Circuit recognition: A single resistor (RB) between the base terminal and VCC. No emitter resistor. Advantage: Circuit simplicity. Disadvantage: Q-point shift with temp. Applications: Switching circuits only.

Base bias characteristics Load line equations: Q-point equations:

A generic dc load line

Plot the dc load line for the circuit

Plot the dc load line for the circuit Plot the dc load line for the circuit. Then, find the values of VCE for IC = 1, 2, 5 mA respectively. IC (mA) VCE (V) 1 9 2 8 5

Example

Question 1

Voltage divider bias This is a very stable bias circuit.

Exact Analysis:

Applying Kirchhoff’s voltage law in the clockwise direction in the Thevenin network, (Substituting IE = (+1)IB)

Determine the values of ICQ and VCEQ for the circuit

Example A voltage-divider bias circuit has the following values: R1 = 1.5 kW, R2 = 680 W, RC = 260 W, RE = 240 W and VCC = 10 V. Assuming the transistor is a 2N3904, determine the value of IE for the circuit. Assume beta to be 100.

Load line for voltage divider bias circuit.