Chapter 4(b) DC Biasing – Bipolar Junction Transistors (BJTs) DMT 121 – ELECTRONIC 1 Chapter 4(b) DC Biasing – Bipolar Junction Transistors (BJTs)

VOLTAGE DIVIDER BIAS The most widely used type of bias circuit. Only one power supply is needed and voltage-divider bias is more stable ( independent) than other bias types. Two methods of analysis, exact and approximate analysis

VOLTAGE DIVIDER BIAS – Exact Analysis Determining RTH. To determine RTH  The voltage source is replaced by a short-circuit equivalent, resulting…….. RTH = R1 ǁ R2

VOLTAGE DIVIDER BIAS – Exact Analysis To determine ETH  The voltage source VCC remained on the network and the open circuit Thevenin voltage can be determined. Determining ETH.

VOLTAGE DIVIDER BIAS – Exact Analysis The Thevenin network is then redrawn and IBQ can be determined by applying Kirchoff’s voltage law. ETH – IBRTH – VBE – IERE = 0, ….substitute IE = ( + 1) IB….. then Almost similar with emitter bias Voltage differences over resistance.

VOLTAGE DIVIDER BIAS – Exact Analysis IC = IB ; IE = ( + 1) IB  IB Substituting between these OR equation in previous slide (from derivation), resulting : If RE >>> RTH/, then… Independent to Beta

VOLTAGE DIVIDER BIAS – Exact Analysis Once IB is known, the rest of the parameters can be determined. VCE = VCC – IC (RC + RE) The remaining equations VE, VC and VB are also similar as obtained in emitter bias configuration. Voltage-divider bias configuration.

VOLTAGE DIVIDER BIAS – Approximate Analysis (Loading Effect) and Ri = ( + 1)RE  RE with condition RE  10R2 Partial-bias circuit for calculating the approximate base voltage VB. If beta times the value RE is at least 10x the value R2, the approximate approach can be applied with high accuracy. Ri = equivalent transistor between base and ground for transistor with an emitter resistor RE

VOLTAGE DIVIDER BIAS – Approximate Analysis(Loading Effect) Once VB is determined, the level of VE can be calculated. VE = VB – VBE And emitter current and IC  IE VCE = VCC –ICRC –IERE but since IE  IC VCE= VCC – IE (RC + RE) Partial-bias circuit for calculating the approximate base voltage VB.

Voltage divider bias - Summary Circuit recognition : The voltage divider in the base circuit. Q-point stability : The circuit Q-point values are stable against changes in β Advantage: The Q-point of voltage divider bias circuit is less dependent on than β that of the base bias (fixed bias). Disadvantage: Requires more components than most other biasing circuits. Applications: Used primarily to bias linear amplifier.

Cont’d Summary Load line equations: Q-point equations:

EXAMPLE 1 Assuming the VBE = 0.7V, determine the voltage divider circuit is stiff or not and find the value of IB , IC , VE and VCE for the circuit. Β = 173.

Example Assuming the VBE = 0.7V, determine the voltage divider circuit is stiff or not and find the value of IC and VCE for the circuit. Β = 50.

Example For pnp circuit, given that VEE = 5V, R1=22kΩ, R2=10k Ω, Rc=2.2kΩ, RE=1.0k Ω and β=150. Find the value of Icand VCE.