DC Biasing of BJTs-Basic Electronics Biasing-Lecture Slides, Slides of Basic Electronics

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Basic Electronic Engineering

Lecture 12

DC Biasing of BJTs

• Complete analysis and design of an electronic amplifier

requires

• dc analysis
• ac analysis
• The dc voltages are provided to the transistor to turn it

on so the transistor can amplify the ac signal

• The application of dc voltages to establish a fixed level of

current and voltage is called biasing of a transistor

DC Biasing of BJTs

DC Biasing of BJTs

• Q-point A
  • Without any bias voltage the Q-point will be at A
  • No current through the transistor and no voltage drop across the transistor
• Q-point B
  • Q-point B will allow the signal to swing without any distortion ( input kept within limits)
  • Point B is most suitable for the purpose of linear amplification

DC Biasing of BJTs

Effect of Temperature on Q-point

• The temperature effect the device parameters such as current gain(β) and leakage current(ICEO)
• Higher temperature results in higher leakage currents
• The bias circuit must provide temperature stability so that there is a minimum change in operating point with the change in temperature

Fixed-Bias Circuit

• A single voltage source VCC

is used for biasing

• VCC is directly connected to

RC and RB

• For analysis with pnp

transistor the direction of

currents will be opposite

and voltage polarities will

be reversed

Fixed-Bias Circuit

Collector-Emitter Loop

• Collector current
• Collector current depends on base

current and is not controlled by RC

• Changing RC will change the value of

VCE

I C  IB

VCE  VCC I RC C

Fixed-Bias Circuit

• Find the values of

a) IC b) RC c) RB d) VCE

Fixed-Bias Circuit

• Find the values of

a) IBQ and ICQ

b) VCEQ

c) VB and VC

d) VBC