Download Chapter4 DC Biasing BJT (part b)

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 shortcircuit 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.
ETH
VCCR 2
 VR 2 
R1  R 2
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
ETH  VBE
IB 
RTH  (   1) RE
Almost similar with emitter bias
Voltage differences over resistance.
VOLTAGE DIVIDER BIAS – Exact Analysis
IB 
ETH  VBE
RTH  (   1) RE
IC = IB ; IE = ( + 1) IB  IB
Substituting between these OR
equation in previous slide (from
derivation), resulting :
ETH  VBE
IE 
RE  RTH / 
If RE >>> RTH/, then…
Independent to Beta
ETH  VBE
IE 
RE
VOLTAGE DIVIDER BIAS – Exact Analysis
Once IB is known, the rest
of the parameters can be
determined.
VCE = VCC – IC (RC + RE)
Voltage-divider bias configuration.
The remaining equations VE, VC
and VB are also similar as
obtained in emitter bias
configuration.
VOLTAGE DIVIDER BIAS – Approximate
Analysis (Loading Effect)
R2


VB  VCC 

R
1

R
2


and
Ri = ( + 1)RE  RE
with condition
Partial-bias circuit for calculating the
approximate base voltage VB.
Ri = equivalent
transistor between
base and ground for
transistor with an
emitter resistor RE
RE  10R2
If beta times the value RE is at
least 10x the value R2, the
approximate approach can be
applied with high accuracy.
VOLTAGE DIVIDER BIAS – Approximate
Analysis(Loading Effect)
Once VB is determined, the
level of VE can be calculated.
VE = VB – VBE
And emitter current
VE
IE 
RE
and IC  IE
Partial-bias circuit for calculating the
approximate base voltage VB.
VCE = VCC –ICRC –IERE but
since IE  IC
VCE= VCC – IE (RC + RE)
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:
I C (sat )
VCC

RC  RE
VCE (off )  VCC
Q-point equations:
R2


VB  VCC 

 R1  R2 
VE  VB  0.7V
VCEQ  VCC  ICQ ( RC  RE )
 VE 
IC  IE  

 RE 
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.