Modified collector to base bias
circuit:
To
improve the level of stability, emitter resistance is connected in this
circuit.
Base circuit:
Applying
KVL to base circuit,
VCC
– (IC + IB ) RC – IBRB – VBE
– IERE = 0
IB
= [VCC – VBE ] / [RB + (1+β) (RC
+ RE)]
IB
= [VCC – VBE]/ [RB
+ β (RC + RE)]
Only
difference between the equation for IB and that obtained for the
fixed bias configuration is the term β (RC + RE).So
feedback path results in a reflection of the resistance RC back to
the input circuit.
In
general,
IB
= V’ / RB + β R’
Where V’
= VCC - VBE
R’ = 0
for fixed bias
R’ = RE
for emitter bias
R’ = RC
for collector to base bias
R’ = RC
+ RE for collector to base bias with RE
Collector circuit:
Applying
KVL to collector circuit,
VCC
– (IC+IB) RC – VCE – IERE
= 0
VCE
= VCC – I E (RC+RE)
Stability factor S for collector to base bias
circuit:
VCC
= IC RC – IB(RB+RC) + VBE
When ICBO,
IB and IC changes with no effect on VCC and VBE,
the equation becomes,
S = [1+β ] /1+β (RC/ (RC+RB))
Collector
to base bias circuit is having lesser stability factor than for fixed bias
circuit. So this circuit provides better stability than fixed bias circuit.
Problem 1:
Locate
the operating point of the given circuit with VCC = 15V, hfe
= 200.
Solution:
IBQ
= [VCC – VBE] / [RB+ (1+β) (RC+RE)]
= 15-0.7
/ 630*103 + (1+200) (4.7*103+680)
ICQ
= β IBQ = 200*8.356*10-6 = 1.6712mA
IEQ
= ICQ + IBQ = 1.6712*10-3 + 8.356*10-6
= 1.68mA
VCEQ =
VCC – IE (RC+RE)
= 15-1.68*10-3
(4.7*103 + 680)
5.96V
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