Modified collector to base bias circuit:
To improve the level of stability, emitter resistance is connected in this 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.
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
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.
Locate the operating point of the given circuit with VCC = 15V, hfe = 200.
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)