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Modified collector to base bias circuit

To improve the level of stability, emitter resistance is connected in this circuit.

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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