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Analysis of 4 Resistor Bias Circuit

VB = VTH

=

Vcc R 2 R1 + R 2

R B = RTH

=

R1 R 2 R1 + R 2

25

Analysis of 4 Resistor Bias Circuit (2)

Applying KVL to the base-emitter circuit of the Thevenized

Equivalent form VB - IB RB -VBE - IE RE = 0 (1)

Since IE = IB + IC = IB + βIB= (1+ β)IB

(2)

Replacing IE by (1+ β)IB in (1), we get

VB VBE IB = RB + (1+ β)RE

(3)

If we design (1+ β)RE >> RB (say (1+ β)RE >> 100RB)

Then

IB

VB VBE (1+ β)R E

(4)

26

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