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Analysis of 4 Resistor Bias Circuit (3)

And IC = IE

VB VBE RE

(for large β) (5)

Hence IC and IE become independent of β!

Thus we can setup a Q-point independent of β which tends to vary widely even within transistors of identical part number (For example, β of 2N2222A, a NPN BJT can vary between 75 and 325 for IC = 1 mA and VCE = 10V)

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4 Resistor Bias Circuit -Example

A 2N2222A is connected as shown with R1 = 6.8 kΩ, R2 = 1 kΩ, RC = 3.3 kΩ,

V R BE = 0.7V. E = 1 k Ω a n d V C C

= 30V. Assume

Compute VCC

and IC for β = i)100

and ii) 300

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