Modified collector to base bias 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,

V_CC – (I_C + I_B ) RC – I_BR_B – V_BE – I_ER_E = 0

I_B = [V_CC – V_BE ] / [R_B + (1+β) (R_C + R_E)]

I_B =    [V_CC – V_BE]/ [R_B + β (R_C + R_E)]

Only difference between the equation for I_B and that obtained for the fixed bias configuration is the term β (R_C + R_E).So feedback path results in a reflection of the resistance R_C back to the input circuit.

In general,

I_B = V’ / R_B + β R’

Where V’ = V_CC - V_BE

R’ = 0 for fixed bias

R’ = R_E for emitter bias

R’ = R_C for collector to base bias

R’ = R_C + R_E for collector to base bias with R_E

Collector circuit:

Applying KVL to collector circuit,

V_CC – (I_C+I_B) RC – V_CE – I_ER_E = 0

V_CE = V_CC – I _E (R_C+R_E)

Stability factor S for collector to base bias circuit:

V_CC = I_C RC – I_B(R_B+R_C) + V_BE

When I_CBO, I_B and I_C changes with no effect on VCC and V_BE, the equation becomes,

S =       [1+β ] /1+β (R_C/ (R_C+R_B))

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 V_CC = 15V, h_fe = 200.

Solution:

I_BQ = [V_CC – V_BE] / [R_B+ (1+β) (R_C+R_E)]

= 15-0.7 / 630*10³ + (1+200) (4.7*10³+680)

I_CQ = β I_BQ = 200*8.356*10^-6 = 1.6712mA

I_EQ = I_CQ + I_BQ = 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