Emitter Bias

Posted by AJ × 01:20

• It can be shown that, including an emitter resistor in the fixed bias circuit

improves the stability of Q point.

• Thus emitter bias is a biasing circuit very similar to fixed bias circuit with an

emitter resistor added to it.

Input loop

• Writing KVL around the input loop we get,

VCC = IBRB + VBE + IERE (1)

We know that,

IE = (β+1)IB (2)

Substituting this in (1), we get,

VCC = IBRB + VBE + (β+1)IBRE

VCC – VBE = IB(RB + (β+1) RE)

Solving for IB:

IB = (VCC – VBE ) /[(RB + (β+1) RE)]

The expression for IB in a fixed bias circuit was,

IB = (VCC – VBE ) /RB

Equivalent input loop:

• REI in the above circuit is (β+1)RE which means that, the emitter resistance that is

common to both the loops appears as such a high resistance in the input loop.

• Thus Ri = (β+1)RE ( more about this when we take up ac analysis)

Output loop

Collector – emitter loop

Applying KVL,

VCC = ICRC + VCE + IERE

IC is almost same as IE

Thus,

VCC = ICRC + VCE + ICRE

= IC (RC + RE) +VCE

VCE = VCC - IC (RC + RE)

Since emitter is not connected directly to ground, it is at a potential VE, given by,

VE = IERE

VC = VCE + VE OR VC = VCC – ICRC

Also, VB = VCC – IBRB OR VB = VBE + VE

All Syllabus