Transistor biasing

Emitter Bias

The analysis of base biased circuits depends on the current gain which can vary in a wide range. In an amplifier we need circuits whose Q-points are immune to changes in current gain. The solution for this problem is the emitter biased circuit:

Fig. 5-1: Emitter biased circuit, b = 100

Find the Q-point:

Given : V_BB = 5V, V_BE = 0.7V, V_CE = 15V

R_E = 2.2KW, R_C = 1KW

Calculation:

V_E = V_BB - V_BE = 5V - 0.7V = 4.3V

I_E = I_C (close approximation)

V_C = VCC - (RC * IC)

= 15V - (1KW - 1.95 mA)

= 13.1V

V_CE = V_C - V_E = 13.1V - 4.3V = 8.8V

Q-point coordinates:

I_C = 1.95 mA

V_CE = 8.8V

An emitter biased circuit is immune to changes in current gain. Analysing summary:

1. get V_E
2. calculate IE
3. find V_C
4. V_CE = V_C - V_E

At no time we need the current gain!

Tip for troubleshooter:

Don't measure direct V_CE, because the common lead of the voltmeter is grounded, so you will short the emitter to ground.

1. Measure V_C
2. Measure V_E
3. Subtract V_CE = V_C - V_E

HO: What is the collector voltage?

Fig. 5-2: Emitter biased CE connection

Solution:

V_E = V_BB - V_BE = 2V - 0.7V = 1.3V

I_E = I_C (approx.)

V_C = V_CC - (R_C * I_C) = 10V - (910 W * 7.2 mA) = 3.4V

HO: What is the collector-emitter voltage?

Fig. 5-3: Emitter biased CE connection

Solution:

V_E = V_BB - V_BE = 1.1V

I_E = I_C (approx.)

V_C = V_CC - (R_C * I_C) = 9.4V

V_CE = V_C - V_E = 8.35V

Effect of Small Changes

For example, tolerances of resistors (+/- 10%) are small changes.

See Fig. 5-4 on the next page.

Fig. 5-4: Emitter biased CE connection

Before we can analyse the effects of small changes we have to find out which values are dependent or independent.

independent values: V_BB, V_CC, b_dc, R_E, R_C
dependent values : V_E , V_C, I_B, I_C, I_E

Suppose the independent values will increase one after another. What will be the effect on the dependent values:

increase ¯		dependent
		VE	IE	IB	IC	VC	VCE
	VBB	U	U	U	U	D	D
	VCC	N	N	N	N	U	U
independent	RE	N	D	D	D	U	U
	RC	N	N	N	N	D	D
	hfe	N	N	D	N	N	N

U = up
D = down
N = no change