Example-2 (Emitter-Resistor Amplifier Design)

Design an emitter-resistor amplifier as shown in fig. 2 to drive a 2 KΩ load using a pnp silicon transistor, V_CC = -24V, β = 200, A_v = -10, and V_BE = -0.7 V. Determine all element values and calculate A_i, R_in, I_CQ and the maximum undistorted symmetrical output voltage swing for three values of R_C as given below:

1. R_C = R_load
2. R_C = 0.1 R_load
3. R_C = 10 R_load

Fig. 2

Solution:

(a) R_C = R_load

We use the various equations derived in previous lecture in order to derive the parameters of the circuit.

From the voltage gain, we can solve for R'_E.

So R'_E = r_e + R_E = 100 Ω

We can find the quiescent value of the collector current I_C form the collector-emitter loop using the equation for the condition of maximum output swing.

Therefore,

This is small enough that we shall ignore it to find that R_E = 100 Ω. Since we now know β and R_E. We can use the design guideline.

R_B = 0.1 β R_E = 2 k Ω

As designed earlier, the biasing circuitry can be designed in the same manner and given by

V_BB = -1.52 V

R₁ = 2.14 K Ω

R₂ = 3.6 K Ω

The maximum undistorted symmetrical peak to peak output swing is then

V_out (P-P) = 1.8 I_CQ (R_load || R_C ) = 13.5 V

Thus current gain A_i = -9.1

and input impedance R_in = 1.82 K Ω

(b)   R_C = 0.1 R_load

we repeat the steps of parts (a) to find

RC =200 Ω	Ri = 390 Ω
ICQ =-57.4 mA	R2 =4.7K Ω
r'e = 0.45 Ω	vout(p-p) = 18.7 V
RB = 360 Ω	Ai = -1.64
VBB = -1.84 V	Rin = 327 Ω

     (C)     R_C =10 R_load

Once again, we follow the steps of part (a) to find

RC =20 K Ω	R1 =3.28K Ω
ICQ =-1.07 mA	R2 = 85.6K Ω
r'e = 24.2 Ω	vout(p-p) = 3.9 V
RB = 3.64K Ω	Ai =-14.5
VBB = -0.886 V	Rin = 2.91K W

We now compare the results obtained Table-I for the purpose of making the best choice for R_C.

	ICQ	Ai	Rin	vout(p-p)
RC = Rload	-7.5 mA	-9.1	1.82K W	13.5 V
RC = 0.1 Rload	-57.4 mA	-1.64	327 W	20.8 V
RC = 10 Rload	-1.07mA	-14.5	2.91W	3.9 V

Table - 1 Comparsion for the three selections of R_C

It indicates that of the three given ratios of R_C to R_load, R_C = R_load has the most desirable performance in the CE amplifier stage.

It can be used as a guide to develop a reasonable designs. In most cases, this choice will provide performance that meets specifications. In some applications, it may be necessary to do additional analysis to find the optimum ratio of R_C to R_load.

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