TUTORIAL 3: MULTISTAGE BJT AMPLIFIERS - Additional

TUTORIAL 3: MULTISTAGE BJT AMPLIFIERS - AdditionalCASCADE CONFIGURATIONQUESTION

3.Figure 3 shows an amplifier in cascode configuration where a common-base stage (Q2) is being driven by a common-emitter stage (Q1).

(a)Draw the circuit under dc condition and determine the quiescent currents ( and ) of both transistors.

(b)Draw the small-signal equivalent circuit of the complete amplifier and perform an ac analysis to show that the approximate overall small-signal voltage gain is given by the expression;

(c)If the peak-to-peak value of the input voltage is 30 mV, determine the approximate peak-to-peak value of the output voltage .

Assume , , , and .

SOLUTION3.(a)The circuit under dc condition and

quiescent currents.

The currents are as labeled in the figure;

Taking the VCCR1R2R3GROUND loop gives us;

Substituting values;

Rearranging;

Taking the R3B1 E1REGROUND loop;

Or;

Substituting values;

Rearranging;

Substituting (2) in (1);

Answer

Answer(b)The small-signal equivalent circuit and voltage gain

At node E2;

Thus;

Substituting for in (3)

Where;

and

When

(c)The approximate peak-to-peak value of the output voltage.

AnswerQUESTION

4.Determine the input impedance and output impedance of the two-stage amplifier shown in Figure 4.

Assume , , , , for both transistors is 0.7 V and .

SOLUTION4.

Taking the base-emitter loop of Q1;

Or;

Substituting values;

For the - collector of - base of - loop;

and

Substituting values;

The input resistance of the amplifier is;

The output resistance of the amplifier is;

Figure 3

Figure 4

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