tutorial 3 - part ii - answers
DESCRIPTION
Tutorial 3 - Part II - AnswersTRANSCRIPT
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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