eee103_2005_exam

DEPARTMENT OF ELECTRONIC AND ELECTRICAL ENGINEERING

Spring Semester 2005 (2 hours)

EEE 103ANALOGUE CIRCUITS 1

Answer THREE questions. Solutions will be considered in the order in which they arepresented in the answer book and no marks will be awarded for an attempt at a fourthquestion. Trial answers will be ignored if they are clearly crossed out. The numbers givenafter each section of a question indicate the relative weighting of that section.

1(i)

Sketch the waveshape of the outputvoltage, VO, for the circuit of figure1 assuming that the load draws aconstant current, IL. Label yoursketch with amplitudes and timesexpressed in terms consistent withthe labels in figure 1. {4}

240V50Hz

VS

C

IL

VO

LOAD

(ii)

Using a suitable model, estimate a

Figure 1value of C that will give a ripple

voltage of less than 1V peak to peak

at a load current of 150mA. Make sure that you state any assumptions or approximationsthat you use. {5}

(iii)What value of VS would be appropriate for a nominal d.c. output voltage of 40V? {2}A second supply of 15V is to be derived from the 40V supply above by using a simple zener dioderegulator circuit consisting of a resistor and zener diode. The load for this second supply maydraw a current anywhere in the range 5mA to 25mA and the zener diode must be biased such thatthe current through it never falls below 5mA.

(iv)Draw a circuit diagram to show how the resistor and zener diode could be added to figure 1

to realise the 15V supply. {2}(v)

What is the maximum value of resistor that can be used in the zener diode regulator circuitif the main supply output voltage is assumed constant at 40V? {4}

(vi)If the zener diode has a slope resistance of 6Ω, estimate the ripple voltage at the output of

the 15V supply. {3}

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TURN OVER

2(a)

The diode in figure 2a has a forward voltagedrop of 0.7V. Determine whether the diode willbe conducting and work out the reverse biasvoltage or the forward conduction current asappropriate. {7}(i)

3kΩD

VA

1kΩ1kΩ

VK

2mA

+

16V

−(b)

(ii)

The circuit of figure 2b initially consists

of R1 and C only, that is, D is initially replaced by a piece of wire and R2 is notconnected. Sketch VO in response to the input pulse shown. If R1 = 3kΩ and C =2nF, what is the voltage reached by VO at the end of the input pulse? Calculate thetime for which VO > 5V. (Remember to think carefully about what will happen to thevoltages across the circuit elements at the instant of transient changes of VI) {6}An R2 of 2kΩ and an ideal

R1VIDdiode, D, are now included

to give the circuit of figure10V10µsCR2VOVI2b. Sketch VO and IC over a

IC0Vtime period long enough to

show all the importantaspects of the circuit’sFigure 2bresponse to VI. Label any

exponential shapes with their time constants and label all peak values in both sketches.Assume that D has a forward voltage drop of 0V. {7}

Figure 2a

3(i)

Redraw the circuit of figure 3 to include only thetransistor and those components involved incontrolling the bias conditions. {3}

Calculate the d.c. conditions VB, VC, VE and ICtogether with the small signal parameters gm andrbe. {6}

signalsource1kΩRS

24V

R191kΩC1

VBT1

R251kΩ

RE4.7kΩ

C2

0V

RLIC

VC

VE4.7kΩC3

(ii)

vo

(iii)Draw a small signal equivalent circuit of figure

3. {4}(iv)Work out the small signal voltage gain, vo/vs, of

the circuit of figure 3. {4} (v)

Which single component, if removed, will leavethe amplifier with a gain, vo/vs of approximately-1? You should briefly justify your answer. {3}

vs

Figure 3

You should assume that a forward biassed pn junction has 0.7V across its terminals and that kT/e

eI

= 0.026V. The transistor has a small signal current gain, β, of 400. The relationships gm = C

kT

β

and rbe = may be assumed without statement but all other assumptions and approximations

gm

used in your solution must be stated and, where appropriate, justified.

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4(a)

Draw a circuit diagram of a practically useful operational amplifier based invertingamplifier. {3}

(ii)What is meant by the term \"inverting\" used in part (i)? {1}

(iii)Explain briefly what is meant by the term \"virtual earth\". Point out the virtual earth

node in the circuit of part (i) and give the reason for virtual earth behaviour. {4}(iv)What relative values of resistors would you use in your circuit if a voltage gain of

-10V/V was required? {2}

(v)What is the input resistance of your circuit? {1}

The following questions relate to the circuit of figure 4 which uses an ideal operationalamplifier.(i)(ii)

What is the gain vo/v1 if v2= v3= 0? {2}What is the gain vo/v2 if v1= v3= 0? {3}

(i)

(b)

(iii)What is the gain vo/vi if v1= v2= vi and v3 = 0? {4}

300kΩ

v1v2

150kΩR1

R210kΩ

R3

−RF

∞+10kΩ

vo

v3

Figure 4

4RCT/DASEND OF PAPER

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