C3 / F5 Electronics Student Work Book

C3 / F5 Electronics – Student Work Book

1).(a)What is an operational amplifier (op-amp)?

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(b)State the general properties of an ideal op-amp.

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(c)An op-amp has an open loop voltage gain of 106. If the supply voltage is ±15V, what is the maximum voltage difference between the two input terminals if the output is not to saturate.

2).(a)Explain what is meant by:-

(i)Voltage Gain

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(ii)Power Gain

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(b)An amplifier has a voltage gain of 25 and an input signal of amplitude 50mV.

Calculate the amplitude of the output voltage.

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3).(a)Explain how negative feedback is able to reduce the voltage gain of an op-amp amplifier system.

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(b)(i)Calculate the voltage gain of the amplifier in the circuit below.

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(ii)Label the Virtual Earth Point on the diagram with the letter P.

(iii)Explain why it is called a Virtual Earth Point.

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(iv)State the input resistance of the amplifier circuit.

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(c)State two ways in which the input and output signals of the amplifier are different.

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4).(a)(i)Calculate the value of the feedback resistor needed to give the amplifier in the circuit diagram below a voltage gain of 4.

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(ii)State the approximate input resistance of the amplifier circuit.

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(b)The diagram below shows the input signal for the amplifier.

Draw the output waveform onto the diagram

5).(a)Explain why an op-amp makes a good comparator

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(b)Summarise how the output voltage of an op-amp used as a comparator is related to its input voltages.

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(c)How does the resistance of an ntcthermistor vary with temperature?

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(d)(i)The circuit below is designed to warn the owner of an incubator if the temperature becomes too warm. The temperature is to be maintained at 25°C and at this temperature the thermistor has a resistance of 50k. A green LED is to light if the temperature is below 25°C and a red one if the temperature is any higher.

Calculate the value for R1.

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(ii)Label the red LED and explain your choice.

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For more examples of comparator questions go to

6).(a)Explain the difference between negative and positive feedback.

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(b)Calculate the value of the switching levels in the Schmitt trigger circuit shown below.

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(c)On the graph below, show how the output voltage varies as the input voltage goes from 0V to 12V.

(d)In a different colour, mark onto the graph above how the output voltage varies as the input voltage goes from +12V to 0V.

(e)Explain what is meant by the term hysteresis.

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7).(a)A digital signal passes through a transmission medium and suffers both attenuation and dispersion. Describe the effect each of these will have on the digital signal:-

(i)attenuation …………………………………………………………………....

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(ii)dispersion ………………………………………………………………......

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(b)The diagram below shows one pulse from the digital signal after it has been received.

What else has happened to the signal during its journey from transmitter to receiver?

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(c)Such a signal would generate significant errors if it were not processed correctly before being used. The options for processing are:-

(i)an amplifier,

(ii)a comparator or

(iii)a Schmitt trigger.

For each of these processors describe the effect on the received signal and then suggest which one should be used to minimize any errors in the received signal.

Use diagrams to assist your descriptions.

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C3 / F5 Electronics – Student Work Book – Responses

1).(a)An integrated circuit containing a voltage amplifier which amplifies the difference between the voltages on its two input terminals.

(b)Very large differential voltage gain (106)

Very large input resistance (109Ω)

Low output resistance (100Ω)

(c)

For the op-amp not to saturate, the output voltage must be less than 15V.

2).(a)(i)

(ii)

(b)

3).(a)With negative feedback a proportion of the output signal is 'fed-back' to the input so that it cancels out some of the input signal and reduces the apparent voltage gain of the whole circuit.

(b)(i)

(ii)The junction of the 10kΩ, 470kΩ resistors and the inverting input of the op-amp.

(iii)The non-inverting input is connected to 0V, usually at earth potential. For the op-amp output not to saturate, the inverting input can only be a few microvolts different in voltage to the non-inverting input, i.e. it is at virtually the same voltage as the non-inverting input. So therefore it is virtually at earth – a virtual earth point.

(iv)Since the inverting input is a virtual earth point, the input resistance of the amplifier circuit is 10kΩ.

(c)Amplified and Inverted

4).(a)(i)

(ii)The input resistance of the circuit is equal to the input resistance of the op-amp
i.e. approximately 109.

(b)

5).(a)It has a very large voltage gain so that unless the inputs are within a few microvolts of each other, the output will be saturated at the supply voltage.

(b)

(c)As the temperature increases the resistance decreases.

(d)(i)The reference voltage for the op-amp is 0V.

So op-amp will switch when V+ = V–

Therefore since the thermistor and R1 form a voltage divider, R1 must have the same resistance as the thermistor = 50k.

(ii)The top LED is the red one and the bottom one the green. When the temperature increases, the resistance of the thermistor decreases, so V+ > V– and the output of the op-amp will go positive, so turning on the top LED.

6).(a)Negative feedback is when some of the output signal from a processor is added to the input signal so that the value of the input signal is reduced. This has the effect of reducing the amplification of the processing system and allowing the output to achieve a steady value.

Positive feedback is when the output signal from a processor is added to the input signal so that the value of the input signal is increased. This has the effect of increasing the amplification of the processing system forcing the output to settle at one of its saturation values.

(b)When the output of the op-amp is at 0V, the two 20k resistors, R2 and R, are in parallel, giving a combined resistance of 10k. This forms a voltage divider with R1 across the 12V supply. Since the two resistances of the voltage divider are equal, the voltage will be split equally, i.e., 6V across each resistor. Therefore the voltage at the non-inverting input of the op-amp is 6V, which is the lower switching level.

When the output of the op-amp is at 12V, the 20k feedback resistor, R, is in parallel with R1 giving a combined value of 6.67k. This now forms a voltage divider with R2 across the 12V supply.

So the upper switching level is 9V.

(c)

(d)As above in red.

(e)When the voltage level needed to make the output go high is different from the voltage level needed to make it go low.

7).(a)(i)amplitude is reduced, signal becomes weaker owing to resistive losses

(ii)signal becomes spread out in time owing to high frequency signals traveling at a different speed to low frequency signals.

(b)The signal has gained noise and interference components.

(c)

The Schmitt trigger, since it restores the logic levels and eliminated noise and false pulses.