Electronics

Question 1

Name the 3 components of an electronic sensor.

Answer 1

1) sensing device
2) processing unit
3) output device

Question 2

What is the function of the sensing device in an electronic sensor?

Answer 2

Sensing devices are used to detect changes in the environment or the properties of a body.
The sensing device changes one of its physical properties (e.g. resistance) with a change in whatever is to be monitored.

Question 3

List 4 sensing devices.

Answer 3

1) Light-dependent resistor (LDR)
2) Thermistor
3) Piezo-electric transducer
4) Strain gauge

Question 4

State the property of a light-dependent resistor (LDR).

Answer 4

The resistance of an LDR decreases as the light intensity increases

Question 5

State the property of a thermistor.

Answer 5

The resistance of a thermistor decreases as temperature increases.

Question 6

State the property of a piezo-electric transducer.

Answer 6

A piezo-electric transducer converts variations in pressure (such as due to a sound wave) into variations in voltage.

Question 7

State the property of a strain gauge.

Answer 7

The strain on a metal wire strain gauge is proportional to its change in resistance.

Question 8

State the 5 properties of an ideal op-amp.

Answer 8

1) infinite input impedance (i.e. no current enters or leaves either of the inputs)
2) infinite open-loop gain (i.e. if there is only a very slight difference between the input voltages, the
output will be saturated - the output will have the same value as the supply voltage); 3) zero output impedance (i.e. the whole of the output voltage is provided across the output load)
4) infinite bandwidth (i.e. all frequencies are amplified by the same factor)
5) infinite slew rate (i.e. there is no delay between changes in the input and consequent changes in
the output).

Question 9

What is feedback?

Answer 9

The process of taking a fraction of the output of the amplifier and adding it to the input.

Question 10

What is negative feedback?

Answer 10

A fraction of the output is combined with the input and subtracted from the input.
Negative feedback is achieved by feeding a fraction of the output back to the inverting input.

Question 11

State the effects of negative feedback.

Answer 11

1) Reduces the gain of the op-amp.
2) Increased bandwidth.
3) Greater operating stability.
4) Less distortion.

Question 12

How does a simple microphone work?

Answer 12

A sound wave consists of a series of compressions and rarefactions. When the wave is incident on a piezo-electric crystal, a varying voltage is produced across the crystal. The voltage is amplified by the amplifier.

Question 13

How does a strain gauge work?

Answer 13

A strain gauge is made by sealing a length of very fine wire in a small rectangle of thin plastic.
When the plastic is stretched, the wire will also be stretched.
This causes the wire’s length to increase and its cross-sectional area to decrease very slightly.
As a result, the resistance of the wire increases.

Question 14

The output voltage Vout of the op-amp is proportional to ___________.

Answer 14

the difference between the voltage V+ at the non-inverting input and the voltage V – at the inverting input.

Question 15

Gain of an amplifier =

Answer 15

output voltage/input voltage

Question 16

Explain the function of a comparator circuit.

Answer 16

A comparator compares the voltages at the inverting input and non-inverting inputs.
The polarity of the output depends on which input is larger.
If V+ > V -, then the output voltage Vout is positive.
If V - > V+ , then Vout is negative.

Question 17

When is an op-amp saturated?

What happens when an op-amp is saturated?

Answer 17

When the two inputs are not almost the same, the output voltage will be larger than the power supply voltage. The op-amp is saturated and so, the output will be equal to the power supply voltage.

Question 18

Inverting amplifier:

The input voltage is connected to the _________ input,

Answer 18

inverting input.

Question 19

Inverting amplifier:

Is the output voltage in phase or out of phase with the input voltage?

Answer 19

Output is 180 degrees out of phase with the input.

Question 20

In the circuit diagram of an inverting amplifier,
there is a point known as virtual earth.
Explain why it is known as virtual earth.

Answer 20

The non-inverting input is connected to earth, so the non-inverting input is at 0 V.
The gain of an ideal op-amp is infinite.
To avoid saturation, the two input voltages must be nearly equal.
Hence, the inverting input must be virtually at zero volts.
Point P is known as virtual earth.

Question 21

Derive an expression for the gain of inverting amplifier.

Answer 21

Check notes.

Question 22

Non-inverting amplifier:

The input signal is applied to the ________ input.

Answer 22

non-inverting input

Question 23

Non-inverting amplifier:

Is the output voltage in phase or out of phase with the input voltage?

Answer 23

In phase.

Question 24

Formula for the gain of a non-inverting amplifier.

Answer 24

Check notes.

Question 25

Formula for the gain of an inverting amplifier.

Answer 25

Check notes.

Question 26

What is the function of a relay?

Answer 26

A relay is an electromagnetic switch that uses a small current to switch on or off a larger current.

Question 27

Describe the structure of a metal-wire strain gauge.

Answer 27

Check notes.