Page 413 - Physics Coursebook 2015 (A level)

Chapter 25: Electronics
Summary
■■ A sensor contains a sensing device and a processor to produce an output voltage.
■■ The resistance of an LDR decreases as the level of the light intensity increases.
■■ The resistance of a negative temperature coefficient thermistor falls as temperature rises.
■■ Thermistors and light-dependent resistors can be used in potential divider circuits to provide output voltages that are dependent on temperature and light intensity, respectively.
■■ A piezo-electric transducer produces a variation in output voltage when a crystal is stressed, for example by pressure changes in sound.
■■ The change in resistance of a metal-wire strain gauge is proportional to extension.
■■ An operational amplifier used as a comparator produces either a high positive or a high negative voltage output, depending on which of two input voltage levels is larger.
■■ An operational amplifier can be used as an inverting or a non-inverting amplifier to amplify voltage signals.
■■ The gain of an inverting amplifier is − Rf Rin
■■ The gain of a non-inverting amplifier is 1 + R1 . R2
■■ Feedback reduces gain but increases bandwidth and stability and reduces distortion.
■■ Output devices which may be connected to an op-amp include relays, LDRs and calibrated meters.
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End-of-chapter questions
1 How can the circuit of Figure 25.17 be altered so that the op-amp switches from +9 V to −9 V when the
temperature rises, instead of when it falls? Draw the circuit diagram. [1]
2 Draw the circuit diagram of a comparator that switches the output voltage from positive to negative
when the light intensity falling on an LDR decreases. [3]
3 Explain what is meant by:
a feedback [1]
b the virtual earth approximation. [2]
4 In the inverting amplifier shown in Figure 25.20, Rin = 10 kΩ, Rf = 200 kΩ and Vs = 12 V. Calculate:
a the gain of the amplifier [2]
b the input voltage when the output voltage is 8.0 V [2]
c the maximum input voltage. [1]
5 a
b Explain why the gain of a non-inverting amplifier cannot be less than 1. [2]
Explain how an inverting amplifier can have a gain of −0.5. [2]