8 (a) Negative feedback is often used in amplifiers incorporating an operational amplifier (op-amp). State (i) what is meant by negative feedback, (ii) two effects of negative feedback on the gain of an amplifier. 1. 2. (b) An ideal op-amp is incorporated into the amplifier circuit shown in Fig. 8.1. [Figure 8.1] (i) Calculate the gain G of the amplifier circuit. G = .............................................. (ii) Determine the output potential difference VOUT for input potential differences VIN of 1. -0.10 V, VOUT = .............................................. V 2. +1.3 V. VOUT = .............................................. V (iii) The gain of the amplifier shown in Fig. 8.1 is constant. State one change that can be made to the circuit of Fig. 8.1 so that the amplifier circuit monitors light intensity levels, with the magnitude of the gain decreasing as light intensity increases.
- (a)(i)what is meant by negative feedback,[2]
- (a)(ii)two effects of negative feedback on the gain of an amplifier. 1. 2.[2]
- (b)(i)Calculate the gain G of the amplifier circuit. G = ..............................................[2]
- (b)(ii)1.-0.10 V, VOUT = .............................................. V[1]
- (b)(ii)2.+1.3 V. VOUT = .............................................. V[1]
- (b)(iii)State one change that can be made to the circuit of Fig. 8.1 so that the amplifier circuit monitors light intensity levels, with the magnitude of the gain decreasing as light intensity increases.[1]
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Mark scheme
Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.
- [M1] (fraction of) output is combined with the input
- [A1] output (fraction) subtracted/deducted from input
- [B2] any two valid points e.g.: greater bandwidth/gain constant over a larger range of frequencies/greater bandwidth / smaller gain
- [C1] gain = (-)9600/800
- [A1] = -12
- [B1] 1.2V
- [B1] -6V
- [B1] replace the 9600 Ω resistor with an LDR
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