Wire up this circuit again using 100 k for R f and 10 k for R i Measure the

# Wire up this circuit again using 100 k for r f and 10

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! Wire up this circuit , again using 100 k ! for R f and 10 k ! for R i . ! Measure the gain, and compare with the predicted voltage gain, which is:
Laboratory exercise 10 10–10 A = v out v in = 1 + R f R i This can again be deduced from the ‘golden rules’. Here, v + = v " = v in , and v ! comes from what is effectively a voltage divider: v " = v out R i R i + R f Simple substitution gives the gain of the amplifier. As with the inverting amplifier, we see that if the op-amp behaves ideally then the gain depends only on the external resistor values and not on the properties of the op-amp chip itself. The positive value means that there is no inversion, and the absolute value of the gain is higher than that of the inverting amplifier by 1. Part D: A simple digital-to-analogue converter Introduction The world around us is full of analogue signals. The availability of sophisticated but cheap digital logic and microprocessors means that it is also essential to have converters, both to digitise analogue signals for processing, and to turn digital results back into analogue. In this part you will first use an op-amp to sum input signals, and then study an electronically controlled switch. Finally, these will be put together to build a simple 4-bit digital-to-analogue converter (DAC) in order to understand some of the basic principles. The DAC constructed in this exercise works by summing currents, each of whose magnitude is proportional to the binary bit that it represents. The binary number is formed by using an electronically-controlled switch for each current, so that each binary bit can be turned on or off. To show this circuit in action, the binary number is taken from a counter so that it varies in a simple and easily understood manner. However, this DAC has very limited performance. For real applications demanding higher precision and speed it is far better to buy ready-made integrated converter chips than to try to design your own circuits. Very high-performance conversion is not easy, and prices go up rapidly as the number of bits and/or the speed increase. For example, the 16-bit DACs used to play CDs were initially state-of-the-art but have now become both more sophisticated and very cheap. Summing amplifier This circuit is a variation on the inverting amplifier of part B, and is shown in figure 11. Since no current flows into the op-amp input (golden rule), the currents in the two input resistors R 1 and R 2 are summed in the feedback loop resistor R f : i 1 + i 2 = i f or in terms of voltage: v 1 R 1 + v 2 R 2 = " v out R f If R 1 = R 2 = R f , then v out = " v 1 + v 2 ( ) .
10–11 Laboratory exercise 10 This means that, apart from the sign, the output depends on the sum of the inputs. ! Build the summing circuit , using ±12 V supplies for the op-amp. The choice of resistance is not critical — 10 k ! is suitable for all three resistors.

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• Summer '09
• Electrical resistance, Electrical network, Voltage drop, Thermometer, Geiger tube

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