OP-AMP CIRCUITS
Question 1. FIGURE 1 shows two amplifier circuits, each of which has one unknown voltage (shown highlighted). Select, from TABLE A, the most appropriate value for the unknown voltages for each circuit. Assume that the op-amps are ideal and that the magnitude of their output voltages is less than their maximum peak output voltage swing (VOM).
|
- 1 mV
|
+
|
1 mV
|
-
|
1 V
|
+
|
1 V
|
|
- 2 mV
|
+
|
2 mV
|
-
|
2 V
|
+
|
2 V
|
|
- 3 mV
|
+
|
3 mV
|
-
|
3 V
|
+
|
3 V
|
|
- 4 mV
|
+
|
4 mV
|
-
|
4 V
|
+
|
4 V
|
|
- 5 mV
|
+
|
5 mV
|
-
|
5 V
|
+
|
5 V
|
|
- 6 mV
|
+
|
6 mV
|
-
|
6 V
|
+
|
6 V
|
|
- 7 mV
|
+
|
7 mV
|
-
|
7 V
|
+
|
7 V
|
|
- 8 mV
|
+
|
8 mV
|
-
|
8 V
|
+
|
8 V
|
|
- 9 mV
|
+
|
9 mV
|
-
|
9 V
|
+
|
9 V
|
|
- 10 mV
|
+
|
10 mV
|
-
|
10 V
|
+
|
10 V
|
|
- 100 mV
|
+
|
100 mV
|
-
|
100 V
|
+
|
100 V
|
|
- 500 mV
|
+
|
500 mV
|
-
|
300 V
|
+
|
300 V
|
TABLE A
Fig 1.
Question 2. A 12-bit DAC (Digital to Analogue Converter) gives an output range of 0 to -1 mA for a digital input word of 0 to 4095. The required output, however, is for a bipolar voltage range of ±5 V.
(a) Design a current-to-voltage converter circuit to achieve the required bipolar output voltage.
(b) Test your design on PSpice and produce suitable graphical output to demonstrate the circuit's operation.
Question 3. FIGURE 2(a) shows a ‘black box' that is to partially synthesise a ‘saw- tooth' waveform of FIGURE 2(b) by using the Fourier series algorithm:
v(t) = 2/Π (sin(ωt) + 1/2sin(2ωt) + 1/3sin(3ωt) + 1/4sin(4ωt)+.... )
(a) Design an operational amplifier circuit to perform the function of the ‘black box'.
(b) Test your design on PSpice and produce a graph of the output waveform.
Fig.2(a)
Fig2(b)
Question 4. FIGURE 3 (on page 7) shows a PIR (passive infra-red) detector and its associated amplifier, as used in burglar alarm systems1.
(a) The detector is powered from a 12 V unregulated supply that needs to be stepped down to 5 V. Design a suitable 12-to-5 V voltage converter using an op-amp and a diode that has the forward characteristics given in FIGURE 3(b).
(b) Estimate the quiescent voltages at the inputs and outputs of the two op-amps and the overall voltage gain (in decibels) of the circuit at the frequency of operation. State any assumptions made.
Fig. 3(a)
Fig. 3(b)