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Design IdeasMarch 3, 1997 |
W Dijkstra, Waalre, The Netherlands
You can use a lowpass filter to simultaneously increase a rectifier's speed and reduce or maintain the ripple voltage. The input voltage of the precision rectifier in Figure 1a is 100 mV at a frequency of 50 Hz. The output with C1 equal to 0 is a double phase-rectified voltage of R2/R1 times the top value of the input voltage. With R2=33 k(omega) and R1=22 kiliohms, the result is 150 mV. If C1=560 nF, the amplitude has an average value of 95 mV, and the ripple is 10 mV p-p. The circuit reaches this output within 80 msec, but the ripple is fairly high. You can increase the value of C1--to 22 µF, for example--which reduces the ripple to 0.3 mV p-p, but the circuit now takes 5 sec to reach the output.
When you feed the output of the rectifier to a lowpass filter (Figure 1b) you can reduce the ripple and increase the rectifier's speed. The filter has a cutoff frequency of 7 Hz and unity gain. Again decreasing the value of C1 in Figure 1a to 560 nF, the ripple voltage at the output of Figure 1b is 0.2 mV p-p, and the output is stable within 120 msec. So, the rectifier is about 40 times faster with approximately the same ripple-voltage level. (DI #1996)
| Figure 1 |
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| Feeding the output of a precision rectifier (a) to a lowpass filter (b) simultaneously keeps the ripple voltage around 0.2 mV p-p and increases the rectifier's speed. The output of the lowpass-filtered rectifier is stable within 120 msec. |
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