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Adaptive AC Null Detector Disclosure Number: IPCOM000073474D
Original Publication Date: 1970-Dec-01
Included in the Prior Art Database: 2005-Feb-22
Document File: 2 page(s) / 31K

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Related People

Levine, JL: AUTHOR


This circuit relates to a narrow-band amplifier advantageously employed as a null detector.

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Adaptive AC Null Detector

This circuit relates to a narrow-band amplifier advantageously employed as a null detector.

Twin-T (narrow-band) amplifiers are used as null detectors in AC bridges to reduce the noise bandwidth. In such use, they are frequently followed by some form of compression circuit to provide a useful indication far from balance. If a very narrow bandwidth is required for maximum sensitivity close to null, the response time of the amplifier will be long. While such long response time may be unavoidable close to balance, the long response time adds undesirably to the balancing time at a point which is far from null. This difficulty is overcome with the circuit shown in A. There is combined in this circuit both tuning and compression.

In this circuit, the elements therein, other than the voltage divider RO and the back-to-back diodes D1 and D2 constitute a simple twin-T network. Ignoring the diodes, the gain at the twin-T null frequency f(o) is given by: G = -R(2)/R(1) (1).

Using the notation shown in B, the bandwidth and response time are given by: BW = 2f(0)(R/R(2) Square root of 2+K Hz (2) tau = 1/tau BW = R(2)/(2 tau f(0) R square root of 2+K) sec. (3)

For a suitable choice of diode and voltage divider ratio, the amplifier shown in A behaves according to equations 1-3 for sufficiently small input voltages, i.e., linear and sharply tuned. For larger voltages, the gain is reduced and the response time is decreased due to the shunting of R(2) by t...