A d.c. and a.c. balance detector with automatic protection from overload
Charles Morton · Journal of Scientific Instruments · 1955
By using two junction transistors in a balanced circuit having a current gain of 50 and power consumption of 13 mW, the sensitive galvanometer normally employed as a d.c. null point detector may be replaced by a robust and inexpensive microammeter. The amplifier is so designed that, when an e.m.f. exceeding ±150 mV is applied to its input terminals, the differential input resistance approaches infinity, thus protecting both the transistors and the meter from injury. In order to use the circuit as an amplifying, phase-selective, a.c. balance detector, an auxiliary e.m.f. of about 4 V, of the same frequency as the bridge voltage, is applied to the transistors; a voltage sensitivity of 15 mA/V (r.m.s.), with automatic protection from overload, is then obtainable.