A transistor high-gain chopper-type d.c. amplifier

In transistor d.c. amplifiers, the sensitivity is mainly limited by drift of operating point caused by changes in ambient temperature. A modulated amplifier is necessary to obtain a high sensitivity without recourse to methods of drift compensation requiring the adjustment of several balancing controls. The paper describes a modulated system consisting of a transistor input chopper, a high-gain transistor a.c. amplifier, and a transistor output chopper. Gains are expressed throughout as transfer impedances (output-voltage/input-current), a convention which is well suited to transistor amplifiers. The a.c. amplifier uses four transistors and has a gain of 20 volts/¿A over a band covering 60c/s¿20kc/s. It contains one capacitor only, giving a single low-frequency time-constant, which simplifies stability problems when feedback is applied. A chopping frequency of 1·6kc/s is used, and the complete system gives an open-loop gain of 50 volts/¿A with a bandwidth extending from direct current to 25 c/s. The peak output is ± 10 volts, and the current drift referred to the input is 4 × 10¿9 amp in the range 20° C¿50° C. The voltage drift at the input is less than 100 ¿V. Despite the low input impedance the virtual-earth principle is still valid, and the amplifier should be useful in analogue-computer applications. The complete amplifier is suitable for amplification of signals from a wide range of source impedances¿ionization chambers and thermocouples being typical extremes.