Abstract :
The sensitivity of a d.c. amplifier using transistors is mainly limited by the variation with temperature of the inter-electrode potentials and currents of the transistors, the variation between any two temperatures being termed the thermal drift. A typical low-frequency germanium transistor connected as a directly-coupled common-emitter amplifier may introduce current and voltage drifts, referred to the input, of 50 ?A and 100 mV, respectively, when the temperature changes from 20 to 500?C. By using two such transistors in a balanced circuit, in which the drifts oppose, the net drifts are reduced to about 1 ?A and 2 mV, respectively, if several interdependent component values are adjusted at different temperatures. By selecting transistors, and using greater care in balancing the amplifier, the drift can be reduced by one or two orders of magnitude, but such a procedure is usually unacceptable. The same transistor connected as a simple modulator introduces a fundamental current drift of only 3 ?A but it still has a voltage drift of 100 mV. It is thus most suitable for use with devices having a high source impedance such as an ionization chamber for measuring radiation intensity. The current error can be reduced by a factor of 10 to 0.3 ?A if two such transistors are used in a balanced modulator which requires only one adjustment at a single temperature. The use of the transistor in a common-emitter chopping circuit results in a current drift of 50 ?A and a voltage drift of only 2 mV, while reversing the functions of emitter and collector reduces these drifts to 3 ?A and 100 ?V, respectively. The latter voltage drift is considerably less than that of most thermionic-valve amplifiers, but the current drift is still large by thermionic-valve standards, and limits the use of the chopper to low-impedance sources. A modification to this chopper eliminates the leakage current, but leaves the voltage drift unchanged at 100 ?V. This voltage drift, acting on the transistor i- mpedance, produces a current drift of only 3 ? 10?8 amp, which makes this chopper suitable for use with high-impedance sources. The chopper drift figures are obtained without recourse to selection or balancing of tcansistors, and are low enough to enable the transistor to contribute its many other advantages to such applications as sensitive null detectors and analogue computers. The current drift can be reduced still further by using higher-frequency transistors, which, in general, have lower leakage currents. For example, the surface-barrier SB100 transistor (fc? ? 50 Mc/s) introduces a maximum current drift of only 3 ? 10?9 amp. The silicon junction transistor, promising even lower leakage currents, should enable direct currents of less than 10?11 amp to be measured.
