Limitations of transistor DC amplifiers

This paper considers the important mechanisms contributing to drift in low-level differential dc amplifiers using matched-pair silicon transistors operated at low collector currents. It is shown theoretically and experimentally that thermally induced drift and sensitivity to power-supply variation are both reduced substantially by operating the two transistors with equal VBEinstead of with equal collector currents. Moreover, by intentionally unbalancing the passive components in the circuit, it is shown to be possible, in principle, to achieve a self-compensating operation with almost perfect cancellation of drift. Experimentally, for a source resistance of 400Ω, it was not difficult to achieve equivalent input drifts of the order of 0.05 µv per °C, 0.2 µv for a 1 per cent variation in supply voltage and 0.3 µv drift over a three day period. The ultimate limitation appeared to be 1/f noise which was on the order of 0.5 µv peak to peak.