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 V
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instead 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.