Low level D.C. amplifier

The input stage of a low-level D.C. amplifier utilises a chopping technique to convert a D.C. input signal into an A.C. signal for initial amplification by an A.C. amplifier (8), the input stage includes a transformer (1) having a centre- tapped input winding (2) with its centre tap (4) connected to receive the input signal, and an output winding (3) connected to the A.C. amplifier (8). The input stage further includes two switching devices (6, 7) arranged for cyclic, antiphase, operation to connect respective ends of the input winding (2) to a signal return line (5), the effect of this being to apply the input signal alternately across the two input-winding halves. As a result, the A.C. amplifier (8) operates during both halves of the chopping cycle and may have considerable loop gain. By making the transformer (1) a step-up transformer, the effective input noise voltage of the A.C. amplifier (8) can be reduced so that the predominant noise voltage is that resulting from the switching-device resistances when conducting.