The circuit can use any general-purpose, low-offset, low-drift op amp, such as the OP-07. The differential signal from the bridge feeds an amplifier that drives an ordinary, rugged ±50-p.A meter. Near the null point, however, the drastically reduced signal level from the bridge requires very high gain to achieve a high null resolution. To provide the variable-gain feature, the op amp`s feedback path needs a dynamic resistance that increases as the input signal drops.

Two common signal diodes, Dl and D2, in an antiparallel configuration in the feedback path supply function for all positive and negative inputs. To stabilize the op amp circuit at high gain, capacitors C3, C5, and C6 reduce response to high frequencies; capacitors Cl and C2 bypass the amplifier"s power supplies.

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