The design of an audio-frequency amplifier for high-precision voltage measurement

The specification and design of an amplifier required for precision audio-frequency measurements is discussed. Formulae are presented for the `ring of three¿ feedback circuit and a detailed analysis is made of appropriate feedback theorems. A multi-gain amplifier which extends the voltage range of the electrostatic voltmeters, used at the N.P.L. as basic a.c./d.c. transfer instruments, is described. An increase in this range of up to 1000 times is provided and enables voltages between 60mV and 60 volts in the frequency range 30c/s¿30kc/s to be measured to a few parts in 104.