A Direct-Current and Audio-Frequency Amplifier

An amplifier suitable for the amplification of small direct-current and low-frequency transient potentials to a usable level is described. In this amplifier two multielement tubes are used as balanced modulators. A carrier voltage is applied in push-pull to two of the grids of these modulators. The plates of these modulator tubes, operating in parallel, are connected to a high-gain amplifier designed for the carrier frequency. Any carrier unbalance appearing in the modulator is amplified to a sufficient level in the carrier-frequency amplifier and applied to a linear detector. The amount of carrier unbalance is determined by the magnitude of the direct-current or low audio-frequency voltage applied to the control grid of one of the modulator tubes. By proper choice of operating conditions a linear relationship between impressed signal voltage and detector output voltage can be obtained. The amplifier is quite stable in its operation and is capable of amplifying signal voltages of the order of a few microvolts.