抄録
Among many types of d-c amplifiers in the process instrumentation or control systems, the self excited oscillating amplifier is widely used.
In this amplifier, the circuit performs a dual function: it amplifies a modulated signal and also it works as a carrier frequency oscillator.
This circuit may be regarded as the standard circuit in the force-balance type d-c current transmitter and the electronic PID controller.
A solid-state electrometer circuit is developed by the author with this oscillating amplifier technique, and it is applied to the β ray thickness gage, the pH meter and the Gas chromatograph.
In this paper, the amplitude modulation type oscillating amplifier is analyzed as one of the nonlinear systems, and the theoretical bases about its gain band width and stability are given.
The results are compared with those of the conventional carrier type d-c amplifier with a separate oscillator.
The equivalent transfer function of the oscillating amplifier is K/γ·1/1+Td/γs
where K: constant, Td: d-c equivalent time constant of resonance circuit that dominates oscillation frequency, γ: an extent of nonlinearity in the positive feed back loop. (γ<<1) The d-c gain and the band width generally depend upon the nonlinearity γ. By decreasing the nonlinearity, a very high d-c gain is obtained, but the band width is reduced.
The characteristics of oscillating amplifier with a chopper modulator or a limitter in the loop are similar to those of the d-c amplifier with a separate oscillator and a tuned carrier amplifier.
It is concluded that the oscillating amplifier is superior to other amplifiers when applied in the industrial instrumentation, because the circuit is simple, the reliability is high and the power consumption is low.
