Disclosed herein are a variety of operational circuits each including an operational amplifier with a plurality of feedback loops. Each of the feedback loops as well as the circuit inputs are connected to the operational amplifier through a high impedance input terminal forming part of a feedback loop switching circuit. The switching circuit includes a field effect transistor circuit, the high impedance input terminals comprising the gate electrodes of the field effect transistors. Signals in the feedback circuits are selectively applied to the inputs of the operational amplifier by selectively shorting each of the feedback loops to ground potential. The selective shorting is accomplished by using bipolar transistors placed in a conductive state by control signals.
A logarithmic compression circuit comprising an operational amplifier and a transistor for logarithmic compression. One of the emitter and collector of the transistor is connected to the inverting input of the amplifier and the other of the emitter and collector of the transistor is connected to the output of the amplifier. A switching device is selectively connected between the base of the transistor and the non-inverting input of the amplifier and between the base and the said one of the emitter and collector of the transistor in accordance with the magnitude of a signal current input.
A negative input voltage comparator that compares a negative input voltage signal against a positive voltage signal in a circuit, such as an integrated circuit. The negative input voltage signal is effectively isolated from the circuit by means of a series resistor in the base-emitter circuit of a transistor which is connected in the output leg of a current comparator.
