In integrated circuits, there frequently arises the need for circuit elements that effectively act as resistance or transresistance elements, particularly such elements that can be voltage-tuned to a prescribed value. Typical of such circuits are continuous-time integrators and continuous-time filters.
A circuit element which has proven attractive for this role has been the familiar MOS transistor, which between its two main current handling electrodes, the source and drain, can be made to exhibit a resistance which is tunable by varying the voltage of its control or gate electrode. One difficulty has been that this resistance tends to be linear over a relatively narrow range of values, which limits its usefulness in some applications, such as in continuous-time integrators and filters.
U.S. Pat. No. 4,509,019 which issued to Banu and Tsividis on Apr. 2, 1985, describes a technique for improving on the basic limitations of the MOS transistor for use as a resistor in particular circuits. This technique capitalizes on the fact that the current of a single transistor may be viewed as comprising to a first order two ccmponents, one of which is sensitive to voltage, and so adapted for voltage tuning of its value, and the other of which is relatively independent of the operating voltages, particularly their signs. Moreover, also utilized is the fact that an MOS transistor can be viewed as a conductance, the conductance being the reciprocal of the resistance. From these principles, there is derived, for use as a variable resistance or conductance, a circuit which employs two MOS matched transistors, whose channels are connected in series with the inverting and noninverting inputs, respectively of a balanced twin-output twin output operational amplifier. Equal and opposite voltages are applied in series, respectively, with the two channels of the transistors, and equal voltages are applied to the gate electrodes and substrates of the two transistors. This circuit provides across the two balanced outputs of the amplifier a relatively linear resistance, which is tunable by the voltage applied to the gate electrodes of the transistors. Basically, these result a circuit whose output conductance is the sum only of the voltage sensitive components of the conductances of the two transistors. The voltage-insensitive components of the conductances of the two transistors substantially cancel as a result of subtraction. While this circuit represents an improvement over a single transistor for most applications, it has shortcomings. One is the need for the complexity of a double-input double-output operational amplifier. Another is the need for two well-matched input voltages of opposite polarities. Moreover, in fact the cancellation of the two other components is not complete because of second order effects.
For many applications, these shortcomings are important and the invention is directed at a circuit arrangement which can be free of one or more of these problems, depending on the particular embodiment of the invention chosen.