The present invention relates to electronic circuits and, more particularly, to integrator circuits.
FIG. 1 illustrates a conventional active RC integrator circuit 10 having an operational amplifier 11, a capacitor 12 and a resistor 13 with resistance R. Resistor 13 has one electrode connected to the inverting input terminal of operational amplifier 11 and the other electrode connected to receive an input signal VIN. Capacitor 12 is connected between the output lead and inverting input terminal of operational amplifier 11. The non-inverting input terminal of operational amplifier 11 is connected to ground. In operation, input signal VIN is converted to a current by resistor 13, which is then integrated by capacitor 12 to provide an output signal VOUT. Such integration circuits are commonly used in a wide variety of applications and may be tuned by manipulating the RC time constant of the feedback loop to operate over a range of frequencies.
In low frequency applications, the RC time constant of the feedback loop must be relatively large in order for the integrator to function properly. For example, in an audio application, the RC time constant is typically on the order of eight milliseconds. Further, the capacitor should be small to minimize the cost of fabricating a monolithic integrated circuit integrator circuit, typically having a capacitance no greater than twenty picofarad. Consequently, the resistor must have a resistance on the order of four hundred mega ohm. Such a large resistor can be fabricated, but conventional resistive elements in integrator applications generally require a relatively large amount of semiconductor real estate, which tends to increase fabrication costs.
In accordance with aspects of the present invention, an integrator circuit having a relatively large RC time constant is provided. In one aspect of the present invention, the integrator circuit includes a resistive element implemented with a field effect transistor operated in a sub-threshold mode. By controlling the size of the field effect transistor in addition to the sub-threshold gate voltage, a desired resistance value can be achieved in a small area and without using bipolar devices, thereby decreasing fabrication costs and complexity.
In a further aspect of the present invention, the integrator circuit is fully differential with the integrating capacitor being implemented as a floating capacitor. In accordance with this aspect of the invention, the integrator circuit is only responsive to differential signals and allows integration to take place on both sides of the floating capacitor.
In another aspect of the present invention, the integrator circuit also includes a bulk drive circuit that uses the differential input signal to bias the bulk keep the bulk quiet. More particularly, the bulk drive circuit uses gate capacitance of matching bulk drive transistors to cancel high frequency noise.