High side current source circuit having improved output impedance to reduce effects of leakage circuit

Abstract

A high side low voltage current source circuit having improved output impedance to reduce effects of leakage current. A current source circuit is described with a transistor having an emitter coupled to an emitter degeneration resistor which is coupled to a power supply voltage. The output of the current source is taken at the collector of the transistor. In one embodiment, the transistor is a PNP transistor device. The base of the transistor is coupled to the output of an operational amplifier. One input of the operational amplifier is coupled in a feedback loop to the emitter of the transistor. A direct current bias voltage is applied to the other input of the operational amplifier. The output impedance (R″o) of the current is source is based on the open loop gain of the operational amplifier (e.g., about 35 dB) and is therefore orders of magnitude larger than the output impedance of other prior art current source designs. Also included is a novel level shifting circuit for shifting (up) the DC voltage level at the emitter of the transistor to improve its dynamic range of operation in low power supply voltage environments. A resistor is also placed in a tail current path of the operational amplifier, rather than a transistor, to further improve the dynamic range of the current source. The present invention finds particular application within a clock generator circuit where its reduced leakage current properties and improved dynamic range help to reduce clock jitter.