The present invention relates, in general, to generating current in an analog circuit, and more particularly, to generating a current that tracks the gain of a transistor.
In a bipolar circuit design, when a transistor is required to run at a particular current, it is often advantageous to be able to predict and supply the corresponding base current required by the transistor. The base current of the transistor can vary greatly as the transistor gain varies. The current gains of individual transistors on the same integrated circuit chip, however, are normally very close in value. Therefore, to supply the base current for one transistor, another transistor can be set to the same collector current and its base current is used to supply the first transistor. Furthermore, several transistors, used as, for example, current sources, with a common base bias can have their base currents supplied by the base current of one transistor suitably multiplied. Without such a compensating current, as more transistors are added, their cumulative base currents can load the circuit supplying the bias voltage, which causes errors in the current source values.
A high supply voltage is usually required for a circuit to generate such a tracking current. For example, a supply voltage of 2V.sub.BE +V.sub.SAT, which is about 1.5 volts, is required when the circuit is comprised of bipolar transistors. It should be noted that V.sub.BE is the forward bias voltage across the base and emitter electrodes of a bipolar transistor and V.sub.SAT is the voltage across the collector and emitter electrodes of a bipolar transistor in saturation mode. However, in the portable electronic devices using a single battery cell, it is preferred for a circuit to operate at supply voltages lower than 1 volt to prolong the usable life of the battery cell.
Accordingly, it would be advantageous to have a circuit that is capable of generating a tracking current that compensates the gain of a transistor. It would be of further advantage for the circuit to operate with a low supply voltage.