GB2212018A - Combined FET/bipolar transistor constant current source - Google Patents

Abstract

A current source circuit incorporates both p-channel field effect transistors and npn bipolar transistors. The field effect transistors provide a current-determining function whilst the bipolar transistors provide both a current-handling function and voltage protection of the field effect transistors. The circuit is suitable for integration. <IMAGE>

Description

IMPROVEMENT IN INTEGRATED CIRCUITS This invention relates to integrated circuits and in particular to circuits containing both bipolar and field effect devices.

A recent development in integrated circuits has been the introduction of circuits incorporating both bipolar and field effect devices. A cost reduced form of this technology has been developed which dispenses with pnp bipolar devices. However, the introduction of this cost reduced technology has been inhibited by the lack of suitable high current source circuitry. Attempts have been made to overcome this probelm by the use of p channel field effect transistors in place of pnp bipolar transistors in a conventional circuit arrangement.

However, this is costly in terms of chip area particularly if currents greater than a few microamps are required.

The object of the invention is to minimise or to overcome the disadvantage.

According to the invention there is provided a current source circuit, said circuit including both p channel field effect transistors and npn bipolar transistors, wherein the field effect transistors provide a current determining function and wherein the bipolar transistors provide both a current handling function and voltage protection of the field effect transistors.

An embodiment of the invention will now be described with reference to the accompanying drawing in which the single figure shows a current source circuit e.g. for use as a constant current source load.

Referring to the drawing, the circuit is composed of field effect (p - channel) and npn bipolar devices. The function of the circuit is to provide a constant current, e.g. lma, through a resistive load RL from a supply voltage V1 which voltage may not be constant. Typically the arrangement provides a current bias for of an amplifier AMP the output of which is coupled to the load resistor RL.

The current through the load resistor is the sum of a controlled current through diode D1 and bipolar transistor Q1, the base of which is supplied with a fixed current Iref, and a correction current ICor supplied via bipolar transistor Q2 and diodes D1 and D2.

The reference current 1ref is determined by p channel field effect transistor M3, coupled to a fixed voltage supply Vcc, and bipolar transistor Q4. The particular value of Iref may be set by resistor R1.

The current through transistor Q4 is mirrored by transistor Q3 which also passes a current 1ref The current passed by field effect transistor M2 is Iref + #I, where #I is an current due to the low output impedance of M2, and this current is mirrored by transistor M1. This mirror current is divided into one part Iref which is shared between diode D1 and the base of transistor Q1, and a second part gI which flows through transistor Q2. Diode D2 and transistor Q2 together form a further current mirror so that a current b I also flows through this diode. The net result is to provide a correction current which, when subtracted from the current through M1, maintains a constant current through diode D1 and the load resistor RL.

The bipolar transistors protect the field effect transistors from excess voltage and also provide the current handling function of the circuit. The circuit is particularly adapted for integration as part of a larger structure.

It will be appreciated that the technique described herein is not limited to the particular circuit embodiment described.

Claims (4)

1. A current source circus', said circuit including both p - channel field effect transistors an: npn bipolar transistors, wherein the field effect transistors provide a current determining function and wherein the bipolar transistors provide both a current handling function and voltage protection of the field effect transistors.

2. A current source circuit including both p-channel field effect transistors and npn bipolar transistors and adapted to provide a constant current through a load to which, in use, the circuit is connected, the circuit including a first bipolar transistor, means for supplying the base of that transistor with a first constant current , a second bipolar transistor arranged to mirror the current passed by the first transistor, the sum of the currents passed by the first and second bipolar transistor providina the constant current through the load, a firs' field effect transistor arranged in the collector circuit of the second bipolar transistor, and current mirror means including one or more further field effect transistors whereby base current is supplied to the second bipolar transistor and is controlled as to maintain the net current through the load at a constant predetermined value.

3. A current source circuit substantially as described herein with reference to and as shown in the accompanying drawing.

4. An integrated circuit incorporating one or more current sources as claimed in any one of the preceding claims.