GB2186141A

GB2186141A - Beta compensating current source circuit

Info

Publication number: GB2186141A
Application number: GB08602232A
Authority: GB
Inventors: Philip Albert Kenneth Moon
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1986-01-30
Filing date: 1986-01-30
Publication date: 1987-08-05
Also published as: JPS62253203A; GB8602232D0; NL8700234A

Abstract

A current source circuit operable at low supply voltage-ie. at voltages lower than 1.4V and down to 0.9V and providing beta compensation. This circuit comprises a reference current source (S) in the collector circuit of a reference transistor (TS) the base of which is connected to the collector of a diode biassed drive transistor (T1). The collector voltage of the reference transistor (TS) is fed to the base of a further transistor (TS') to control current applied to the diode (TD) biassing the drive transistor (T1). The collector of the further transistors (TS') may be connected directly to the diode (TD) or may be applied to the base of an additional transistor (TS'') which transistor (TS'') in turn feeds current to the diode (TD). PNP and NPN drive transistor variants of this circuit are described. <IMAGE>

Description

SPECIFICATION Beta compensating current source circuit Technical Field The present invention concerns improvements in or relating to current source circuits, especially those that provide compensation for variance in transistor current gain( ).

The need often arises to compensate for beta effects in circuit design. This is particularly relevant on low supply voltages due to restrictions imposed on circuit flexibility.

Background Art Fig. 1 shows a known method of generating a reference current dependant on beta. As it stands this current is not very useful because it is clamped at a voltage Vbe above ground.

Putting the reference source instead into the emitter circuit has not got this limitation, as is shown in Fig. 2. The base current, however, is not now just beta dependant, but beta plus one, which can cause a large error with low beta transistors such as integrated circuit PNP's. To turn this current into a source entails a circuit such as Fig. 3 where a diodebiassed transistor T1 is shown. This now has a limitation on the minimum supply voltage, in this case approximately 1.4V.

Typically a 1.5V (nominal) battery would be used for the above circuit (Fig. 3). Battery replacement, however, becomes necessary once the supply voltage falls below circa 1.4V, despite the fact that such a battery could supply power down to an end-of-life voltage circa 0.9V. There is thus a need for current source circuits that will provide beta compensation but which nonetheless would be capable of operation at low supply voltage le. at voltages less than 1.4V and down to say 0.9V or below.

Disclosure of the Invention The present invention is intended as a remedy and thus provides a beta compensating current source circuit operable at low supply voltage.

In accordance with the invention there is provided a beta compensating current source circuit of the tyupe comprising a reference transistor and a reference current source connected in the collector circuit thereof, characterised by a feedback and drive sub-circuit including: a diode bias and a drive transistor, the base of which is connected to the diode, the collector of which is connected to the base of the reference transistors; and, at least one further transistor, the base of which is connected to the collector of the reference transistor, the collector of which is connected to supply bias current to the diode.

Preferably, in the circuit as defined above, the diode is provided by a transistor the base and collector of which are connected together.

Hereinafter this will be referred to as a diode transistor.

In this circuit the collector of the further transistor aforesaid may be connected direct to the base of the drive transistor and the diode. Alternatively, the collector of this transistor, may instead be connected to a bias impedance and to the base of an additional transistor which transistor in turn is connected to the base of the drive transistor and the diode.

The circuit may be constructed for controlling operation of one or more PNP drive transistors, in which case sub-circuit drive transistor would likewise be of PNP type and the reference transistor and said further transistor woul be of NPN type. The diode transistor and said additional transistor (each optional) would then be of PNP type.

Alternatively, the circuit may be constructed for controlling operation of one or more NPN drive transistors, in which latter case said subcircuit drive transistor would be of NPN type and the reference transistor and said further transistor would be of PNP type. The diode transistor and said additional transistor (each optional) would then be of NPN type.

Brief Introduction of the Drawings In the drawings accompanying this specification: Figures 1 to 3 are circuit diagrams of conventional current source circuit arrangements; and, Figures 4 and 5 respectively, are circuit diagrams each for a current source circuit that is in accord with this invention, one for controlling PNP type, the other for controlling NPN type, drive transistors.

Description of Preferred Embodiments A current source circuit for controlling PNP drive transistors is shown in Fig. 4. This circuit comprises an NPN reference transistor Ts which has in its collector circuit a reference current source S. The base of this reference transistors Ts is connected to the collector of a first PNP drive transistor T1. The base of the latter transistor T1 is connected to the base of a diode transistor TD and to the bases of all following PNP drive transistors T2, TN.

The collector of the reference transistors T5 is connected to the base of a further transistor T5,, also an NPN type transistor. The collector of this further transistor is, as shown (chainlink outline), connected directly to the diode transistor TD and to the bases of the drive transistor T1 to TN.In this circuit, reference current Is flows and is divided between the collector of the reference transistors T5 and the base of the further transistor T5,. This causes current to flow around a feedback loop, via thus the collector of the further transistors T5,, the diode transistor TD, the first drive transistor T1 and the base of the reference transistor T5. The loop is balanced when the current in the collector of the reference transistor T5 is equal to the reference current 15. The base current of the reference transistor, also the collector current of the first drive transistor T1 is then equal to 15/ss. All other drive transistors T2 to TN will have proportionate collector currents, the constant of proportionality in each case depending on the size relationship of these drive transistors T2 to TN to the first drive transistor T1. All the beta losses in the PNP current mirror comprised of the diode transistor TD and the drive transistors T1 to TN are compensated by feedback, andconsequently PNP beta is not a problem.

Also where the following drive transistors T2 to TN are connected each via a diode to ground VEE, as may be the case in practical application, the collector currents 12 to IN of these transistors T2 to TN will also be compensated for early effect, such as arise upon supply voltages rise.

In the alternative configuration of the circuit shown in Fig. 4 (broken outline) the collector of the further transistor T5, is connected to a bias impedance and to the base of an additional transistor T5,,, a PNP-type transistor.

The diode transistor TD is now instead connected to the eimiter of this additional transistor T5".

The PNP/NPN inverse equivalent of the circuit Fig. 4 may also be constructed and used thus for controlling NPN drive transistors. This inverse circuit is shown in Fig. 5.

Technical Application The immediate application is an integrated circuit designed for Radio Paging. The circuit design for pagers is not straight forward due to the low battery voltage, end life 0.9 Volts, and low current consumption. The need to battery economise the circuit, also entails the extensive use of current mirrors in the circuit design.

The invention although designed for pagers, could be used with advantage on any low supply, low current circuit.

Claims

1. A beta compensating current source circuit of the type comprising a reference transistor and a reference current source connected in the collector circuit thereof, characterised by a feedback and drive sub-circuit including: a diode bias and a drive transistor, the base of which is connected to the diode, the collector of which is connected to the base of the reference transistor; and, at least one further transistor, the base of which is connected to the collector of the reference transistor, the collector of which is connected to supply bias current to the diode.

2. A circuit, as claimed in claim 1, wherein the diode is provided by a transistor the base and collector of which are connected together.

3. A circuit, as claimed in either claims 1 or 2, wherein the collector of the further transistor aforesaid is connected direct to the base of the drive transistor and the diode.

4. A circuit as claimed in either claims 1 or 2 wherein the collector of the further transistor aforesaid is connected to a bias impedance and to the base of an additional transistor which transistor in turn is connected to the base of the drive transistor and the diode.

5. A circuit, as claimed in any one of the preceding claims, wherein the drive transistor is PNP type, and the reference transistor and the further transistor are both NPN type.

6. A circuit, as claimed in any one of the preceding claims 1 to 4 wherein the drive transistor is NPN type and the reference transistor and the further transistor are both PNP type.

7. A beta compensating current source circuit configured, arranged and adapted to perform substantially as described hereinbefore and with reference to either Figs. 4 or 5 of the accompanying drawings,