GB2195211A

GB2195211A - Improved transconductor

Info

Publication number: GB2195211A
Application number: GB08622294A
Authority: GB
Inventors: Simon Atkinson
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1986-09-16
Filing date: 1986-09-16
Publication date: 1988-03-30
Anticipated expiration: 2006-09-16
Also published as: DE3731130A1; GB2195211B; DE3731130C2; GB8622294D0

Description

1 GB 2 195211 A 1

SPECIFICATION

Improved transconductor This invention relates to transconductors. 70 Transconductors are devices used in electronic circuits when it is required to convert an input signal of varying voltage to an output current related to the input signal. A transconductor has a parameter called transconductance; the symbol---9m--- is used. The transconductance is the slope of the linear part of a graph of current out against voltage in.

A known transconductor is shown in Figure 1 of the accompanying drawings. It is derived from a configuration known as a long tailed pair. The input voltage signal is applied across the bases 10 of a pair of transistors 12 and the varying output current is taken from the collector of one of the pairs. The relationship between the input voltage signal Vin and the output current 1,,u, is shown in Figure 2 of the accompanying drawings. It will be seen that the transfer characteristic is linear over only a very limited range illustrated on the graph as between -25mV and +25mV. Below and above this range of input voltage signals, the output current will not bear a linear relation ship to the input voltage.

One known way of increasing the linear 95 range of the transfer characteristic is shown in Figure 3 of the accompanying drawings. Re generating resistors 14 are placed in the emit ter leads 16 of a long tailed pair of transistors 18. The extent of linearity of the transfer characteristic is increased, as shown in Figure 4 of the accompanying drawings, but the slope of the charateristic is decreased so that the range of output currents of the device remains substantially the same. Further, if the tail current is low in such a device, the resistors 14 need to have a correspondingly high value to obtain the required increased linearisation of the transfer characteristic. If there is any mismatch between the resistors 14, the whole circuit is imbalanced and the transfrer characteristic becomes assymetrical about the origin.

It is an object of the present invention to provide an improved transconductor wherein the linearity of the transfer characteristic is increased in extent without a corresponding decrease in the slope i.e. in the transconductance gm of the device.

According to the present invention, there is provided a transconductor comprising a pair of matched first transistors coupled together in a long tailed pair configuration, input means for applying a varying voltage input signal across the bases of the first transistors, each first transistor having a second transistor con nected in parallel thereto and voltage supply means for supplying a voltage, intermediate the voltage applied to either of the bases of the first transistors, to the bases of the sec- 130 vice.

ond transistors.

The invention will be described further, by way of example, with reference to Figures 5 to 9 of the accompanying drawings, in which:- Figure 5 is a schematic diagram of a first form of transconductor in accordance with the present invention; Figure 6 is a diagram, similar to Figure 5, of a modified transconductor in accordance with the present invention; Figure 7 is a graph illustrating the extended linear transfer characteristic obtainable with transconductors according to the present invention; Figure 8 is a diagram, similar to Figures 5 and 6 of a transdocnductor according to the present invention, operated as a linear voltage gain device; and Figure 9 is a diagram of a preferred form of transconductor according to the present invention.

Referring firstly to figure 5, there is shown a transconductor constructed according to the present invention. The transconductor corn- prises a pair of transistors 20 connected in a long tailed pair configuration. Current sources 22 feed the collectors 24 of the transistors from a supply rail 26. The emitters 28 of the transistors 20 are connected together and through a resistance constituting a current source 30 to ground. Signal input means 34 are connected to the bases 32 of the transis tors 20 to connect the bases across a varying a.c. voltage signal source.

In accordance with the invention, each tran sistor 20 is paralleled by a second transistor 36. Each base 32 of a first transistor 20 is connected to the base 38 of the parallely con nected second transistor 36 through a resistor 40. The resistors 40 are valve matched as closely as possible and the bases 38 are con nected together. The voltage thus applied to the bases 38 through the voltage divider con stituted by the resistors 40 is half the voltage appearing on the base of one of the transistors 20.

In operation over the normal linear range of the transconductor, very little difference is seen in the transfer characteristics. However, when the normal range is exceeded, the transconductance of the first transistors 20 becomes non-linear and reduces. At this stage, the second transistors 36, operating at half the signal voltage on their bases, are still within the linear transfer range and thus source sufficient extra current to mantain the transfer characteristic substantially linear.

If the area of the emitters 42 of the second transistors 36 is the same as that of the tran- sistors 20, there is an effective doubling of the total emitter area and a consequent near doubling of the input signal voltage range whilst maintaining linearity of the transfer characteristic and transconductance gm of the de- 4 2 GB 2 195 211 A Figure 6 shows a modification of the transconductor shown in Figure 5. In this embodiment, the bases 38a of the second transistors 36a are connected together and to a reference voltage source 44. The reference voltage of the source 44 is maintained at the centre of the range of input signal voltage from the source 34a.

In this way, the second transistors 36 always operate within the linear part of the transfer characteristic input voltage range.

Figure 7 illustrates the transfer characteristic obtainable with transconductors according to the present invention. The conventional response is shown in this graph for purposes of comparison. As in Figure 2, the input signal voltage range is shown at 25mV for linear transfer. However, on extending the range using transconductors in accordance with the in- vention, the transconductance gm remains substantially unchanged.

If an effective increase in emitter area to twice that presented by a long tailed pair, is obtained by using pralleled second transis- tors, the range of the linear transfer characteristic is more than doubled. If second, third and fourth transistors (having equal emitter areas) were paralleled with each first transistor, then a substantially linear transfer charac- teristic is obtained over a range of input voltage signal of 20OmV, nearly a 10 x improvement factor.

If the second (or second and third) paralleled transistors have a greater emitter area than the first transistors then an increase dependent upon the sum of the emitter areas of the parallel transistors is obtained.

As shown in Figure 8, the transconductor according to the invention may also be used as a voltage gain device with improved linearity. The device shown in this figure includes load resistors 46 in the collector leads of the paralleled first and second transistors. Voltage variations developed across these resistors can be utilised as the output of the device.

Figure 9 is a schematic diagram of a preferred transconductor according to the present invention. The device here shown has second and third transistors 36b and 48 paralleled with each first transistor 20b. A voltage divider constituted by four substantially equal value resistors 50 is connected across the bases of the first transistors 20b and hence across the signal source 34b. The bases of the second transistors have a voltage equal to three quarters of the signal voltage applied thereto whilst the bases of the third transistors have a voltage equal to half the signal voltage applied thereto.

The invention is not confined to the precise details of the foregoing example and variations may be made thereto.

For instance, either NPN or PNP transistors may be employed in the device providing that each pair, first transistors, second transistors or third transistors and so on are of the same type. However, the transconductor of the present invention is eminently suitable for fabrication by integrated circuit technology and hence similar type transistors are preferred. Other variations are possible within the scope of the present invention.

Claims

1. A transconductor comprising a pair of matched first transistors coupled together in a long tailed pair configuration, input means for applying a varying voltage input signal across the bases of the first transistors, each first transistor having a second transistor connected in parallel thereto and voltage supply means for supplying a voltage, intermediate the voltage applied to either of the bases of the first transistors, to the bases of the second transistors.

2. A transconductor as claimed in claim 1 including a voltage divider connected across the input means, the bases of the second transistors being coupled to a tap on the vol- tage divider.

3. A transconductor as claimed in claim 2 wherein the tap on the voltage divider is arranged such that the voltage applicable to the bases of the second transistors is half the voltage across the input means.

4. A transconductor as claimed in claim 1 wherein the input signal has a predetermined range, and the bases of the second transistors are connectable to a reference voltage source equal to the midpoint of such range.

5. A transconductor as claimed in any preceding claim including a third transistor connected in parallel with each first and second transistor, and wherein the voltage supply means is arranged to supply a voltage, intermediate the voltage across the input means, to the bases of the third transistors.

6. A transconductor as claimed in claim 5 wherein the voltage supply means is arranged to supply a voltage, different from the voltage supplied to the bases of the second transistors, to the bases of the third transistors.

7. A transconductor as claimed in claim 5 or claim 6 wherein the emitter area of the second transistors and/or of the third transistors is greater than the emitter area of the first transistors.

8. A transconductor as claimed in any preceding claim including resistors arranged in the collector circuits of the first and second transistors or, when appendant to anyone of claims 5 to 7, arranged in the collector circuits of the first, second and third transistors, and the output of the transconductor is ar- ranged to be taken as the voltage variations across said resistors for enabling the transconductor to act as a linear voltage gain device.

9. A transconductor substantially as herein- before described with reference to and as il- 2:

i 3 GB 2195211 A 3 lustrated in Figure 7 and Figure 5, Figure 6, Figures 8 or Figure 9 of the accompanying drawings.

10. An integrated circuit device comprising a transconductor according to any one of claims 1 to 9.

Published 1988 at The Patent Office, State House, 66/71 HighHolborn, London WC1 R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BF15 3RD. Printed by Burgess & Son (Abingdon) Ltd. Con. 1/87.