Transconductance amplifier

Abstract: A transconductance amplifier comprises first and second input transistors (TO1,TO2) to the base electrodes of which an input voltage (Vi) is applied. A constant current source (10) is connected to the emitters of the input transistors. Outputs are derived from their collector electrodes. A tapped impedance having two or more segments (RO to RN,RN to RO) is connected between the base electrodes of the input transistors (TO1,TO2). The base of a further transistor (T1 to TN,TN to T1) is connected to a respective tap (a1 to aN,aN to a1) of the tapped impedance. The emitter areas (e1 to eN,eN to e1) of these further transistors increase from each end of the tapped impedance towards the centre (28) of the tapped impedance. The emitters of the further transistors (T1 to TN,TN to T1) are connected to the current source (10) and the collectors are connected to a junction (26) which is connected to means (30) for computing the combined base currents of the further transistors (T1 to TN, TN to T1) from the combined collector currents at said junction (26). The computed current is supplied by a current mirror circuit (32) to the centre (28) of the tapped impedance to pull up the voltage at the centre (28). The tapped impedance can have an increased resistance without unduly loading a preceding stage.

Description: The present invention relates to a transconductance amplifier for converting an input voltage to an output current. Depending on the intended application of the amplifier, the transfer characteristic between the input voltage to the output current may be linear when the circuit is used in an adjacent channel filter and may be square law when used in an automatic gain control circuit based on sum of squares of quadrature related signals. Transconductance amplifiers are known per se from EP-B1-0 234 655. In a simplified form they may be regarded as a long tail pair circuit in which the control electrodes, for example base electrodes, are interconnected by a resistor. In developments of this simple circuit, the resistor is symmetrically tapped at various points beginning at each end and proceeding to the centre. Whilst tapping the resistor in this way increases the range of input voltages which can be handled, the problem arises that there is a practical limit to the overall value of the resistor because the higher the value of resistance, the greater the error caused by the flow of base currents. This error is small if the resistance has a relatively small value but this resistance also forms the output load of the preceding stage so its current consumption will be increased compared to when the resistance has a higher value. Accordingly a transconductance amplifier of such a design is less attractive to use in circuits for inclusion in battery powered equipments. Figure 12 of EP-B-0 234 655 discloses a base current compensation technique which although theoretically perfect can suffer from the drawback of having to work with unacceptably small currents of the order of nanoamps so that the leakage currents of the transistors set the limit. An object of the present invention is to increase the range of signal input voltages which can be handled by a transconductance amplifier whilst not increasing the overall current consumption. According to the present invention there is provided a transconductance amplifier comprising a first and a second input transistor, each input transistor having emitter, base and collector electrodes, inputs for receiving an input voltage connected to the base electrodes of the first and second input transistors, a current source connected to the emitter electrodes of the first and second transistors, and outputs connected to the collector electrodes of the first and second transistors, and a tapped impedance connected between the base electrodes of the first and second input transistors, characterised in that the tapped impedance has a centre point, in that further transistors are provided having base electrodes located at at least those taps of the tapped impedance other than the centre point, in that emitter electrodes of the further transistors are connected to the current source, and in that means are coupled to a collector electrode of the or at least one further transistor at each tap for computing the combined base current of the further transistors from the collector currents of the or at least one further transistor at each tap and for supplying without distribution a current representative of the total base currents of all the further transistors to the centre point of the tapped impedance. By accurately compensating for the error(s) at the centre point of the tapped impedance and less precisely compensating for errors at other taps between the centre and the ends of the tapped impedance using current which was previously lost to the positive rail, it is possible to increase the value of the tapped impedance tenfold before the errors which exist become so large as to be unacceptable. However not only has the range of input signal voltages been extended but also the loading by the transconductance amplifier on the preceding stage is within acceptable limits. In the case of there only being one further transistor at each tap, the total of the collector currents is used to compute the combined base current which is duplicated and supplied to the central point of the tapped impedance. In certain situations a fraction of the collector current of the further transistors which in the known circuits was wasted is used for another purpose leaving the balance or remaining fraction of the collector current to be used for compensating for errors. The remaining fraction is used to compute a corresponding fraction of the combined base current of all the further transistors which base current is multiplied by the reciprocal of said remaining fraction to obtain substantially the total base current which is supplied to the central point. In order to obtain fractions of the currents at each tap at least two further transistors are provided with the ratio of the emitter areas at each tap being the same. If desired a current mirror may be used to duplicate or provide the current supplied to the central point. The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a schematic circuit diagram of an embodiment of a class - AB linear transconductance amplifier made in accordance with the present invention, Figure 2 is a graph illustrating the typical voltage along the tapped impedance in the quiescent state, Figure 3 is a schematic diagram of a part of a class - AB linear transconductance amplifier having an impedance at the centre of the tapped impedance, Figure 4 is a schematic diagram of a part of a class AB linear transconductance amplifier having a transistor at the centre of the tapped impedance, and Figure 5 is a schematic diagram of a class AB linear transconductance amplifier in which only a fraction of the combined collector currents is used to compute the current fed back. In the drawings the same reference numerals have been used to indicate corresponding features. Referring to Figure 1 the transconductance amplifier circuit comprises NPN input transistors TO1 and TO2 having emitter areas eO. Emitter electrodes of these transistors are connected to a constant current source 10. An input signal is applied to terminals 12,14 connected respectively to base electrodes of the transistors TO1,TO2. The collector electrodes of the transistors TO1,TO2 provide a differential output. The difference between the collector currents of the transistors TO1,TO2 can be obtained by means of a differential to single ended converter circuit 16 comprising PNP transistors 18,20. The emitter electrodes of the transistors 18,20 are connected together and the junction is connected to the positive supply line 22. The base electrodes of these two transistors are connected together and the base electrode of the transistor 18 is connected to its collector electrode which in turn is connected to the collector electrode of the transistor TO1. The collector electrode of the transistor 20 is connected to the collector electrode of the transistor TO2 from which a single ended output is derived on a terminal 24. A voltage divider formed by a tapped impedance, sometimes termed the base line resistor, is connected between the base electrodes of the transistors TO1,TO2. The tapping of the impedance is such that beginning from the base electrodes of the input transistors TO1,TO2 and progressing to the centre 28, segments RO to RN and RN to RO of the impedance symmetrically increase in resistance value towards the centre. Junction points a1 to aN and aN to a1 are connected respectively to base electrodes of NPN transistors T1 to TN and TN to T1. The emitter areas ek, where k = 1 to N, of these transistors increase symmetrically towards the centre. For each value N, the ratio between the emitter areas ek, where k = 0 ... N, of the transistors TO1,T1 ... TN,TN ... T1,TO2 and the ratio between the resistance values Wk, where k = 0 to N, of the segments RO to RN,RN to RO can be selected in such a way that the difference between the collector currents of the transistors TO1 and TO2 has a linear relationship with the input voltage Vi over an as large as possible range. The emitter electrodes of these transistors T1 to TN and TN to T1 are connected jointly to the constant current source 10. The collector electrodes of the transistors T1 to TN and TN to T1 are connected to a junction 26 at which the collector currents are summed. The collector current at the junction 26 is used to provide compensation for the voltage drop which normally occurs at the centre 28 of the tapped resistor due to the current flowing through the base electrodes of the transistors T1 to TN and TN to T1. The compensating current is designed to compensate substantially for the voltage drop at the centre 28 of the tapped impedance relative to its ends. The actual voltage drop varies as a result of the actual proportion of the current I flowing through the tapped impedance relative to that flowing through the transistors TO1 and TO2. In the quiescent state, the transistors T1 to TN and TN to T1 drain a part of the current I from the current source 10, so that the bias current through the transistors TO1 and TO2 is only a fraction of the current I. As the input voltage Vi increases the sum of the currents flowing in the transistors T1 to TN and TN to T1 comprises an increasingly smaller part of the current I, so that the bias current through one of the transistors TO1 and TO2 increases. The current at the junction 26 is coupled to a NPN transistor 30 which computes the combined base currents of the transistors T1 to TN and TN to T1 from the combined collector currents of these transistors at the junction 26. The base electrode of the transistor 30 is connected to a current mirror circuit 32 which duplicates the combined base currents and supplies this current without distribution to the junction 28. The current mirror circuit 32 comprises PNP transistors 34,36 whose emitter electrodes are jointly connected to the positive supply line 22. The base electrodes are joined together with a further connection from the common connection to the collector electrode of the transistor 34. The collector electrode of the transistor 36 is connected to the junction 28. In the embodiment illustrated the emitter areas of the transistors 34,36 are equal. The effect of applying this form of current compensation to the transconductance amplifier is to make any errors tolerable. Consequently the resistance value of the tapped impedance or base line resistor can be increased, for example by a factor of 10 before the inherent errors in the circuit are unacceptable. However increasing the value of the tapped impedance or base line resistor reduces the load on the preceding stage thereby decreasing its current drain. Figure 2 illustrates in full lines the variation of the voltage along the tapped impedance or base line resistor without the current being fed to the junction 28 and in broken lines the approximate voltage distribution achieved by "pulling-up" the voltage at the junction 28 as indicated by the arrow X. Figure 3 illustrates a variant of the current mirror circuit 32 in order to cope with the tapped impedance having an odd number of elements and as a result having a single central resistance element RNC with a resistance value of 2N. Compared to Figure 1, the transistor 36 either comprises two transistors 36A,36B or has two collector electrodes, with the collector electrodes being connected to either side of the resistance element RNC. When the transistor 36 comprises two transistors 36A,36B the emitter areas of the transistors 34 and 36 are scaled such that the current flowing in the transistor 36A equals the current flowing in the transistor 36B which current equals half the current flowing in the transistor 34. In the alternative arrangement of the transistor 36 having two collectors the current flowing in each collector contact of the transistor 36 equals half the current flowing in the transistor 34. Figure 4 illustrates another variant of the transconductance amplifier to that shown in Figure 1 in which there are an odd number of transistors. In Figure 4 the tap at the centre 28 of the tapped impedance is connected to the base electrode of a transistor TN having an emitter area eN. The emitter electrode of this transistor is connected to the current source 10 and the collector electrode is connected to the junction 26. The operation of the circuit is as described with reference to Figure 1. Figure 5 illustrates a transconductance amplifier in which a fraction 1/Z of the combined collector current is used to provide the base current compensation whilst the remaining current is used for some other (unspecified) function. At each of the junction points or taps a1,a2, a2 and a1 of the tapped impedance, pairs NPN transistors T1a,T1b; T2a,T2b; T2b,T2a and T1b and T1a are provided. The emitter areas of the pairs of transistors are equal but the combined emitter areas at successive taps vary in the manner described with reference to Figure 1. The emitter electrodes of all these pairs of transistors are connected to the current source 10. The collector electrodes of the transistors T1a,T2a, T2a and T1a are connected to a terminal 40 from where the current can be supplied for some other use. The collector electrodes of the transistors T1b,T2b, T2b and T1b are connected to the junction 26. In this example the current at the junction 26 will be 50 per cent (or 1/Z, where Z = 2) of that which is present in the embodiment shown in Figure 1. In order to compensate for the combined base currents, the current mirror circuit is arranged to multiply the current computed by the transistor 30 by Z, where Z = 2, and to supply the current to the junction 28. This multiplication by Z can be effected by either the emitter area of the transistor 36 being Z times that of the transistor 34 or the transistor 36 comprising Z parallel connected transistors each having the same emitter area as the transistor 34. It is possible for the emitter areas of each pair of the transistors T1a,T1b; T2a,T2b; T2b,T2a and T1b,T1a to be unequal but in the same ratio 1/Z, where Z has any suitable value, including non-integer values. However once the value of Z has been determined then the multiplication by the transistor 36, however configured, is set. From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of transconductance amplifiers and component parts thereof and which may be used instead of or in addition to features already described herein.

Claims: A transconductance amplifier comprising a first and a second input transistor(TO1 ,TO2), each input transistor having emitter, base and collector electrodes, inputs(12,14) for receiving an input voltage connected to the base electrodes of the first and second input transistors(TO1 ,TO2), a current source(10) connected to the emitter electrodes of the first and second transistors, and outputs connected to the collector electrodes of the first and second transistors, and a tapped impedance(R0 to RN) connected between the base electrodes of the first and second input transistors(TO1 ,TO2), characterised in that the tapped impedance has a centre point(28), in that further transistors(T1 to TN) are provided having base electrodes located at at least those taps of the tapped impedance other than the centre point, in that emitter electrodes of the further transistors are connected to the current source(10), and in that means(30,32) are coupled to a collector electrode of the or at least one further transistor at each tap for computing the combined base current of the further transistors from the collector currents of the or at least one further transistor at each tap and for supplying without distribution a current representative of the total base currents of all the further transistors to the centre point(28) of the tapped impedance. A transconductance amplifier as claimed in claim 1, characterised in that the centre point(28) comprises the junction of two impedances (RN, R2). A transconductance amplifier as claimed in claim 2, characterised in that the centre point(28) comprises the base electrode of another transistor(TN) whose emitter electrode is connected to the current source(10) and collector electrode is coupled to said means(30,32) for computing the combined base current. A transconductance amplifier as claimed in claim 1, characterised in that the centre point(28) comprises a single impedance(RNC) and in that said current representative of the total base currents is divided and applied substantially equally to each end of the single impedance(RNC). A transconductance amplifier as claimed in Claim 1 or 2, characterised in that at least two further transistors (T1a, T1b, T2a, T2b) are provided at each tap (a1, a2) of the impedance, in that the ratio of the emitter areas of the at least two further transistors at each tap is the same, in that the collectors of those further transistors having substantially the same fraction of collector currents are combined and supplied to said means (30, 32) for computing the combined base currents of those further transistors. A transconductance amplifier as claimed in any one of Claims 1 to 5, characterised in that said means for computing the combined base current comprises a transistor (30) of the same conductivity type as said further transistors (T1 to TN), means for supplying the collector currents of the or at least one further transistor at each said tap (a1, a2) to an emitter electrode of the transistor (30), and a current mirror circuit (32) having a current input connected to the base of the transistor (30) and a current output connected to the central point (28) of the tapped impedance. A transconductance amplifier as claimed in Claim 6, having at least two said further transistors at each tap, characterised in that the transistor computes a fraction 1/Z of the total base currents of the further transistors and supplies it to the current mirror which is arranged to multiply by the reciprocal of said fraction and to supply a current representative of substantially the total base currents of all the further transistors to said central point (28).

Inventors: CUSDIN ANTHONY RICHARD, MOORE PAUL ANTHONY, CUSDIN, ANTHONY RICHARD, MOORE, PAUL ANTHONY
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS UK LTD, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS ELECTRONICS UK LIMITED
Application year: 1993
Main classification: H03F3
IPC codes: H03F1, H03F3