Transconductor circuit with high-linearity double input and active filter thereof

The invention relates to a transconductor circuit with a double input and a single output, comprising two input transistors (M1, M2) whose primary conduction terminals (D1, S1, D2, S2) are respectively connected together; in this way, variations in load current and voltage can be made lower, thereby also lowering distortion from changes in their transconductance.

CROSS-REFERENCE TO RELATED APPLICATION 
This application claims priority from EPC App'n 94830324.3, filed 06/30/94, 
which is hereby incorporated by reference. However, the content of the 
present application is not necessarily identical to that of the priority 
application. 
BACKGROUND AND SUMMARY OF THE INVENTION 
This invention relates to double-input transconductor circuits, and to 
active filters which incorporate them. 
A transconductor is a voltage-controlled variable-transconductance stage, 
and is an integral part of the operational transconductance amplifier 
("OTA") which is a voltage-controlled current-source amplifier. A 
well-known desideratum of transconductor circuits is linearity. 
Transconductors are used in active filters, and also in gyrators, 
oscillators, and circuits for impedance transformation. See generally 
J.Scott, ANALOG ELECTRONIC DESIGN (1991), which is hereby incorporated by 
reference. Some specific examples of the literature on transconductor 
designs, and their application to con- tinuous-time filters, includes the 
following, all of which are hereby incorporated by reference: 
Silva-Martinez et al., "A large-signal very low-distortion transconductor 
for high-frequency continuous-time filters," IEEE JOURNAL OF SOLID-STATE 
CIRCUITS vol.26, no.7 p.946-55 (July 1991); Tanimoto et al., "Realization 
of a 1-V active filter using a linearization technique employing plurality 
of emitter-coupled pairs," 26 IEEE JOURNAL OF SOLID-STATE CIRCUITS vol.26, 
no.7 p.937-45 (July 1991); Castello et al., "A very linear BiCMOS 
transconductor for high-frequency filtering applications," in the 1990 
IEEE INTERNATIONAL SYMPOSIUM ON CIRCUITS AND SYSTEMS vol.2, pp. 1364-7; 
Perry, "A flexible transconductor-capacitor filter demonstrator," in the 
1989 IEEE INTERNATIONAL SYMPOSIUM 0N CIRCUITS AND SYSTEMS vol.2, p.1075-8; 
Haigh et at., "Continuous-time and switched capacitor monolithic filters 
based on current and charge simulation," 137 IEE PROCEEDINGS G (Circuits, 
Devices and Systems) 147 (1990); de Heij et at., "Transconductor and 
integrator circuits for integrated bipolar video frequency filters," 1989 
IEEE INTERNATIONAL SYMPOSIUM ON CIRCUITS AND SYSTEMS vol. 1 p. 114-17; 
Perry, "An integrated continuous-time bipolar transconductor-capacitor 
filter," 24 IEEE JOURNAL OF SOLID-STATE CIRCUITS 732 (1989); Nedungadi et 
al., "High-frequency voltage-controlled con- tinuous-time lowpass filter 
using linearised CMOS integrators," 22 ELECTRONICS LETTERS 729 (1986); 
Czarnul et al., "MOS tunable transconductor," 22 ELECTRONICS LETTERS p. 
721 (1986); Wang and Guggenbuehl, "A voltage-controllable linear MOS 
transconductor using bias offset technique," 25 IEEE JOURNAL OF 
SOLID-STATE CIRCUITS 315 (1990); Van de Plassche, "A wide-band monolithic 
instrumentation amplifier," 10 IEEE JOURNAL OF SOLID-STATE CIRCUITS 424 
(1975); Pookaiyaudom and Surakampontorn, "An integratable precision 
voltage-to-current converter with bilateral capability,"13 IEEE JOURNAL OF 
SOLID-STATE CIRCUITS (June 1978); Blauschild, "An open loop programmable 
amplifier with extended frequency range", 16 IEEE JOURNAL OF SOLID-STATE 
CIRCUITS 626 (1981); all of which are hereby incorporated by reference. 
Patent Application GB-A-2 175763, which is hereby incorporated by 
reference, addresses (inter alia) the need for linearity, and proposes as 
a preferred embodiment the half-circuit shown in FIG. 1. This comprises a 
MOS transistor denoted by M which has its source terminal connected to a 
reference potential GND (usually ground) and its gate terminal connected 
to the circuit input; and a bipolar transistor having an emitter terminal 
connected to the drain terminal of the transistor M and a base terminal 
connected to a reference bias potential UDC; a bias current IDC flows 
through the collector of transistor Q. 
The transconductance G of the circuit is given by 
EQU G=K*H*V.sub.DS, 
where V.sub.DS is the drain-to-source voltage of the transistor M, K is a 
coefficient dependent on the manufacturing process used for the transistor 
M and on the gate-to-source voltage V.sub.GS, and H is a coefficient 
dependent on the geometry of transistor M. Note that the drain-source 
voltage V.sub.DS can be expressed as 
EQU V.sub.DS =UDC-Vbe. 
The base-emitter voltage Vbe is dependent on the current being passed by 
the bipolar, according to the well-known relation 
##EQU1## 
In order to limit the distortion due to the coefficient K, the 
aforementioned application proposes the use of differential circuits 
formed of fully symmetrical half-circuits. 
To limit the distortion due to the presence of the term V.sub.DS, the 
aforementioned Application proposes, as a preferred embodiment, that the 
transistor Q be used to lower the output impedance as seen from the 
transistor M on the drain terminal, as shown in the half-circuit of FIG. 
1; in fact, when the voltage V.sub.GS of transistor M varies, its (output) 
drain current also varies, and consequently, so does the (output) voltage 
on the load applied to the drain terminal, which corresponds to the term 
V.sub.DS. As second choice, the aforementioned Application proposes that 
the transistor Q can be replaced with an output stage consisting of a 
feedback connected circuit which has a much lower input impedance and much 
higher output impedance. 
Such an output stage necessarily requires a fairly complicated circuit 
implementation, and becomes even more complicated where several 
transconductor circuits must be used, as is the case with active filters, 
and especially if they are to be integrated to a chip. The feedback 
scheme, moreover, restricts the structure utility range by placing limits 
on its frequency. 
It is an object of this invention to provide a circuit with improved 
linearity, which is particularly beneficial in high-frequency active 
filters. 
By using a two-input transconductor wherein the two input transistors have 
output terminals connected together, the effect of variations in the 
output current, and ensuing distortion, can be greatly reduced. 
The proposed transconductor is especially useful for low-voltage 
continuous-time filters. The transconductor of FIG. 1 realizes the 
voltage-to-current conversion. In order to realize a double-input stage, 
two single-input stages would conventionally be connected at their output 
node, i.e. at the collector of the bipolar transistor Q shown in FIG. 1. 
However, in the circuit of FIG. 1, the major cause of distortion is the 
dependance of VDS on the variation of VBE in the bipolar transistor. The 
composition of two stages like that of FIG. 1 does not reduce this 
nonlinearity. 
In the innovative scheme of FIG. 2, the summation of the two input currents 
is performed earlier, namely at the emitter of a single bipolar 
transistor. If the two inputs are exactly in phase, the peak current 
across the bipolar will simply be the sum of the peak currents due to the 
separate inputs; but if the two inputs are not in phase, then the average 
signal current across the bipolar will be reduced accordingly. This 
reduction follows from the well-known inequality .vertline..SIGMA.i.sub.k 
.vertline..ltoreq..SIGMA..vertline.i.sub.k .vertline.. (The quiescent 
current across the bipolar will merely be the sum of the quiescent 
currents across the MOS devices.) The percentage of modulation of the 
emitter current will therefore be reduced, which implies a reduced 
varation in VBE, and hence a reduced variation in VDS, and hence a 
reduction in distortion. 
This solution is specially effective where the signal voltages to the two 
inputs are approximately equal in modulo and offset in phase by a large 
amount (e.g. 90-180 degrees). Understandably, best performance would be 
achieved with input transistors which are as far as possible identical.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The numerous innovative teachings of the present application will be 
described with particular reference to the presently preferred embodiment 
(by way of example, and not of limitation), in which: 
The transconductor circuit of this invention, shown in FIG. 2, has a double 
input on first I1 and second I2 input terminals and a single output on an 
output terminal OP; it comprises a first input transistor MI of the MOS 
type and a second input transistor M2, also of the MOS type, which have 
their gate terminals, G1 and G2, respectively connected to the input 
terminals I1 and I2, source terminals 1 and 2 connected together and to a 
first node ND1, and drain terminals D1 and D2 connected together and to a 
second node ND2; it further comprises a third transistor QO of the BJT 
type which has its emitter terminal E connected to the node ND2, base 
terminal B connected to a reference bias potential UDC, and collector 
terminal C connected to a third node ND3. In addition, a bias current 
generator GDC is connected to the node ND3, with the output terminal OP 
being also connected to the node ND3. 
The potential at the node ND2 can be controlled through the transistor QO. 
With the above assumption, and neglecting the bias signals, it is easily 
seen that the signal current being flowed through the transistor QO will 
be small, so that small will also be variations caused by the 
base-to-emitter voltage VBE. In fact, this current is given by the signal 
voltage on the input I1 multiplied by the transconductance of the 
transistor M1 plus the signal voltage on the input I2 multiplied by the 
transconductance of the transistor M2. 
The node ND2 potential is, therefore, quite constant, and the 
drain-to-source voltage VDS of the transistors M1, M2 and, hence, the 
transconductance variations, will be limited. This results in distortion 
being low. 
The circuit shown in FIG. 2 lends itself to several modifications, among 
which are the transistors M1, M2 and QO, which may either be of the MOS 
type or the BJT type. 
The node ND 1 is usually held at a predetermined fixed potential, often at 
ground potential. The node ND 1 could be connected, however, to another 
bias current generator. 
The assumption made (signal voltages to the inputs approximately equal in 
modulo and offset by about 180 degrees) is frequently met in actual 
practice. 
A conventional active filter of the first order is shown in Figure 3a to 
comprise two transconductor circuits TT of a traditional type connected in 
series. The center tap of that link is connected to ground through a 
capacitor CC and to directly to the filter output, thereby forming a 
feedback path. 
A similar type of circuit, but implemented with a transconductor TD 
according to this invention, is shown in FIG. 3b. The transconductor TD 
has a first input connected to the filter input, and a second input 
connected to the output of the transconductor TD; this output is connected 
to ground through a capacitor CC and to the filter output by a direct 
link. 
The circuit of FIG. 3b illustrates that the assumption has been met, as a 
first approximation, at least at frequencies below the circuit cutoff 
frequency. 
This circuit provides a transconductance building block having two inputs 
and having low distortion to be used in filters; the performance of the 
filter of FIG. 3b is the one of an inverting buffer having low distortion 
for signals having a bandwidth well below the pole given by the capacitor 
CC. 
FIG. 4a shows an active filter of the second order implemented with 
transconductors TT of conventional design, a first capacitor C1 and a 
second capacitor C2. It is essentially the equivalent of having two 
filters of the first order, similar to the one shown in FIG. 3a, feedback 
cascade connected. 
FIG. 4b shows the same type of filter of the second order as implemented 
with transconductors TD according to the invention. This corresponds 
essentially to having two filters of the first order like that shown in 
FIG. 4a cascade connected and fed back. 
As will be recognized by those skilled in the art, the innovative concepts 
described in the present application can be modified and varied over a 
tremendous range of applications, and accordingly the scope of patented 
subject matter is not limited by any of the specific exemplary teachings 
given. For example, as will be obvious to those of ordinary skill in the 
art, other circuit elements can be added to, or substituted into, the 
specific circuit topologies shown.