Integrated circuit comprising a plurality of voltage-current converters

An integrated circuit comprising a plurality of adjustable voltage-current converters, of which one converter is employed as a reference converter. The reference converter is included in a control loop. To the input of the reference converter an input voltage is applied in that a first current is passed through a reference resistor connected to said input. A second current, which is in a fixed ratio to said first current, is compared with the output current of the reference converter and this reference converter is then biassed so that its output current corresponds to said second current. In this way the transconductance of the reference converter is determined by the reference resistor and the ratio of the first and the second current, so that this transconductance is highly independent of process, temperature and supply-voltage variations. The other converters are biassed by signals derived from the bias signals for the reference converter and thus have corresponding transconductances.

The invention relates to an integrated circuit comprising a plurality of 
voltage-current converters (1, 2, 3, 4), each having an input (11, 21, 31, 
41) and an output (12, 22, 32, 42) and a bias-signal input (13, 23, 33, 
43) for controlling the transconductances (a.sub.1 G.sub.0, a.sub.2 
G.sub.0, a.sub.3 G.sub.0, a.sub.4 G.sub.0) of said voltage-current 
converters by means of a bias signal on said bias-signal input, the 
transconductances of the voltage-current converters being in a 
substantially fixed ratio to each other. The IC also includes a 
bias-signal source (5, 51, 52, 53, 54) for applying bias signals, which 
are in a substantially fixed ratio to each other, to said bias-signal 
inputs, and a control network for controlling the bias-signal generator in 
such a way that the transconductances of the voltage-current converters 
are highly insensitive to process variations and variations in operating 
conditions, at least one of the voltage-current converters (1) being 
included in the control network as a reference converter. 
Voltage-current converters are inter alia employed in analogue filters. For 
this application it is of importance that the transconductances of said 
voltage-current converters are highly constant or can be adjusted in a 
reliable manner, are highly independent of process variations during 
manufacture of the integrated circuits and are also highly independent of 
variations in operating conditions, such as supply voltage and 
temperature. On the basis of the fact that in an integrated circuit 
various parameters may vary substantially but that the mutual ratios of 
the parameters of similar components vary to a comparatively small extent, 
said independence can readily be achieved in an integrated circuit by the 
steps mentioned in the preamble. Such an integrated circuit is known from 
the publication "Electronic Design" of Feb. 15, 1978, pages 26-32, in 
particular the circuit arrangement shown on page 30. In this integrated 
circuit two voltage-current converters operating as an integrator are 
arranged as a voltage-controlled oscillator. The output frequency of the 
oscillator thus formed is compared with a reference frequency in a phase 
comparator and the voltage-controlled oscillator is controlled so that the 
oscillating frequency of said oscillator corresponds to the reference 
frequency. The control voltage applied to the oscillator is also applied 
to an analogue filter comprising voltage-current converters operated as 
integrators, so that the filter frequency corresponds to the reference 
frequency and is thereof highly independent of process and temperature 
variations. In such a circuit arrangement the voltage-current converters 
are stabilized indirectly and their use is limited to a range of 
application where a stable frequency is available. Moreover, the known 
stabilizing method demands comparatively complex circuitry. 
It is an object of the invention to provide an integrated circuit of the 
type mentioned in the preamble, in which a number of voltage-current 
stabilizers can simply be stabilized in respect of process variations and 
variations in operating conditions. 
To this end the invention is characterized in that the control network 
comprises a first (6) and a second (7) current source whose currents 
(I.sub.1 and nI.sub.1 respectively) are in a substantially fixed ratio to 
each other. A terminal (9) is coupled to the input (11) of the reference 
converter (1) for coupling to a reference resistance element (8), which 
input (11) is included in the output circuit of the first current source. 
A comparing control amplifier (10), which is included between the output 
of the reference converter and a control input of the bias-signal 
generator for comparing the output current of the reference converter and 
the current from the second current source and controlling the bias-signal 
generator in such a way that a substantially fixed ratio is maintained 
between the output currents of the reference converter and the second 
current source. 
The invention is based on the recognition that, by applying a first 
current, whose magnitude is unknown per se and which is subject to 
variations, to the input of the reference converter and a second current, 
which is in a fixed ratio to said current--which is easy to realize in 
integrated circuits--to a comparator in order to compare the output 
current of the reference converter with said second current, and by 
controlling said reference converter in such a way that its output current 
is in a fixed ratio, specifically is equal, to said second current, the 
transconductance of said reference converter, regardless of the absolute 
value of the first and the second current, is inversely proportional to 
the resistance of a reference resistor arranged at the input of said 
reference converter--which in practice is frequently a precision resistor 
which is externally connected to the integrated circuit, or a resistor 
which is simulated by means of switched capacitances whose equivalent 
resistance is determined by a switching frequency. With the control signal 
thus obtained or with the signals which are in a fixed ratio thereto the 
transconductances of all the other voltage-current converters are then 
also related to said reference resistance.

FIG. 1 shows the block diagram of an integrated circuit in accordance with 
the invention. It comprises a number of voltage-current converters, four 
of which 1, 2, 3 and 4 are shown, which each have an input 11, 21, 31 and 
41, an output 12, 22, 32 and 42 and a bias input 13, 23, 33 and 43 
respectively. Via their bias inputs the voltage-current converters receive 
bias signals from a bias-signal generator 5, which supplies mutually 
related bias signals. The converters 1, 2, 3 and 4 have transconductances 
a.sub.1 G.sub.0, a.sub.2 G.sub.0, a.sub.3 G.sub.0 and a.sub.4 G.sub.0 
respectively, which are in a fixed ratio a.sub.1 :a.sub.2 :a.sub.3 
:a.sub.4 to each other, said ratio being determined by the mutual 
structure of said converters and/or the mutual realtionship of the control 
signals supplied by the bias generator 5. In practice, said 
transconductances will frequently be selected to be equal (a.sub.1 
=a.sub.2 =a.sub.3 =a.sub.4 =1). 
Voltage-current converter 1 is selected as a reference converter. Via a 
resistor 8 having a resistance R.sub.0, which resistor may be connected to 
a pin 9 outside the integrated circuit, its input 11 is connected to 
ground in the present example. The integrated circuit further comprises a 
first (6) and a second (7) current source which are coupled to each other. 
These current sources 6 and 7 supply currents of magnitudes I.sub.1 and 
nI.sub.1 respectively, which are in a fixed ratio 1:n to each other. The 
current I.sub.1 flows via resistor 8 and produces a voltage I.sub.1 
R.sub.0 on input 11 so that the reference converter 1 supplies an output 
current I.sub.u =a.sub.1 G.sub.0 R.sub.0 I.sub.1. In a comparing amplifier 
10 this current is weighted by a factor m--in practice generally equal to 
1--and compared with the current nI.sub.1. This comparing amplifier may 
for example be a differential voltage amplifier with resistors at its 
inputs, but alternatively, as in the example of FIG. 2, a high-ohmic 
voltage amplifier whose input is a junction between the current source 7 
and the output of the reference converter. The output signal of the 
comparing amplifier controls the bias-signal generator 5 so that in the 
case of an adequate loop gain in the control loop constituted by the 
comparing amplifier 10, the bias-signal generator 5 and the reference 
converter 1, mI.sub.u =nI.sub.1, which yields the relationship a.sub.1 
G.sub.0 =n/m.multidot.1/R.sub.0 for the transconductance of the reference 
converter 1. In the practical case that n=m=1 and a.sub.1 =a.sub.2 
=a.sub.3 =a.sub.4 =1, this yields G.sub.0 =1/R.sub.0. This relationship is 
only subject to variations of the parameters a.sub.1, n and m, which 
represent ratios and are subject to a comparatively small spread or 
variation in integrated circuits. The same applies to the other converters 
owing to the coupling via the bias-signal generator 5. 
The reference resistor 8 with a resistance value R.sub.0 may also be 
simulated by means of a switched capacitance, so that the apparent 
resistance R.sub.0 is determined by the value of said capacitance and the 
switching frequency, which may for example be of practical importance in 
said filter applications if a clock signal is present. Since such filter 
circuits employ capacitances and the resistance R.sub.0 is also determined 
by such a capacitance a significant advantage accures in that the absolute 
value of the various capacitances is no longer of importance. Indeed, a 
variation of the filter capacitance is then compensated for in that the 
apparent resistance R.sub.0 of the simulated resistor 8 varies in a 
compensating sense. An example of a resistor simulated by means of a 
switched capacitance is inter alia described in "IEEE Transactions on 
Circuits and Systems", Vol. CAS-26, no. 2, February 1979, pages 140-144. 
FIG. 2 shows essentially the same circuit arrangement as FIG. 1, the 
voltage-current converters being shown with differential inputs by way of 
illustration. In this example the inverting input 14 of the reference 
converter is internally connected to ground, so that for the resistor 8, 
which is included between the differential inputs 11 and 14, only one 
additional external pin 9 is required because an earthing pin is already 
available. 
The bias-signal generator 5 is constituted by a controlled multiple current 
source with p-channel transistors 51, 52, 53 and 54, whose source 
electrodes are connected to a positive power supply terminal +V.sub.B, 
whose drain electrodes are connected to the respective bias inputs 13, 23, 
33 and 43, and whose gate electrodes are connected to the gate and drain 
electrode of a p-channel transistor 55, whose source electrode is 
connected to the power supply terminal +V.sub.B. Thus, the transistors 51, 
52, 53 and 54 each form a current mirror with transistor 55, enabling any 
desired non-unity current ratios to be realized by suitable geometry 
ratios. The current sources 6 and 7 are also constituted by p-channel 
transistors arranged as current mirrors using a transistor 16 with 
interconnected gate and drain electrodes as input transistor, which 
transistor is included in series with a resistor 17 between the power 
supply terminals 15 and +V.sub.B. The current ratio n is again determined 
by the geometry ratios of the transistors which are arranged as current 
sources 6 and 7. The comparing amplifier 10 is constituted by an n-channel 
transistor 18, whose source electrode is connected to ground and whose 
gate electrode is connected to the output 12 of the reference converter 1 
and to the output of the current source 7. Comparison is effected in that 
the gate electrode of transistor 18 draws no current, so that I.sub.u 
=nI.sub.1. The drain electrode of transistor 18 is connected to the 
positive supply terminal +V.sub.B via input transistor 55 of the bias 
signal generator and thus controls said bias generator in such a way that 
the relationship I.sub.u = nI.sub.1 is satisfied. 
The voltage-current converters 1, 2, 3 and 4 may be designed in various 
manners. 
FIG. 3 shows a simple embodiment of such a voltage-current converter in 
C-MOS with a current-controlled transconductance. It comprises two 
p-channel transistors 25 and 26 connected as a differential pair, the 
drain current being coupled out via a current mirror comprising n-channel 
transistors 27 and 28. The bias signal is applied to the common source 
electrodes (13) of the transistors 25 and 26 as a current, the gate 
electrodes constituting the differential input, so that the 
transconductance of this converter is determined by the "slopes" of 
transistors 25 and 26.