Voltage-to-current converters are implemented in many conventional applications. Related transconducting devices are electronic building blocks characterized by a current output derived from a voltage input in a linear relationship lout = Gm * Vin, where Gm is transconductance, and where Seimens (S) is the standard unit of Gm in Amps/Volt. Voltage-to-current converters form the basic input structure of instrumentation amplifiers and are used as basic Gm building blocks in frequency shaping devices. Generally, the value of Gm in a conventional voltage to current converter is fixed and is determined by the circuit elements. Thus, the Gm is fixed when the circuit is manufactured. It is possible, however, to vary the Gm using programmable means on a circuit chip. In such a configuration, the Gm could be varied by such programmable means according to a run-time configuration, rather than a predetermined configuration. These devices are used in programmable linear filters, gain amplifiers and other general purpose linear processing elements.

Such transconducting devices, being building blocks of fixed and programmable components, have a large impact on the performance of the device in which they are incorporated. Conventional devices are limited in the manner and quality in which the Gm value is set or otherwise programmed. Performance factors such as the accuracy with which the Gm is set, the range over which the Gm is set, the resolution over which the Gm is set, and the linearity of the Gm at a particular point are all factors that affect the performance of the Gm device. Therefore, improving these factors in a Gm device would improve the device's performance, as well as the component or other device within which it is incorporated. Therefore, there exists a need for Gm devices having improved performance factors. As will be seen below, the invention accomplishes improved performance factors in an elegant manner.

Summary of Invention The invention provides a method and apparatus for performing a voltage to current conversion. In particular, the invention provides a voltage to current converter configured to vary its Gm. Such a converter is configured to receive a voltage input signal combined with a reference voltage signal to be converted to a current output.

Optionally, the reference voltage signal may be provided by a parabolic impedance network that includes a bank of resistors and a plurality of corresponding current sources. Each current source corresponds to each node between two resistors, and may be varied in order to program changes in the comparator's Gm. Each resistor and corresponding current source is configured to create an individual reference voltage reference having a value that occurs in a parabolic manner in relation to other voltage references occurring across the impedance network. The converter further includes a plurality of comparators corresponding to the plurality of voltage reference signals.

The reference voltage is summed together with an input voltage to an input of each corresponding comparator. Each comparator is configured to receive an enabling signal source for receiving an enabling signal. The enablement signal source is configured to provide a variable input signal to further control the Gm by varying the input current of the converter. The comparator includes a comparator output that outputs a signal when the comparator is enabled.

A circuit embodying the invention may include a plurality of transistor devices, wherein each device includes a pair of transistors. The discussion that follows describes a field effect transistor ("FET") device. The invention, however, is not limited to such a device. For example, a bipolar device may also be used with the base substituting for the gate, the collector substituting for drain and the emitter substituting for the source may be used. Each transistor device has individual gate, source and drain connections, and each of the pair of transistors is configured to receive a separate input at each of their respective source connections. Furthermore, each of the pair of transistors has a commonly connected gate. The other gate of each of the respective transistors is commonly connected with corresponding gates of transistors contained in each of the other transistor devices. The circuit further

includes a voltage source configured to provide a reference voltage to the drain of one of the pair of transistors, and each of the other transistor devices also includes a corresponding voltage source at the drain of one of the pairs of the transistors of the respective device. Further, the device includes a current source connected with the gates of the two transistors that are commonly connected. The current source is configured to produce a variable current at the common junction of the two transistors, providing the ability to vary the Gm of the voltage to current converter.

Such a circuit embodying the invention may include a parabolic bank of resistors configured to provide the reference voltage to the gate of one of the pair of transistors. When a voltage is applied to this bank of resistors, a different reference voltage is applied to the gate of one of a pair of transistors of each of the plurality of transistor devices. The parabolic bank of resistors may be further configured to provide a reference voltage in a manner that would produce reference voltages of any desirable character. In this configuration, the reference voltage provided to a first transistor device is of a relatively higher value than the reference voltage provided to an intermediate transistor device. Furthermore, the intermediate transistor device is of a minimum voltage value relative to the other transistor devices.

In an alternative configuration, the reference voltage provided to a first transistor device is of a relatively lower value than the reference voltage provided to an intermediate transistor device. The intermediate transistor device is of a maximum voltage value relative to the other transistor devices. Thus, if the output voltages were presented in graphical form, the graph would be in parabolic form, with a relative minimum or maximum value occurring at the center, depending on whether the parabola is in a concave upward or downward orientation respectively.

Brief Description of Drawings Figure 1 is a diagrammatic view of a voltage to current converter according to the invention; Figure 2 is a diagrammatic view of a long tailed pair circuit utilized in Figure 1 ; Figure 3 is a graph showing the output signals of the long tailed pair circuit of

Figure 2; Figure 4 is a graph showing the overall differential output of the converter of Figure 1 when only a single given comparator is enabled, versus the difference in input signals to that comparator; Figure 5 is a graph illustrating the relationship between the two current sources and the input voltage of the circuit of Figure 1; Figure 6 is a graph illustrating the relationship between the output current and the input voltage of the circuit of Figure 1; Figure 7 is a graph illustrating the relationship between the output current and the input voltage of the circuit of Figure 1 relative to the curve of Figure 6 when the tail current is reduced; Figure 8 is a diagrammatic view of the circuit of Figure 1 incorporating a parabolic impedance network at the input; Figure 9 is a graph illustrating the increase in linearity of the voltage to current converter of Figure 8; Figure 10 is a graph illustrating the result of varying the tail current of a long- tailed pair and the input reference voltage of a voltage to current converter according to the invention; and Figure 11 is a graph illustrating several results of varying the tail current of a long-tailed pair and the input reference voltage of a voltage to current converter according to the invention.

DETAILED DESCRIPTION In one embodiment, a voltage to current converter is provided that includes a converter input for receiving a voltage input signal and a reference voltage signal to be converted to a current output. Alternatively, the converter may include a parabolic impedance network that includes a bank of resistors, a plurality of nodes occurring between each resistor and a plurality of current sources, where each current source corresponds to each node. Each resistor and corresponding current source is configured to create an individual voltage reference having a value that occurs in a

parabolic manner in relation to other voltage references occurring across the impedance network. The converter further includes a plurality of comparators corresponding to the plurality of voltage reference signals. The parabolic impedance network is configured to provide parabolic reference voltage inputs to each comparator input. According to the invention, the reference voltage may be varied in a manner to vary the transconductance of the comparator. Each comparator is also configured to receive an enabling signal and to output a signal when the comparator is enabled. According to the invention, an enablement signal source is configured to provide a variable input signal to also vary the Gm of the comparator.

The result is a voltage to current converter configured to accurately vary and set the Gm of such a converter. Furthermore, the resolution and range over which the Gm can be set is markedly improved. Also, the linearity of such a device is increased over this extended range. Improving these performance characteristics of the converter likewise improves the performance of devices that incorporate such converters. The invention is described below in one for use in converting voltage input signals to current output. It will be appreciated by those skilled in the art, however, that other useful applications of the invention may be implemented in applications where voltage to current converters are useful, without departing from the spirit and scope of the invention, where the scope is defined in the appended claims.

Figure 1 is a diagrammatic view of a circuit embodying the invention.

Converter circuit 100 is an example of a plurality of comparators 104, Cl, C2, C3,...

CN in a converter 100 that can be selectively enabled. According to the invention, the signal that enables the comparators can be individually varied to vary the transconductance of the converter as a whole. The outputs of comparators Cl, C2, C3, ... CNcan then be compounded together to form the output of the converter 100. The manners in which the outputs can be compounded are well known in the art. Those skilled in the art will appreciate that there exists many method by which the outputs of the comparators may be summed or otherwise combined to produce an output indicative of the result of a voltage to current converter. Accordingly, the invention is not limited as such. Each of the plurality of comparators Cl, C2, C3,... CN consists of

a connection of components that has a linear output in proportion to a given input. In operation, the comparators are configured to each receive a voltage input Vi,, summed together with a corresponding reference voltage Vl, V2... V7. According to the invention, the reference voltage steps between Vl, V2... V7 may be varied to vary the transconductance of the converter as a whole. The specific components making up the plurality of comparators, 104, Cl, C2, C3,... CN in Figure 1 is not limiting. It should be apparent to those skilled in the art in light of the following detailed description of the circuit in both Figures 1 and 8 that the comparators Cl, C2, C3,... CN may be configured differently according to the invention to improve their linear input-output characteristic. Such a principle can be applied to any voltage to current converter having a plurality of comparators configured to vary the transconductance of the converter.

Each of the plurality of comparators Cl, C2, C3,... CN outputs a quantity li, out= Iz Es, where 1 < i < N, and Ii is the difference between the output currents Hilft and 7//, f. In this equation, Ei is the value of an enabling signal. The enabling signal is a current that is varied in amplitude, all the comparators typically receiving the same enabling current. If the outputs Ii, t of all of the comparators Cl, C2, C3,... CN are added together as a group, the total output fout is : N IoUt = ILeft-IRight = E aft riglt) (1)<BR> =1 According to the invention, if the value of E for all the comparators is varied between, 1.5 and 2 for example, the Gm of the converter can be varied.

Referring again to Figure 1, a converter 100 includes a bank 104 of N comparators Ci. Each comparator includes a pair of three-terminal semiconductor devices Mal and Mi, 2, 1 < i < N. In a preferred embodiment, the low-impedance connections of the two devices are connected in common to one of a bank of current sources, Si, 106. The bank of current sources 106 provides enabling signals for the individual comparators Ci. The three-terminal devices Mil and Mi2 can be, for example, field-effect transistor ("FET") or bipolar junction transistor ("BJT") devices.

In any such specific configuration, the comparator Ci will be responsive to the voltage difference at the gates or bases of the pair of devices, depending on how the device is configured. In a preferred embodiment, the high impedance connection is utilized, whether it is the gate or the base. Where each of Mil and Mi2 consists of an n- channel-type metal-oxide-semiconductor FET ("NMOS"), the sources of those devices would be connected together to a current source Si In operation, the current from current source Si would be split between the two devices Mil and Mi2 depending on the relative gate voltage of devices Mil and Mi2. Each pair of devices Mil and Mi2 together forms a comparator Ci responsive to the voltage difference applied between the gates of the pair of devices Mil and Mi, 2. The voltage difference is in turn provided by an input impedance network. The drains of all devices Mi'l are connected together to provide an output current Ilft, and the drains of all devices Mi2 are connected together to provide an output current Irig, t. The output of converter 100 can be considered the difference between the output currents Ileft and Irig, t, which are at nodes comprising sufficiently low impedance points to hold the appropriate voltage bias conditions.

In this example, each of the comparators Cl, C2, C3,... CN is connected to corresponding voltage sources V1, V2, V3,... VN provides voltage having a value Vs.

The voltage sources provided a reference voltage to be combined with Vu, t to apply a voltage across the comparators as an input voltage Vi,,. According to the invention, the reference voltage Vs may be varied in order to vary the Gm of the corresponding comparator.

The input voltage Vu, t is the difference between the signals Volez and Vright. As the signal Vin is varied, the comparator with a zero input signal (i. e. the one that has no voltage difference between the gates) will vary dependant on the particular values of the voltage sources Vn. In this way, the comparators Cl, C2, C3,... CN are responsive to the input signal Vi,,. In the example illustrated, N=7. However, those skilled in the art will appreciate that numerous variations in component numbers are possible without departing from the invention.

Referring again to the circuit in Figure 1, in operation, each of the current sources Si is able to provide an enabling signal in the form of a current that enables

the pair of devices Mil and Mi2 that make up comparator Ci. Referring to Figure 2, a schematic drawing of a three terminal component 200 known in the art as a long- tailed pair is illustrated. A long-tailed pair is a device made up of two three terminal elements such as FET or BJT type transistors that have their respective low impedance connections connected in common to a node that provides current. The long-tailed pair 200 is one of a compound set of long-tailed pairs that make up device 100. The long-tailed pair 200 shown in Figure 2 includes two transistors 202,204, each having a drain connection 206,208, a gate 210,212, and a source 214,216. The source connections 214,216 are connected in common, and are further connected to current sinking device 218. The current sinking device 218 is configured to draw a current from the drain 206 through transistor 202 to the source 214 and to ground connection 220. Similarly, the device 218 is further configured to draw a current from source 208 through transistor 204 to the source 216 and to ground connection 220. The device 200 can be characterized as a basic voltage to current converter. In operation, the device 200 is configured to receive voltage inputs Vin and Vinb at gates 210 and 212 respectively. These two terminals are subject to a voltage difference of the two voltage inputs, which may be of a varying nature. The device may be biased such that one voltage is at ground potential, and the other voltage is at another potential. If both voltages are equal, the input is zero volts. The current sinking device 218 provides an enabling signal to enable the operation of the transistors 202, 204 respectively. Once enabled, the transistors 204,206 produce output currents lout and Ioutb at drains 206,208 respectively. Similar to the input of the device, the output is the difference between the current values at drains 206,208 respectively. If the input voltage is zero, the current drawn is split between the devices evenly, assuming that they are equivalent devices. Thus, they are both enabled to the same degree by the enabling current drawn by the current sinking device. According to the invention, the enabling current that is drawn using current sinking device 218 may be varied to vary the Gm of the long-tailed pair.

Figure 3 is a graph illustrating the relationship of the currents Iout and Ioutb present at the drains 206,208 of devices 202,204 respectively, shown on the vertical axis, versus a range of input voltage differences (Vdiff = Vin-Vinb) between the gates

210,212 respectively, which are shown on the horizontal axis. The enablement signal provided to the device is a 10.0 uA current from current sinking device 220. It can be seen that, when viewed over the entire range of possible input voltage differences, the output current in the drains 206,208 of the devices 202,204 respectively, is not linear. This is clear looking from one extreme, where all of the current flows from current sinking device 218 to the drain 206 of device, to the other extreme, where all of the current flows to the drain 208 of device 204. However, if the circuitry making up long-tailed pair 200, this pairing of transistors 202,204 operates only near the center of the transfer characteristic in the range close to zero, where the current splits approximately equally between the transistors 202,204, increased linearity occurs where slight deviations from the center are linear, recognizing that there is a limited range of voltage differences that can be applied to the gates 210,212 of the devices 202,204 before one or the other of those devices saturates. For example, consider the region from the point where the output current changes from 40% of current flowing through device 202 and 60% of the current flowing through device 204, to the point where the output current changes from 60% of current flowing through device 202 and 40% of the current flowing through device 204. In this region, in practice, the linearity can be better than 1 %.

Referring again to Figure 1, when the drains of all devices Mil are compounded together to provide output current Hilft and the drains of all devices Mi2 are compounded together to provide output current Iright, 1 < i < N. This compounding can be used to increase the region of linearity in the overall transfer characteristic.

Figure 4 shows the same data in Figure 3, but the output is now considered to be the difference between the total drain currents. According to the invention, the region of linearity in this output signal in increased as a result of the overlapping of the transfer characteristic of the individual long tailed pair devices.

Referring again to Figure 1, and also referring to Figures 6 and 7, it can be seen how the Gm of the series of transistors pairs can be varied. In the circuit of Figure 1, the long-tailed pairs are offset such that the center of each device's response is slightly offset from its neighboring device. This may be achieved by a particular setting of the voltage sources Vn shown in Figure 1. According to the invention, in

lining up the long-tailed pairs in this manner, a degree of programmability is possible.

Referring to Figure 6, an illustration of the output current to input voltage relationship of the same device is illustrated where the tail current supplied by the current sinking device is reduced to lOuA. A marked change in the linear range is illustrated in the curve of Figure 6. The Gm is 400mV/40pA = 100, uS. Referring to Figure 7, a comparison with the curve of Figure 6 is illustrated in contrast to yet another curve.

In this comparison, the voltages of the same device are recorded when the tail current supplied by the current sinking device is reduced to 5usa. And, the Gm has been reduced to 501lS.

Thus, according to the invention, this method of varying the tail currents in the long tailed devices accomplishes the ability to program the Gm of a device. Again, the compound set of long-tailed pairs is arranged such that the transfer characteristics overlap. Thus, in the device of Figure 1, the Gm may be programmable by varying the tail current of the compound arrangement of long-tailed pairs. Those skilled in the art will understand various methods of programming the variable tail current of the compound arrangement, and the invention is not limited to any particular method.

Referring to Figure 8, an alternative embodiment of a circuit embodying the invention is shown having a parabolic impedance network bank 802. In this example, each of the comparators Cl, C2, C3,... CN made up of compounded long-tailed pairs of transistors is placed across a corresponding resistor Rl, R2, R3,... RN, each having a value R. These resistors are connected in series and define between them nodes from each of which a corresponding current source Gl, G2, G3,... GN draws an equal current having a value Ise In operation, drawing from a bank 802 of current sources and corresponding resistors creates a parabolic profile of reference voltage signals. This parabolic profile ensures that only one of the comparators has a zero input voltage and that this comparator with a zero input voltage changes with the input signal Vi,,. The parabolic bank of transistors is configured to provide the reference voltage to the gate of the one of the pair of transistors. When a voltage is applied to this bank of resistors, Vleft- Vright, a different reference voltage is applied to the gate of one of a pair of transistors of each of the plurality of comparators. The parabolic bank of resistors may be

further configured to provide a reference voltage in a manner that would produce reference voltages in a parabolic manner. In this configuration, the reference voltage provided to a first comparator is of a relatively higher value than the reference voltage provided to an intermediate comparator. Furthermore, the intermediate comparator is of a minimum voltage value relative to the other comparators.

In an alternative configuration, the reference voltage provided to a first comparator is of a relatively lower value than the reference voltage provided to an intermediate comparator. The intermediate comparator is of a maximum voltage value relative to the other comparator. Thus, if the output voltages were presented in graphical form, the graph would be in parabolic form, with a relative minimum or maximum value occurring at the center, depending on whether the parabola is in a concave upward or downward orientation respectively. The upward or downward concave characteristic is a function of the amplitude and the sign of the current sources Gn. For example, if the sources Gn are current sinking, the parabola bends downward. In contrast, if the sources Gn are current sourcing, the parabola bends upward. In the limiting case if the sources Gn are all set to zero, the characteristic voltages present at the comparator inputs are linearly separated between Vleft and Vright. More particularly, in excel format, one example of the solution for Vleft and Vright, where both are set at zero volts, is (power (M/2-I, 2) +M/2-i)/2- (POWER (M/2, 2) +M/2)/2.

The input voltage Vin is the difference between the signals Vift and Vright. The comparator at the vertex of the parabolic profile will vary as the input signal Vin is varied. In this way, the comparators Cl, C2, C3,... CN are responsive to the input signal Vin. Even though Figure 8 shows the comparators connected to an input impedance network 802 configured to produce parabolic profile of reference voltage signals, the comparators Cl, C2, C3,... CN can easily also be connected to an input impedance network in the same manner as other input impedance networks known in the art, without deviating from the principles of this invention.

Referring again to Figure 4, the graph shows the currents present at the drains of devices M6, 1 and M6, 2 in response to a 10.0 pA current from current source S6 on the vertical axis, versus a range of input voltage differences between the gates of devices

M6, 1 and M6, 2 shown on the horizontal axis. It can be seen that, when viewed over the entire range of possible input voltage differences, the output current in the drain of the devices M61 and M6, 2 is not linear-from one extreme of all the current from current source S6 flowing to the drain of device M6, 1 to all of the current flowing to the drain of M6, 2 ; however, in the circuitry making up converter 800, this pairing of devices M6, 1 and M6, 2 operates only near the"center"of the transfer characteristic where the current splits approximately equally between the devices M6, 1 and M6, 2. This occurs where slight deviations from the"center"are linear, recognizing that there is a limited range of voltage differences that can be applied to the gates of the devices M6, 1 and M6, 2 before one or the other of those devices saturates. When the drains of all devices Mi, l are compounded together to provide output current Hilft and the drains of all devices Mi, 2 are compounded together to provide output current Irightv 1 < i < N. This compounding increases the region of linearity in the overall transfer characteristic.

Altering the Gm of the circuit 800 of Figure 8 may now be described.

Referring again to Figure 8, as discussed above, the parabolic input circuit 802 is interconnected with the series 804 of compound long-tailed pair devices Cl, C2, C3,...

C7. The input circuit includes a series of current sources Gl, G2, G3,... G6. According to the invention, the device 800 may be programmed to vary the current generated by the individual devices, thus varying the Gm of the compound device. Similarly, source bank 806 comprising current sources Sl, S2, S3,... S7 may also be varied according to a programmed device 800. It will be understood by those skilled in the art that various methods exist that may be employed to program the current level in each of the current sources current sources Sl, S2, S3,... S7 and Gl, G2, G3,... G6. The invention is not limited to any particular method or program configuration.

Referring to Figure 9, a graph of input voltage to output current of the circuit 800 of Figure 8 is illustrated. Figure 9 illustrates the relationship between the voltage input to circuit 800 and its current output for one of the series of compounded long- tailed pair devices. The circuit 800, unlike the similar circuit 100 of Figure 1, requires no offset voltage source providing an offset voltage to the individual gates. In this example, offset voltages are provided by the parabolic impedance network, where the voltages are provided in a parabolic manner across the series of resistors Rl, R2, R3,...

R7. In this example, the current input, S6 for example, to the long-tailed pair is set to 10. A, and the current input provided by the current input from the parabolic impedance network, current source G6 for example, is set to lOgA as well. The resistor, R6 for example, is set to 30kQ. As is evident from the graph of Figure 9, the Gm has increased in linear range, and the Gm value is lOOgS as before. This ability to vary both the long-tailed pair current sources as well as the parabolic input current sources provides an added degree of freedom to vary the Gm of the circuit. Thus, varying the current source across the corresponding elemental resistor in the impedance network and separately varying the current source connected to the individual long-tailed pair can affect the Gm. Referring to Figure 10, a graph is shown comparing the curve of Figure 9 with a second curve where the current from the impedance network current source occurring across the corresponding resistor is decreased. The result is a decrease in the Gm. Thus, the Gm of a given long-tailed pair can be decreased by increasing the corresponding current source within the parabolic impedance network. Thus, according to the invention, altering the Gm of the comparators is accomplished by varying both the incoming current from a current source from the bank of current sources 802 or a current source from the other bank of current sources 806.

Referring to Figure 11, a series of curves, 1101 through 1107, is illustrated that each depict the relationship of input voltage versus output current of the circuit 800, Figure 8. The currents measured at the long-tailed pair input range from 28usa to 2pA, and the currents measured at the parabolic impendence network current source range from 2, uA to 28uA respectively. In any particular graph, the total current is 301lA. It can be seen from this series of graphs that the range of the Gm varies from 1. zits to 150pu, a dynamic range of 100 to 1. These samples were taken from a circuit built and measured according to that of Figure 8. In a circuit with even more comparators, the range would be even higher. It will be appreciated by those skilled in the art that the invention can be embodied in a number of different circuit configurations to achieve different results.

As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this

invention without departing from the spirit or scope thereof.

The invention has been described with reference to a high performance voltage to current converter utilizing comparator circuits configured to produce outputs of greater resolution, better linearity and better accuracy. This is accomplished by alternatively varying the reference voltage provided by the impedance network and the current sinking device delivering current to the long- tailed pairs, thus varying the converter's transconductance, Gm. It will be appreciated by those skilled in the art, however, that the invention has broader utility. Other embodiments may be implemented according to the invention without departing from the spirit and scope of the invention, the scope of which is to be construed in accordance with the substance defined by the following claims.