Abstract:
Instrumentation driver apparatus, including a main driver, coupled to receive an alternating input signal and having a main circuit structure, which is adapted to generate, in response to the alternating input signal, a main output signal with alternating voltage. The apparatus includes a mirror driver, coupled to receive a direct voltage input and having a mirror circuit structure located in proximity to the main circuit structure, which is adapted to generate a mirror output signal in response to the direct voltage input, such that a variation in an operating condition of the main driver causes a corresponding variation in the mirror output signal. The apparatus further includes a feedback circuit, coupled to receive the mirror output signal, which provides in response to the mirror output signal a feedback stabilization input to the main driver so as to stabilize the main output signal.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to instrumentation equipment, and specifically to high-accuracy drivers for automatic testing equipment. 
     BACKGROUND OF THE INVENTION 
     Present-day very large scale integrated (VLSI) circuits are routinely rated at operating frequencies of the orders of hundreds of MHz. Testing systems for these circuits must of necessity be able to switch significantly faster than the rate of the systems they are testing, so that the testing system is not a limiting factor in the testing process. The testing systems must also be able to apply accurate voltage levels to circuits being tested. Thus testing systems which are able to switch at high frequency rates of at least 1 GHz or even several GHz, and which are also able to accurately control the voltage swings of the signals, are necessary. 
     FIG. 1 is a schematic electronic diagram of a last stage  10  of an automatic test equipment (ATE) driver, as is known in the art, for producing signals comprising high-frequency controlled voltage swings. System  10  comprises a driver  12  and an external feedback circuit  14 . Driver  12  receives opposite phase switching signals from a preamplifier  16 . The preamplifier output signals are applied to the respective gates of differential pair transistors  18  and  20 , comprised in driver  12 , which have their emitters coupled together in an emitter coupled logic (ECL) stage. Transistors  18  and  20  generate opposite phase output signals OUT and OUTN at their collectors. Both outputs have a swing between controlled upper and lower levels as explained below. 
     The collector of a control transistor  26  is connected to the coupled emitters of transistors  18  and  20 , so that transistor  26  acts to control the current through transistors  18  and  20 , and thus the upper and lower levels of OUT and OUTN. The emitter of transistor  26  is connected in series with a reference resistor  22 , and a reference voltage is measured across the resistor for use by feedback circuit  14 . 
     Feedback circuit  14  comprises an operational amplifier  24 , which reads the reference voltage generated across resistor  22  and feeds the voltage, via the inverting input of the amplifier, to the gate of control transistor  26 . Amplifier  24  also receives at its non-inverting input a swing control voltage which sets the swing voltage, i.e., the peak-peak voltage, of signals OUT and OUTN. 
     Typically, some or all components of last stage  10  are built on a single chip, although some or all of the components may be off-chip and/or discrete components. Furthermore, each of transistors  18 ,  20 , and  26  may be replaced by a respective plurality of transistors in parallel, in order to increase the current that can be passed in each of the respective paths. Alternatively or additionally, the emitter area of transistors in the path is increased so as to increase the current carrying capacity. Preferably, transistors  18 ,  20 , and  26 , or the respective pluralities replacing the transistors, are bipolar. However, the basic concepts of the operation of the last stage also apply if the transistors are CMOS transistors. Most preferably, the bipolar transistors are implemented in silicon-germanium, or other heterostructure technology. Preferably, amplifier  24  is an operational amplifier implemented from conventional bipolar or MOSFET transistors. 
     The accuracy of the feedback loop of systems such as last stage  10  is limited because intrinsic variations in parameters within driver  12  can only be indirectly sensed by the external feedback circuit. The variations, such as changes in current gain, base-emitter voltage, or modulation of the base width (the Early effect) of transistors  18 ,  20 , and  26 , can not be properly sensed by the external circuit. 
     SUMMARY OF THE INVENTION 
     In preferred embodiments of the present invention, an instrumentation driver comprises both a main driver and a mirror driver, preferably connected by a feedback amplifier. The main driver receives an input alternating signal, and generates a corresponding alternating output signal. The mirror driver receives a substantially fixed voltage, and generates a corresponding, approximately-fixed output signal. The mirror driver is subject to substantially the same intrinsic variations in operating conditions and voltage levels as is the main driver. The mirror driver effectively senses these variations and cooperates with the feedback amplifier to stabilize the output alternating signal of the main driver. Thus, the instrumentation driver achieves significantly higher accuracy in its high-speed, alternating output signals than do instrumentation drivers known in the art. 
     The mirror driver is implemented to have electrical properties substantially similar to those of the main driver, and is maintained in the same operating environment as the main driver. Preferably, at least some stages of the main driver and corresponding stages the mirror driver are implemented using substantially the same numbers of corresponding elements. The approximately-fixed output signal from the mirror driver is used as an input to the feedback amplifier, so that the mirror driver and the amplifier together comprise a feedback path or the main driver. 
     Since the main driver and the mirror driver have substantially similar electrical properties and are in the same environment, variations in parameters of the main driver and variations in corresponding parameters of the mirror driver will be substantially similar. Since the mirror driver is in the feedback path, variations in the main driver, which do not directly show in the feedback path of prior art instrumentation drivers, are directly incorporated into the feedback path of preferred embodiments of the present invention. These factors contribute to the high accuracy of output signals that the present instrumentation driver achieves. 
     In some preferred embodiments of the present invention, the main driver and mirror driver are both driven by substantially similar preamplifiers operating in the same environment, so that variations in corresponding parameters of the preamplifiers are also substantially similar. The main driver preamplifier receives an alternating signal from an external source, and generates a corresponding alternating signal as an input to the main driver. The mirror driver preamplifier receives a substantially fixed voltage, and generates, as an input to the mirror driver, a corresponding approximately fixed voltage which reflects changes in the environment of the driver preamplifier. 
     In some preferred embodiments of the present invention, the mirror driver preamplifier is a simplified version of the main driver preamplifier. The simplified mirror driver preamplifier duplicates the conditions at the output of the main driver preamplifier. 
     There is therefore provided, according to a preferred embodiment of the present invention, instrumentation driver apparatus, including: 
     a main driver, coupled to receive an alternating input signal and having a main circuit structure, which is adapted to generate, responsive to the alternating input signal, a main output signal with alternating voltage; 
     a mirror driver, coupled to receive a direct voltage input and having a mirror circuit structure located in proximity to the main circuit structure, and adapted to generate a mirror output signal responsive to the direct voltage input, such that a variation in an operating condition of the main driver causes a corresponding variation in the mirror output signal; and 
     a feedback circuit, coupled to receive the mirror output signal and to provide, responsive thereto, a feedback stabilization input to the main driver so as to stabilize the main output signal. 
     Preferably, the feedback circuit includes an amplifier which is coupled to receive a swing control voltage and to vary the main output signal responsive thereto. 
     Further preferably, the feedback circuit is coupled to provide the feedback stabilization input to the mirror driver. 
     Preferably, the main circuit structure includes a plurality of main elements, and the mirror circuit structure includes a corresponding plurality of mirror elements coupled in a substantially similar manner to the plurality of main elements included in the main circuit structure. 
     Preferably, the apparatus includes: 
     a main driver preamplifier which supplies the alternating input voltage to the main driver; and 
     a mirror driver preamplifier which supplies the direct voltage input to the mirror driver, wherein the main driver preamplifier is implemented in a substantially similar environment to that of the mirror driver preamplifier, such that a variation in an operating condition of the main driver preamplifier causes a corresponding variation in the mirror output signal. 
     Further preferably, the main driver preamplifier includes a plurality of stages including a main power output stage, and the mirror driver preamplifier includes a mirror power output stage substantially similar in number of elements and coupling of the elements to the main power output stage. 
     Further preferably, the mirror driver preamplifier includes a plurality of mirror elements coupled in a substantially similar manner to coupling of a corresponding plurality of main elements included in the main driver preamplifier. 
     Further preferably, the main driver preamplifier is coupled to receive an overshoot feedback input in order to limit an overshoot in the main output signal. 
     Preferably, the main output signal includes signals having frequencies greater than approximately 1 GHz. 
     Preferably, the main output signal includes substantially rectangular signals having a transit time between an upper and a lower level less than approximately 200 ps. 
     Preferably, the main output signal includes substantially rectangular signals including an upper level and a lower level having an accuracy of the order of 10 mV or less. 
     Preferably, the main circuit structure includes a main differential pair of transistors which provide the main output signal at a collector of one of the pair of transistors. 
     Further preferably, the mirror circuit structure includes a mirror differential pair of transistors having substantially similar characteristics to the main differential pair of transistors. 
     Further preferably the main differential pair of transistors include respective pluralities of transistors coupled in parallel. 
     Further preferably, the mirror circuit structure includes a non-differential transistor which is coupled in a substantially similar manner and which has substantially similar characteristics to one of the plurality of transistors coupled in parallel. 
     Preferably, the apparatus is constructed so that at least some elements of the mirror circuit structure operate in a substantially similar environment to that of at least some elements of the main circuit structure. 
     Further preferably, the environment includes a single chip containing the at least some elements of the mirror circuit structure together with the at least some elements of the main circuit structure. 
     There is further provided, according to a preferred embodiment of the present invention, instrumentation driver apparatus, including: 
     a first main driver, coupled to receive a first alternating input signal and having a first main circuit structure, which is adapted to generate, responsive to the first alternating input signal, a first main output signal with alternating voltage; 
     a first mirror driver, coupled to receive a first direct voltage input and having a first mirror circuit structure located in proximity to the first main circuit structure, and adapted to generate a first mirror output signal responsive to the first direct voltage input, such that a variation in an operating condition of the first main driver causes a corresponding variation in the first mirror output signal; 
     a first feedback circuit, coupled to receive the first mirror output signal and to provide, responsive thereto, a first feedback stabilization input to the first main driver so as to stabilize the first main output signal; 
     a second main driver, coupled to receive a second alternating input signal and having a second main circuit structure substantially similar to the first main circuit structure, which is adapted to generate, responsive to the second alternating input signal, a second main output signal with alternating voltage; 
     a second mirror driver, coupled to receive a second direct voltage input and having a second mirror circuit structure substantially similar to the first mirror circuit structure, located in proximity to the second main circuit structure, and adapted to generate a second mirror output signal responsive to the second direct voltage input, such that a variation in an operating condition of the second main driver causes a corresponding variation in the second mirror output signal; and 
     a second feedback circuit, coupled to receive the second mirror output signal and to provide, responsive thereto, a second feedback stabilization input to the second main driver so as to stabilize the second main output signal, so that the first main driver, the first mirror driver, and the first feedback circuit, are electrically independent of the second main driver, the second mirror driver, and the second feedback circuit, and so that the first main output signal and the second main output signal are combined to form a tri-level output. 
     Preferably, the apparatus includes a power supply which powers the first main driver, the first mirror driver, the first feedback circuit, the second main driver, the second mirror driver and the second feedback circuit. 
     Alternatively, the apparatus includes: 
     a first power supply which powers the first main driver, the first mirror driver, the first feedback circuit; and 
     a second power supply which powers the second main driver, the second mirror driver and the second feedback circuit. 
     There is further provided, according to a preferred embodiment of the present invention, a method for generating a signal, including: 
     generating, in a main driver having a main circuit structure, a main output signal with alternating voltage, responsive to an alternating input signal; 
     providing a mirror driver, coupled to receive a direct voltage input and having a mirror circuit structure located in proximity to the main circuit structure; 
     generating a mirror output signal in the mirror circuit structure responsive to the direct voltage input such that a variation in an operating condition of the main driver causes a corresponding variation in the mirror output signal; and 
     providing a feedback stabilization input to the main driver responsive to the mirror output signal so as to stabilize the main output signal. 
     Preferably, providing the feedback stabilization input includes providing a feedback amplifier which receives a swing control voltage and which varies the main output signal responsive thereto. 
     Preferably, the mirror circuit structure includes a plurality of mirror elements coupled in a substantially similar manner to a corresponding plurality of main elements comprised in the main circuit structure. 
     Preferably, the method includes: 
     supplying the alternating input voltage to the main driver from a main driver preamplifier; 
     supplying the direct voltage input to the mirror driver from a mirror driver preamplifier, and 
     implementing the main driver preamplifier in a substantially similar environment to that of the mirror driver preamplifier, so that a variation in an operating condition of the main driver preamplifier causes a corresponding variation in the mirror output signal. 
     Further preferably, the main driver preamplifier includes a plurality of stages including a main power output stage, and the mirror driver preamplifier includes a mirror power output stage substantially similar in number of elements and coupling of the elements to the main power output stage. 
     Further preferably, providing the feedback stabilization includes coupling the main driver preamplifier to receive an overshoot feedback input so as to limit an overshoot in the main output signal. 
     Preferably, generating the main output signal includes generating signals comprising frequencies greater than approximately 1 GHz. 
     Preferably, generating the main output signal includes generating substantially rectangular signals having a transit time between an upper and a lower level less than approximately 200 ps. 
     Preferably, generating the main output signal includes generating substantially rectangular signals including an upper level and a lower level having an accuracy of the order of 10 mV or less. 
     Preferably, the main circuit structure includes a main differential pair of transistors, and the mirror circuit structure includes a mirror differential pair of transistors having substantially similar characteristics to the main differential pair of transistors. 
     Further preferably, the main differential pair of transistors include respective pluralities of transistors coupled in parallel, and the mirror circuit structure includes a non-differential transistor which is coupled in a substantially similar manner and which includes substantially similar characteristics to one of the plurality of transistors coupled in parallel. 
     Preferably, providing the mirror driver includes operating at least some elements of the mirror circuit structure in a substantially similar environment to that of at least some elements of the main circuit structure. 
     Further preferably, the environment includes a single chip. 
     The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which: 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic electronic diagram of an instrumentation driver known in the art; 
     FIG. 2 is a schematic block diagram of an instrumentation driver, according to a preferred embodiment of the present invention; 
     FIG. 3 is a schematic electronic diagram of an alternative instrumentation driver, according to a preferred embodiment of the present invention; 
     FIG. 4 is a schematic electronic diagram of an instrumentation driver preamplifier, according to a preferred embodiment of the present invention; 
     FIG. 5A is a schematic block diagram showing a tri-level instrumentation driver, according to a preferred embodiment of the present invention; and 
     FIG. 5B is a schematic graph of an example of output from the instrumentation driver of FIG. 5A, according to a preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is now made to FIG. 2, which is a schematic block diagram of an instrumentation driver  30 , according to a preferred embodiment of the present invention. Driver  30  comprises a main driver  32 , a mirror driver  42 , and an external feedback stabilizer circuit  34 . Main driver  32  is implemented and operates generally as main driver  12 , as described in the Background of the Invention, except for differences described hereinbelow. Thus, transistors  38 ,  40 , and  64  correspond respectively to transistors  18 ,  20 , and  26  of driver  12 . Differential pair transistors  38  and  40  are coupled at their respective collectors to 50 ohm resistors  46 A and  46 B, which act as impedance matching resistors for respective signals OUTN and OUT. The OUT and OUTN signals are most preferably coupled to final receivers by 50 ohm impedance cables (for simplicity the cables and final receivers are not shown in FIG.  2 ). The collector of control transistor  64  is connected to the coupled emitters of transistors  38  and  40 , and the emitter of transistor  64  is preferably connected via a series resistor  70  to a constant level Vee. Alternatively, the emitter of transistor  64  is connected directly to Vee. 
     Driver  30  is most preferably powered by a highly stable external power supply  31  having one terminal coupled to ground, and the other terminal supplying Vee. A variable supply Vup, provided by power supply  31 , determines the required offset voltage for the “up” level of the generated signal. Most preferably, Vup has an accuracy of 1 mV. Typically the potential supplied by power supply  31  is of the order of 3.6±5% volts. 
     Active elements in driver  30  are most preferably implemented so as to be able to support rates of switching of alternating signals input to a preamplifier  36 , described in detail with respect to FIG. 4 below. Preferably, the input signals alternate at a frequency of the order of 2.5 GHz, and the signals have rise/fall times of the order of 50-100 ps, in which case transistors described hereinabove in driver  30  are preferably implemented as bipolar silicon-germanium transistors. Resistors  46 A and  46 B are preferably implemented off-chip so as to achieve an accuracy of 1%. Other elements of driver  30  are preferably implemented on-chip, except where indicated otherwise hereinbelow. 
     Main driver  32  receives input signals from preamplifier  36 . The input signals from the preamplifier are most preferably out of phase complementary rectangular signals, alternating between level  0  and level  1 , which are input to the bases of transistors  38  and  40 , thus generating corresponding alternating output signals OUT and OUTN. Signals OUT and OUTN will vary between a low level, corresponding to transistor  38  or  40  conducting, and a high level, corresponding to the transistors being cut off. It will be appreciated that the high voltage level is substantially the same as the high voltage level provided by power supply  31 , Vup. 
     Mirror driver  42  is preferably implemented so that its physical components are substantially similar to, and are coupled in substantially the same manner as, those of driver  32 . For example, a control transistor  66  “mirrors” control transistor  64 , and a transistor  50  (one of a differential pair) mirrors transistor  40 . The emitters of differential pair transistors  48  and  50 , substantially similar to transistors  38  and  40 , are coupled together. Similarly, transistors  48  and  50  are connected at their respective collectors to resistors  56 A and  56 B. Values for resistors  56 A and  56 B for preferred embodiments of the present invention are given below. A preamplifier  60 , preferably substantially similar in construction to preamplifier  36 , is coupled to respective gates of transistors  48  and  50 . Alternatively, preamplifier  60  is implemented to have a generally similar output stage as preamplifier  36 , so as to provide substantially similar output levels as preamplifier  36 . The collector of control transistor  66  is connected to the coupled emitters of transistors  48  and  50 , and the emitter of transistor  66  is preferably connected via a series resistor  72  to Vee. Alternatively, the emitter is connected directly to Vee. 
     In contrast to the input signals from preamplifier  36  to main driver  32 , preamplifier  60  inputs constant level signals to the gates of transistors  48  and  50 . The gate of transistor  48  receives a level 0 signal, and the gate of transistor  50  receives a level 1 signal. Thus transistor  48  is substantially cut-off, while transistor  50  conducts. An output Vfeedback is taken from the collector of conducting transistor  50 , and is used as an input to stabilizer  34 . It will be appreciated that Vfeedback corresponds to the low level output of transistors  38  and  40 . 
     The collector of a preamplifier control transistor  62  is coupled to preamplifier  36 , and current through the transistor generates a feedback current in the preamplifier which is used to limit signal overshoot in driver  32 . The emitter of transistor  62  is preferably connected via a resistor  74  to Vee. Alternatively, the emitter is connected directly to Vee. The gates of all three control transistors  62 ,  64 , and  66 , are coupled together, and the three gates receive a feedback signal from external feedback stabilizer circuit  34 . 
     External feedback stabilizer circuit  34  preferably comprises an operational amplifier  60  which receives Vfeedback at its non-inverting input. Alternatively, other types of external feedback circuits may be used in this context, including even a simple wire or conductor. The inverting input of amplifier  60  is supplied by an external DC control voltage which controls the voltage swing of outputs OUT and OUTN, as explained below. The output of amplifier  60  is fed via a current limiting resistor  68  to the gates of control transistors  62 ,  64 , and  66 . 
     During operation of driver  30 , external feedback stabilizer circuit  34  acts to stabilize the output signals of main driver  32 . Any changes within main driver  32 , such as temperature changes, which intrinsically cause the output signals to change, will only be responded directly to by a circuit such as circuit  34  to a second order degree, or may not be responded to at all. However, since mirror driver  42  is in a feedback loop comprising circuit  34 , any such changes will be reflected in the feedback provided by circuit  34 . Thus, since mirror driver  42  is substantially similar to main driver  32  in construction and implementation, any changes causing current variation in driver  32  appear as substantially similar variations in mirror driver  42 , and these changes are incorporated in the feedback loop. 
     It will be appreciated that driver  30  operates by mirror driver  42  behaving with a substantially similar electrical response as main driver  32 . Typically, each transistor  38 ,  40 , and  64  of main driver  32  is respectively implemented as substantially similar parallel transistors, in order to increase the current carrying capacity of the respective transistors. For example, each of transistors  38 ,  40 , and  64 , is implemented as six transistors in parallel. In this case mirror driver  42  may be implemented so each transistor  48 ,  50 , and  66 , is also implemented as six transistors in parallel. Alternatively, each mirror driver transistor may be implemented as a different number, most preferably fewer than six, of substantially similar transistors as those of main driver  32 . Further alternatively or additionally, areas of emitters of transistors in mirror driver  42  are set to be different from areas of emitters of the corresponding transistors of main driver  32 . Since transistor  48  is substantially cut-off during operation of driver  30 , in some preferred embodiments of the present invention, transistor  48  and its collector resistor  56 A are not implemented. Alternatively, transistor  48  is implemented and its collector is coupled directly to ground. 
     The values of each resistor  56 A (when implemented),  56 B, and  72  are most preferably adjusted so that the current density, i.e., current/emitter area, via corresponding paths of driver  32  and mirror driver  42  are substantially similar. For example, if transistors  38 ,  40 , and  64 , each have total emitter areas of A e , then areas of corresponding transistors  48 ,  50 , and  66  may be set to each have emitter areas of A e /N, where N is any convenient number. In this case, resistor values in main driver  32  and mirror driver  42  are adjusted accordingly. For example, if resistors  46 A and  46 B have value R, and resistor  72  has value R e , then values for resistors  56 A,  56 B, and  70  are set to be substantially equal to R.N, R.N, and R e /N respectively, in order to equalize the gain and current density in respective transistors fed by the resistors. Resistors  56 A (when implemented) and  56 B are preferably implemented off-chip so as to achieve high accuracies. 
     FIG. 3 is a schematic electronic diagram of an instrumentation driver  80 , according to an alternative preferred embodiment of the present invention. Driver  80  is of generally the same form as driver  30 , and in FIG. 3 elements having substantially the same function as those of driver  30  are indicated by the same numerals. Except where otherwise indicated hereinbelow, elements of driver  80  are most preferably implemented on-chip. Thus, transistor  38  is implemented as six substantially similar parallel connected transistors  88 , transistor  40  is implemented as six substantially similar parallel connected transistors  90 , and transistor  64  is implemented as six substantially similar parallel connected transistors  104 . Transistors  88 ,  90 , and  104 , are active elements in a main driver  102 , which corresponds to main driver  32  of driver  30 . 
     A first plurality of six parallel diodes  108  is connected in series with a second plurality of six parallel diodes  110 . Diodes  108  are connected in series with the collectors of transistors  88 . Similarly, a third plurality of six parallel diodes  112  is connected in series with a fourth plurality of six parallel diodes  114 . Diodes  112  are connected in series with the collectors of transistors  90 . Diodes  108 ,  110 ,  112 , and  114 , most preferably formed from transistors by shorting emitters to collectors, serve as breakdown protection diodes by reducing the collector-emitter voltage of transistors  88  and  90 . Diodes  110  are connected to the variable supply Vup via an off-chip 50 ohm resistor  116 , and diodes  114  are connected to Vup via an off-chip 50 ohm resistor  118 . 
     In main driver  102  input signals are received from preamplifier  36  at the gates of transistors  88  and transistors  90 . Output signal OUT is taken from the junction of resistor  118  and diodes  114 , and output signal OUTN is taken from the junction of resistor  116  and diodes  110 . 
     Driver  80  comprises a mirror driver circuit  120  and a preamplifier stage mirror circuit  122 , corresponding respectively to mirror driver  42  and preamplifier  60 . Mirror driver circuit  120  comprises four transistors  124 ,  126 ,  128 , and  130 , coupled in series. Transistor  124  and non-differential transistor  126  respectively mirror one of the plurality of transistors  104  and one of the plurality of differential pair transistors  90 . Transistors  128  and  130  each have their gates shorted to their collectors, and respectively mirror one of the plurality of diodes  112  and one of the plurality of diodes  114 . An off-chip resistor  132  is connected from ground to the shorted collector of transistor  130 , and the Vfeedback signal is taken from the junction of resistor  132  and transistor  130 . Resistor  132  is most preferably set to a value six times the value of resistor  118 , i.e., 300 ohms, so that the current in transistors  124  and  126  substantially corresponds to the current in one of transistors  90  and one of transistors  104 . 
     In operation, non-differential transistor  126  is consistently biased “on,” mirroring one of differential pair transistors  90  or transistors  89  in their on state, by preamplifier  122 . Mirror driver  120  does not include another transistor mirroring transistors  90  or  88 , since such a transistor would be consistently biased “off” to mirror the off state of transistors  90  and  88 , and so would generate a level substantially equal to ground. 
     Preamplifier mirror circuit  122  comprises three transistors  134 ,  136 , and  138 , and two resistors  140  and  142 , coupled in series. The transistors and resistors of preamplifier  122  substantially mirror elements of an output stage of preamplifier  36 , but comprise adjustments in values to the resistors to accommodate the fact that, as described below, there are six transistors corresponding to transistor  134  in preamplifier  36 . Thus, resistors  140  and  142  are most preferably set at values of 120 ohms each, and act as a voltage dividing network corresponding to a voltage dividing network of preamplifier  36 . 
     Driver  80  further comprises a preamplifier control transistor  144 , corresponding to transistor  62  of driver  30 . The collector of transistor  144  is connected to preamplifier  36 , and the emitter of the transistor is coupled to Vee. 
     The bases of transistor  144 , transistors  104 , and transistor  124  are connected together, as for the corresponding transistors in driver  30 . The coupled bases receive a feedback signal from an operational amplifier  160  via a current limiting resistor  168 . Amplifier  160  and resistor  168  are preferably implemented as off-chip components. Alternatively amplifier  160  is implemented on-chip, in which case it is most preferably constructed from a combination of field effect and bipolar transistors, as is known in the art. Amplifier  160  is coupled its non-inverting input to the Vfeedback signal generated at the junction of transistor  130  and resistor  132 . The inverting input of amplifier  160  is coupled to an external DC control voltage that determines the required swing. As for driver  30 , the DC voltage sets the size of the output voltage swing at the junction of resistor  118  and diodes  114 , and at the junction of resistor  116  and diodes  110 . 
     FIG. 4 is a schematic electronic diagram of preamplifier  36 , according to a preferred embodiment of the present invention. Preamplifier  36  comprises an input buffer stage  170 , a swing modification stage  172 , and a power output stage  174 . Input stage  170  comprises a differential pair of transistors  180  and  182 , which receive at their gates rectangular input signals. Current through the coupled emitters of transistors  180  and  182  is controlled by a control transistor  192 . Transistor  192  is in turn controlled by its base being coupled to the junction of a resistor  188  connected to a diode  190  (in the form or a transistor with collector and emitter shorted). The collectors of transistors  180  and  182  are respectively connected to substantially equal resistors  184  and  186 , and outputs of stage  170  are taken from the collectors. 
     Swing modification stage  172  changes the swing of the signal fed to stage  174 . Stage  172  receives the outputs from stage  170  at emitter follower transistors  194  and  196 , connected respectively to emitter resistors  198  and  200 , and transfers the buffered signals derived from the transistors to the bases of differential pair transistors  208  and  210  respectively. The collector of a transistor  206  is coupled to the emitters of transistors  203  and  210 , and transistor  206  and a transistor  204  and a resistor  202  have similar functions to, and are connected as, transistors  192  and  190  and resistor  188  described above. Most preferably, the junction of the collector of transistor  206  and the emitters of the differential pair transistors is coupled to transistor  62  of driver  30 , or to transistor  144  of driver  80 , so as to receive overshoot feedback from the respective transistor. In each case, the overshoot signal is used to control differential transistors  208  and  210  so as to limit the overshoot. 
     Stage  174  comprises a pair of substantially similar emitter follower stages. Each emitter follower stage is used to buffer the respective output of stage  172  and also to suppress ranging at the output of the preamplifier. A first emitter follower stage comprises a transistor  214 , resistors  216  and  223 , transistors  218 , and a voltage divider formed from resistors  220  and  222 . Resistors  220  and  222  most preferably have values of approximately 20 ohms. The voltage divider is introduced to suppress ringing at the output, which is taken from the junction of resistors  220  and  222 , and to reduce the output impedance to approximately 11 ohms. A second emitter follower stage comprises a transistor  224 , resistors  226  and  233 , transistors  228 , and a voltage divider formed from resistors  230  and  232 . Components of the second emitter stage perform substantially tasks and have substantially the same values as the corresponding components of the first stage. 
     Six transistors  234  have their respective bases and collectors connected so as to form six parallel diodes. The diodes reduce the voltage across stages  170 ,  172 , and  174  to approximately 2.7 V, so that the current drawn by the emitter followers of stage  174  is correspondingly reduced. 
     Returning to FIG. 3, preamplifier mirror circuit  122  is implemented to reflect one of the emitter follower stages of stage  174 , when the emitter follower stage is in high level ouptut condition. Thus transistor  134  corresponds to transistors  234 , and transistors  136  and  138  correspond respectively to transistors  218  and  214 . 
     Also, resistors  142  and  140  correspond to resistors  220  and  222  respectively. It will be appreciated that the differences in values between the resistors in preamplifier  122  and stage  174  reflect the different currents carried by the preamplifier and stage  174 . 
     FIG. 5A s a schematic block diagram showing a tri-level instrumentation driver  250 , and FIG. 5B is a schematic graph of an example of output from instrumentation driver  250 , according to a preferred embodiment of the present invention. Instrumentation driver  250  comprises a first driver  30 A and a second driver  30 B, which are substantially similar to instrumentation driver  30  described hereinabove, and which are independent of each other. Drivers  30 A and  30 B are respectively supplied by a first preamplifier  36 A and a second preamplifier  36 B, which are substantially similar to preamplifier  36  described hereinabove, and which operate independently. Preamplifier  36 A and preamplifier  36 B in turn receive separate alternating voltage inputs Din 1  and Din 1 _not, and Din 2  and Din 2 _not, respectively. Most preferably, driver  30 A comprises a preamplifier mirror circuit which mirrors preamplifier  36 A and driver  30 B comprises a preamplifier mirror circuit which mirrors preamplifier  36 B, as described above for preamplifier mirror circuit  60 . Preferably, a power supply  252  supplies drivers  30 A and  30 B and preamplifiers  36 A and  36 B. Alternatively, power supply  252  is split so as to power driver  30 A with preamplifier  36 A, and driver  30 B with preamplifier  36 B, separately. 
     Driver  30 A receives an input voltage swing level Vswing 1  which sets an output swing voltage level of V_outswing 1 . Similarly, driver  30 B receives an input voltage swing level Vswing 2  which sets an output swing voltage level of V_outswing 2 . Vout of driver  30 A is connected to Vout of driver  30 B, effectively as a parallel output connection. Since the drivers are independent, the output swings generated by each driver are independent. Thus the output is a tri-state output voltage varying between an upper level Vup, a first level  254 , and a second level  256 . First level  254  is separated from Vup by adjustable swing V_outswing 1 . Second level  256  is separated from first level  256  by adjustable swing V_swing 2 . Driver  250  enables each of the tri-state levels to be determined to an accuracy of about 5 mV, each level being set within a window of approximately 0 V to 3 V. It will be appreciated that a similar output to that described hereinabove can be achieved if the outputs are connected in series. 
     It will be appreciated that the preferred embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.