Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Current driver
Reexamination Certificate
2000-11-17
2002-11-05
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Current driver
C327S563000
Reexamination Certificate
active
06476649
ABSTRACT:
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 inc
Goren David
Papae Donald J
Zelikson Michael
Darby & Darby
Wells Kenneth B.
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