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Developments in high bandwidth power amplifier technology for
compact cost effective calibrator applications.
Paul C. A. Roberts - Applied Technology Manager,
Fluke Precision Measurement Limited, UK.
Abstract:
The workload of a modern calibration laboratory has placed ever greater demands on the
Calibration Source, both in terms of functionality and the combination of voltage, current and
frequency required to calibrate today's diverse range of instruments.
This need for high voltage and high current at high frequency into demanding loads such as
clamp meters and current coils, combined with a desire to produce ever more economic and
compact calibrators, has resulted in a number of significant developments in the transition from
the vacuum tube designs common in the past to today's solid state designs employing the latest
semiconductor and transformer technologies.
This paper explores the development of technologies enabling the design of high bandwidth
power amplifiers compact enough to be integrated within the calibrator in a single enclosure,
eliminating the need for a separate Boost Amplifier, and their impact on calibration applications.
Introduction:
During the 1970s a number of alternating voltage calibrators were successfully developed and
deployed, but where a high voltage output was required a separate power or boost amplifier
was always involved. These designs were based on vacuum tubes, often utilizing tubes designed
as UHF radio transmitters because of their availability in suitable power ratings and their
physical suitability for mechanical mounting. Reliability was poor and many manufacturers
products earned a well deserved reputation for fragility - and with one exception those
manufacturers either collapsed or chose to leave the calibration business. The most successful
AC voltage calibrator/amplifier partnership continued into the 1980s and was the competitive
target for a new generation of AC voltage calibrator released in 1984 - the Datron 4200.
This new design employed fully solid state technology with an integrated power amplifier
capable of delivering 1000V at frequencies up to 30kHz extending to 100kHz at 750V. The
size of this unit was identical to its closest competitor which could only provide 200V - the
additional vacuum tube boost amplifier required to obtain 1000V capability occupied almost
twice the volume of its partner. Analysis of workloads revealed that high voltage capability at
high frequency was an essential requirement for complete calibration of the precision DMMs
appearing on the market at that time - one in particular which required 700V at 100kHz. And
this even in the days before ISO9000 was driving the need for complete calibrations!. However,
output current requirements were within the capabilities of semiconductors - vacuum tube
designs were overkill for the actual workload in this respect.
The technology which enabled such a compact power amplifier design was the power
MOSFET transistor. The following sections of this paper discuss the design requirements and
their implementations in more detail. Initially, for the voltage amplifier used in the Datron 4700
series multifunction calibrators launched in 1986 (developed from the original AC voltage
calibrator design) and employed with further development in the Wavetek 4800 series
introduced in 1991. The paper continues by exploring the current source amplifier used within
the recently introduced 9100 universal calibrator capable of delivering 20Amps at 10kHz from
an enclosure smaller than the original AC voltage only design whilst coping with the demanding
load that an external current coil can present.
Design and Development of the Solid State Voltage Power amplifier:
The design requirements for the multifunction calibrator (4800 series) were to provide a power
amplifier capable of sourcing the output voltages for the DC and AC 100V and 1000V ranges.
This required a high bandwidth design capable of providing up to 200V RMS at 100kHz
reducing to 20V at 1MHz directly from the same amplifier that could also operate as a low
noise DC source for the DCV ranges and drive a step up transformer to provide the 1000V
range at frequencies up to 30kHz, extending at reducing voltage to 100kHz at 750V. (In the
1000V DC range the power amp/transformer provide a signal which is rectified and filtered to
generate the DC output). Additional constraints were weight, size and load into which these
levels must be driven. The most stringent requirement is the 1000V capability requiring delivery
internally of 180V RMS at 1A into the 6:1 step up transformer primary to satisfy the required,
mainly capacitive, load current. (At high frequency the dominant load is the capacitance of
cables connecting the calibrator to its workload and internal signal path capacitance.
The design chosen utilizes complementary MOSFETs, chosen over bipolar devices for their
reliability, lower distortion and more forgiving thermal characteristics - unlike bipolars which
exhibit thermal runaway to destruction, MOSFET characteristics reduce drain current for a
given gate voltage as junction temperature rises. Low distortion is important to ensure output
sinewave purity and eliminate errors in any mean sensing DMMs which appear in the calibration
workload. (The true RMS value of the calibrator output is controlled by internal sense and
leveling circuits, but mean sensing instruments are calibrated in terms of a pure sinewave and
relatively modest amounts of distortion introduce significant errors due to the response of the
mean sensing circuitry to harmonics.) The amplifier output stage topology is shown in Figure 1.
Supplies of plus and minus 400V are required to allow sufficient output voltage swing, which
demands use of devices with 500V ratings series connected with voltage sharing provided by
the resistors marked Rx. Devices are paralleled to increase the current capability. Those familiar
with high performance audio design will recognize elements of this topology. The voltage
capability of P-channel MOSFETs is much more restricted than N-channel devices due to
semiconductor physics - the lower charge carrier mobilities of P-type material impacts device
geometry and has limited currently available devices to 500V. The N channel devices employed
are actually rated at 800V and are chosen not for voltage rating but to match the capacitance of
the P-channel devices and provide a truly complementary design. The high input capacitance of
MOSFETs, typically 1nanofarad per device, poses a difficult load for the driver stages and
imposes a limit on the achievable slew rate which is more restrictive than attaining the bandwidth
target - if the amplifier or any of its parts slew rate limit distortion will be introduced into the
output waveform which the local negative feedback around the amplifier will not be able to
reduce - in effect the amplifier fails to keep up with itself. Other novel features include a
distortion canceling circuit which cancels distortion introduced in a pre-driver cascode stage
resulting from the drain-gate capacitance of another MOSFET which varies non linearly with
voltage. The achieved Volt-Hertz profile is presented in Figure 2.
Design and Development of the Current Power amplifier:
Design objectives for the 9100 universal calibrator were to produce a precision high current
source capable of delivering up to 20A at DC with compliance voltages up to 4V, and up to
20A AC from 10Hz to 10kHz (30kHz at lower currents) into a 700