Page 1200 - The Mechatronics Handbook

a standard feedback-control loop to maintain a constant-voltage envelope on the RF signal output from
the power amplifier.
Some of the signal conditioners employed within the RF module include the following:
1. The RF-voltage pickoff at the output of the power amplifier. The pickoff employs a half-wave
rectifier, followed by a simple capacitive chain divider for voltage scaling.
2. A precision peak detector, which obtains the peak value of the output from the voltage divider
during a modulation cycle and presents that value to the feedback loop.
3. An amplifier, having digitally selectable gain, which amplifies the output of the peak detector.
4. A sample-and-hold amplifier, used to hold the amplified output from the peak detector during
the “off-time” of the modulator. The SHA is needed since the time constant of the peak detector
is not sufficient to prevent significant “droop” during the off-time of the modulator.
5. An integrator (an example of frequency-selective filtering), which develops the control voltage for
the AGC loop from the output of the differencer.
6. A current shunt, not shown in Fig. 47.2, which is used to monitor the DC current supplied to the
power amplifier.
As can be seen from this simple example, several signal-conditioning functions may be employed
within a single system, and the system itself might not even be an instrument!

Defining Terms

Common-mode rejection (CMR): CMRR given in decibels. CMR = 20 log|CMRR|. CMR is a nonlinear
function of common-mode voltage and depends on other factors such as temperature.
Common-mode rejection ratio (CMRR): The ratio of the differential gain to the common-mode gain
of an amplifier.
Gain-bandwidth product (GBWP): The product of an amplifier’s highest gain and its corresponding
bandwidth. Used as a rough figure of merit for bandwidth.
Slew rate: The maximum attainable time rate of change of an amplifier’s output voltage in response
to a large step change in input voltage.

References

Cuthbert, T. R. 1983. Circuit Design Using Personal Computers. John Wiley & Sons, New York.
Graeme, J. G. 1973. Applications of Operational Amplifiers. McGraw-Hill, New York.
Graeme, J. G. 1977. Designing with Operational Amplifiers. McGraw-Hill, New York.
Graeme, J. G., Tobey, G. E., and Huelsman, L. P. (Ed.) 1971. Operational Amplifiers. McGraw-Hill, New York.
Gregory, B. A. 1973. An Introduction to Electrical Instrumentation. Macmillan, London.
Horowitz, P. and Hill, W. 1989. The Art of Electronics, 2nd ed. Cambridge University Press, New York.
Roberge, J. K. 1975. Operational Amplifiers. John Wiley & Sons, New York.
Sedra, A. S. and Brackett, P. O. 1978. Filter Theory and Design: Active and Passive. Matrix, Beaverton, OR.
Sheingold, D. H. (Ed.) 1976. Nonlinear Circuits Handbook. Analog Devices, Norwood, MA.
Stephenson, F. W. 1985. RC Active Filter Design Handbook. John Wiley & Sons, New York.
Stout, D. F. and Kaufman, M. (Ed.) 1976. Handbook of Operational Amplifier Circuit Design. McGraw-
Hill, New York.
Van Valkenburg, M. E. 1960. Introduction to Modern Network Synthesis. John Wiley & Sons, New York.

Further Information
IEEE Transactions on Instrumentation and Measurement. Published bimonthly by the Institute of Electrical

and Electronics Engineers.
IEEE Transactions on Circuits and Systems—II: Analog and Digital Signal Processing. Published monthly
by the Institute of Electrical and Electronics Engineers.

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