Page 1195 - The Mechatronics Handbook

Amplification
Amplification is generally accomplished by an operational amplifier, an instrumentation amplifier, or an
isolation amplifier.

Operational Amplifiers
A conventional operational amplifier (op amp) has a differential input and a single-ended output. An
ideal op amp, used often as a first approximation to model a real op amp, has infinite gain, infinite
bandwidth, infinite differential input impedance, infinite slew rate, and infinite common-mode rejection
ratio (CMRR). It also has zero output impedance, zero noise, zero bias currents, and zero input offset
voltage. Real op amps, of course, fall short of the ideal in all regards.
Important parameters to consider when selecting an op amp include:
1. DC voltage gain K 0 .
2. Small-signal gain-bandwidth product (GBWP) f T , which for most op amps is f T K 0 f 1 , where f 1
is the lower break frequency in the op amp’s transfer function. The GBWP characterizes the closed-
loop, high-frequency response of an op-amp circuit.
3. Slew rate, which governs the large-signal behavior of an op amp. Slew rates range from less than
1 V/µs to several thousand volts per microsecond.
Other parameters, such as input and output impedances, DC offset voltage, DC bias current, drift voltages
and currents, noise characteristics, and so forth, must be considered when selecting an op amp for a
particular application.
There are several categories of operational amplifiers. In addition to “garden-variety” op amps there
are many op amps whose characteristics are optimized for one or more classes of use. Some categories
of op amps include:
1. Low-noise op amps, which are useful in the portions of signal conditioners required to amplify
very-low-level signals.
2. Chopper-stabilized op amps, which are useful in applications requiring extreme DC stability.
3. Fast op amps, which are useful when large slew rates and large GBWPs are required.
4. Power op amps, which are useful when currents of greater than a few mA must be provided to
the op amp’s load.
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5. Electrometer op amps, which are used when very high (>10 Ω ) input resistances and very low
(<1 pA) input bias currents are required.
An introduction to op amps and basic circuit configurations occurs in essentially any modern text on
circuit theory or electronics, and the reader can find detailed theoretical developments and many useful
configurations and applications in Roberge (1975), Graeme et al. (1971), Graeme (1973, 1977), Horowitz
and Hill (1989), and Stout and Kaufman (1976).

Instrumentation Amplifiers
Instrumentation amplifiers (IAs) are gain blocks optimized to provide high input impedance, low output
impedance, stable gain, relatively high common-mode rejection (CMR), and relatively low offset and
drift. They are well suited for amplification of outputs from various types of transducers such as strain
gages, for amplification of low-level signals occurring in the presence of high-level common-mode
voltages, and for situations in which some degree of isolation is needed between the transducer and the
remainder of the instrument.
Although instrumentation amplifiers can be constructed from conventional op amps [a three-op-amp
configuration is typically discussed; see, for example, Stout and Kaufman (1976)], they are readily available
and relatively inexpensive in IC form. Some IAs have digitally programmable gains, whereas others are
programmable by interconnecting resistors internal to the IA via external pins. More-basic IAs have their
gains set by connecting external resistors.

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