handbook of modern sensors

5.2 Amplifiers 157
(A)
(B)
Fig. 5.5. General symbol of an operational amplifier (A) and gain/frequency characteristic of
an OPAM (B).
• Low intrinsic noise
• A broad operating frequency range
• A low sensitivity to variations in the power-supply voltage
• A high environmental stability of its own characteristics
For detailed information and application guidance, the user should refer to data books
published by the respective manufacturers. Such books usually contain selection
guides for every important feature of an OPAM. For instance, OPAMs are grouped
by such criteria as low offset voltages, low bias currents, low noise, bandwidth, and
so forth.
Figure 5.5A depicts an operational amplifier without any feedback components.
Therefore, it operates under the so-called open-loop conditions. An open loop gain,
A OL , of an OPAM is always specified but is not a very stable parameter. Its frequency
dependence may be approximated by the graph in Fig. 5.5B. The A OL changes with
the load resistance, temperature, and the power-supply fluctuations. Many amplifiers
have an open-loop gain temperature coefficient on the order of 0.2–1%/ ◦ C and a
power-supply gain sensitivity on the order of 1%/%. An OPAM is very rarely used
with an open loop (without the feedback components) because the high open-loop
gain may result in circuit instability, a strong temperature drift, noise, and so forth.
For instance, if the open-loop gain is 10 5 , the input voltage drift of 10 µV would cause
output drifts by about 1V.
The ability of an OPAM to amplify small-magnitude high-frequency signals is
specified by the gain–bandwidth product (GBW) which is equal to the frequency
f 1 where the amplifier gain becomes equal to unity. In other words, above the f 1
frequency, the amplifier cannot amplify. Figure 5.6A depicts a noninverting amplifier
where resistors R 1 and R 2 define the feedback loop. The resulting gain,
A = 1 + R 2 /R 1 , is a closed-loop gain. It may be considered constant over a much
broader frequency range (see Fig. 5.5B); however, f 1 is the frequency-limiting factor
regardless of the feedback. The linearity, gain stability, the output impedance, and
gain accuracy are all improved by the amount of feedback and now depend mainly
on characteristics of the feedback components. As a general rule for moderate accu-