An operational amplifier or op-amp is an electronic circuit module (normally built as an integrated circuit) which has a non-inverting input (+), an inverting input (-) and one output. The output voltage is the difference between the + and - inputs multiplied by the open-loop gain: vo = (vp - vn) * Gopenloop. Since op-amps have uniform parameters and often standardized packaging as well as standard power supply needs, they help in designing an application fast.
A typical circuit symbol for an op-amp looks like this:
Its terminals are:
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2 AC Behaviour 3 Applications 4 See also |
DC Behaviour
Open-loop gain is defined as the amplification from input to output without any feedback applied. For most practical calculations, the open-loop gain is assumed to be infinite. This allows the gain in the application to be set simply and exactly by using negative feedback. Of course theory and practice differ, since op-amps have limits that the designer must keep in mind and sometimes work around.
AC Behaviour
The op-amp gain calculated at DC does not apply at higher frequencies. This effect is due to limitations within the op-amp itself, such as its finite bandwidth, and to the AC characteristics of the circuit in which it is placed. The best known stumbling-block in designing with op-amps is the tendency for the device to resonate at high frequencies, where negative feedback changes to positive feedback due to parasitic lowpasses.
Applications
The operational amplifier is so called because it performs mathematical operations by using voltage as an analogue of another quantity. This is the basis for the analogue computer.
The generic opamp has two inputs and one output. The output voltage is a multiple of the difference between the two inputs: vo = G(vp-vn), where G is the open-loop gain of the op-amp.
If the output is connected to the inverting input, after being scaled by a voltage divider, then
Most single, dual and quad op-amps available have a standardised pinout which permits one type to be substituted for another without wiring changes. A specific op-amp may be chosen for its open loop gain, bandwidth, noise performance, input impedance, power consumption, or a compromise between any of these factors. Historically, the first integrated op-amp to become widely available was the Fairchild UA-709, in the late 1960s, but this was rapidly superseded by the much better performing 741, which is easier to use, and probably ubiquitous in electronics - all of the main manufacturers produce a version of this classic chip. The 741 is a bipolar design, and by modern standards has fairly average performance. Better designs based on the FET arrived in the late 1970s, and MOSFET versions in the early 1980s. Many of these more modern devices can be substituted into an older 741-based circuit and work with no other changes, to give better performance.