Operational amplifier
An operational amplifier or op-amp is an electronic circuit module (normally built as an integrated circuit, but occasionally with discrete transistors or vacuum tubes) 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: Vout = (V+ − V−) * 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:
Notation
The power supply pins (VS+ and VS−) can be labeled many different ways. For FET based op-amps, the positive, common drain supply is labeled VDD and the negative, common source supply is labeled VSS. For BJT based opamps, the VS+ pin becomes VCC and VS− becomes VEE. They are also sometimes labeled VCC+ and VCC−, or even V+ and V−, in which case the inputs would be labeled differently. The function remains the same. Often these pins are left out of the diagram for clarity, and the power configuration is described or assumed from the circuit.
The input pin polarity is often reversed in diagrams for clarity. In this case, the power supply pins remain in the same position; the more positive power pin is always on the top, and the more negative on the bottom. The entire symbol is not flipped; just the inputs.
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 op-amp has two inputs and one output. (Some are made with floating, differential outputs.) The output voltage is a multiple of the difference between the two inputs:
- Vout = G(V+ − V−)
If the output is connected to the inverting input, after being scaled by a voltage divider K = R1 / (R1 + R2), then:
- V+ = Vin
- V− = K Vout
- Vout = G(Vin − K Vout)
- Vout / Vin = G / (1 + G K)
This negative feedback connection is the most typical use of an op-amp, but many different configurations are possible, making it one of the most versatile of all electronic building blocks.
When connected in a negative feedback configuration, the opamp will tend to output whatever voltage is necessary to make the input voltages equal. This, and the high input impedance, are sometimes called the two "golden rules" of opamp design (for circuits that use feedback):
- No current will flow into the inputs
- The input voltages will be equal to each other
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.
DC imperfections:
A component level diagram of the common 741 op-amp
Although it is useful and easy to treat the op-amp as a black box with a perfect input/output characteristic, it is important to understand the inner workings, so that one can deal with problems that will arise due to internal parasitic capacitances, etc.
Though designs vary between products and manufacturers, all op-amps have basically the same internal structure, which consists of three stages:
The sections in red are current mirrors. The top left current mirror allows large common-mode voltages on the inputs without exceeding the active range of any transistor in the circuit. The top right current mirror provides a constant current load for the output circuitry, regardless of the output voltage. The lower current mirror has a very low collector current, because of the 5 kΩ resistor. It is used as a high-impedance connection to the negative power supply, to provide a reference without loading the input circuitry.
The capacitor is used as part of a low pass filter (on the base of an emitter follower) to reduce the frequency response of the amp to prevent oscillations.
The green section is a voltage level shifter. It provides a constant voltage drop between the top and the bottom regardless of supply voltage. If the base current to the transistor is zero, and the voltage between base and emitter (and across the 7.5 kΩ resistor) is 0.625 V (a typical value for a BJT), then the current flowing through the 4.5 kΩ resistor will be the same, and will produce a voltage of 0.375 V. This keeps the voltage across the transistor, and the two resistors at 0.625 + 0.375 = 1 V. This serves as a bias for the two output transistors, to prevent crossover distortion. In some amps this function is achieved with diodes.
The offset null pins are used to remove any offset voltage that would exist at the output of the op-amp when zero signal is applied to the inputs.
The output in cyan is a push-pull amplifier. The output range of the amplifier is about 1 volt less than the supply voltage, since the collector-emitter voltage of the output transistors can never go completely to zero. The resistors in the output mean that the current provided by the output is limited (about 25 mA for the 741), and the output resistance is not zero without feedback. With negative feedback it approaches zero.
The resistors used in these configurations are typically in the kΩ range. <1 kΩ range resistors cause excessive current flow and possible damage to the device. >1 MΩ range resistors cause excessive thermal noise and bias currents.
Zout for all of the amplifiers is ideally 0 Ω. Realistically, it is 1 Ω to 1 kΩ, depending on the device.
Op-amp limitations
Although the design of most op-amp circuits relies on the "golden rules" above, designers should also be aware that no real op-amp can match these characteristics exactly. Listed below are some of the limitations of real op-amps, as well as how this affects circuit design.
AC imperfections:
Nonlinear imperfections:
Power considerations:
Internal circuitry
From the diagram, the blue section is a differential amplifier. The base current of the inputs is not really zero, giving the 741 an input impedance of about 2 MΩ.Common Configurations
Inverting amplifier
Non-inverting amplifier
Voltage follower
Difference amplifier
Summing amplifier
Integrator
(Vinitial is the output voltage of the integrator at time t = 0.)
Differentiator
Comparator
Schmitt trigger
Inductance gyrator
See also
External links