Operational amplifiers (op-amps) are complex circuits made up of capacitors, resistors and transistors that can be used to amplify and compare different voltages. However, when used with other external components, they can be used to make many different types of circuits and, as a result, are found in nearly every single electronic circuit in existence.
Choosing an op-amp for an application is no trivial task, so what parameters can affect op-amp circuits and why should designers care?
Op-amp characteristics
The ideal op-amp is a component that has two voltage inputs and a single output that will multiply the difference between the voltage inputs by infinity. Ideal op-amps also have infinite input resistance and zero output resistance, which would make them the perfect measurement circuit. However, real op-amps are made up of real components, which means that their characteristics are far from ideal and these non-ideal characteristics will have a noticeable impact on circuits they are used in.
Simple circuits such as inverters and basic amplifiers can, in most cases, ignore these non-ideal properties, but more advanced circuits that measure small signals will need to consider them (for example, a DIY ECG that measures heart rate would require a specialized instrumentation amplifier with a high-input impedance (i.e. high input resistance).
The op-amp symbol and the internal schematic of the 741 op-amp.
Input voltage range
Input voltage range is the range of voltages that the op-amp can accept on its input pins. This characteristic typically has two components that need to be carefully considered: near-ground sensing and near-rail sensing. While the input of an op-amp may be able to reach or cross 0 V, the op-amp may not be able to sense voltages close to ground (for example lower than 10 mV). If small signals are being measured by an op-amp, then near-ground sensing will be a must. The same applies to the power rails whereby input voltages may not be detectable if they approach the voltage of the op-amp supply. A common industrial op-amp, the LT1493, has near-ground sensing but cannot amplify voltages greater than VCC = 1.5 V.
Open loop gain/bandwidth
When discussing op-amps, gain refers to the amplification factor of the op-amp. For example, a gain of 10 would be an amplification factor of 10 such that a signal of 1V is amplified to 10V. The ideal gain of an op-amp when in an open-loop configuration (i.e., no feedback) would be infinity, but real op-amps have a finite open loop gain. Open loop gain can be used to determine how sensitive a comparator is such that, for example, a voltage difference of 0.1 mV may produce a voltage output change of 5 V, which may not be appropriate. But open loop gain is more commonly linked with bandwidth, which determines how fast an op-amp can operate when used in closed-loop amplifier circuits. Op-amps with a lower unity-gain bandwidth will have a smaller gain at higher frequencies and, therefore, negative feedback amplifiers will not operate as expected at those frequencies. The AD8608, for example, has a gain bandwidth product of 10 MHz, which is significantly higher than the LT1493, which has a gain bandwidth product of 1.2 MHz.
Output voltage swing
The output voltage swing of an op-amp is the range of output voltages that the op-amp can provide. Older designs based on BJTs (such as the 741) are able to get an output voltage range of ±2 V of the supply such that if the op-amp was powered by a ±15-V supply, the maximum/minimum voltage would be ±13 V. Newer designs based on MOSFETs may include a rail-to-rail output stage, which allows the output to swing fully to either supply. For example, the AD8574TRUZ op-amp has rail-to-rail capability, with the output voltage ranging between 1 mV and 4.998 V when powered with a 5-V supply and an output load of 10 kΩ.
Output resistance
While the ideal op-amp has zero output resistance, real op-amps have a finite non-zero output resistance. This can cause problems in scenarios in which an amplifier circuit needs to amplify a small signal and then drive a secondary circuit. An ideal secondary circuit would have infinite resistance so that all of the voltage from the op-amp is transferred to the secondary circuit, but because that stage will also have a finite input resistance, only a portion of the output voltage from the op-amp will be transferred. The output resistance of an op-amp also depends on its configuration and if negative feedback is being used (negative feedback often lowers the output resistance). Op-amps such as ADA4075-2 with 40mA output current capability enable very low closed loop output impedances, such as < 1 Ohm up to 100kHz.
Output short-circuit current
This characteristic of op-amps is important for determining if an op-amp circuit is capable of driving a secondary circuit. For example, an op-amp comparator may be used in a circuit that detects when a PIR sensor detects an intruder and then turns on a relay enabling the security lights. However, relays can require large coil drive currents (such as 30 mA on the V23105A relay), and therefore, an op-amp with a too-small short-circuit current will not be able to drive the relay. As such, an external driver stage will be needed, which could be either a specialized driver IC or even another op-amp in a unity gain configuration (i.e. an amplifier with a gain of 1). The output short-circuit current may also be different for sourcing and sinking. For example, the LTC6363IMS8 has a sinking short-circuit capability of up to 40 mA but can source up to 90 mA.
Power consumption/supply current
The supply current to an op-amp can be a very important consideration in low-power applications being that current consumption is highly linked to both power consumption of a component as well as wasted energy from linear regulators that may be regulating the power supply. However, an op-amp that consumes a large amount of current will not necessarily have a large power consumption, which is why these two figures should be looked at when choosing an op-amp. If low power consumption is important, then op-amps labeled with “nanopower” offer incredibly small power consumptions due to their low quiescent current consumption and, therefore, improve battery life. For example, the AD8500 is a nanopower amplifier that is ideal for portable equipment including smoke alarms and PIR motion sensors for security.
Conclusion
Choosing the right op-amp for the job can be a challenge. One op-amp, for example, may have the perfect input resistance characteristics but could consume too much current, while another op-amp is capable of sensing near-ground but cannot swing its output fully. Some designers may choose to combine multiple op-amps to produce a circuit with all of the required characteristics, which is one of the reasons why instrumentation amplifiers are popular. Either way, there is always an op-amp for every job!