Op-amp parameters explained

It can’t be shown often enough - the most elegant building block in electronics:

In an ideal op-amp …

• the V+ and V- inputs have infinite resistance
• there is zero current into and out of both V+ and V-
• if V+ is above V-, however slightly, then Vout will be Vs+
• if V- is above V+, however slightly, then Vout will be Vs-
• when V- is V+, Vout will be halfway between Vs+ and Vs-
• the gain of the amplifier is infinitely large
• when V+ and V- change by the same amount, Vout stays the same
• as long as in-/outputs are within Vs+ and Vs-, changes to Vs+ and Vs- have no effect
• the behaviour is identical for any frequency, temperature, and time span

Back on earth, things are a bit less rosy. Here are the main types of imperfections in real-world op-amps (with page references to The Art of Electronics book, 3rd edition):

Input Bias Current - TAoE p302

There will be a small current flowing into or out of the V+ and V- pins, called the bias current. This matters with high resistance values, i.e. with a 10 MΩ + 1MΩ 11:1 input divider, we are dealing with currents in the order of 0.1 to 1 µA through these resistors (when measuring a few volts). A bias current of 1 µA would completely mess up our measurements.

For a simple run-of-the-mill LM324 unit, the bias current is 15 nA typ, and 100 nA max (0.1 µA).

The equally-common CMOS-based LMC6482 has a minute 0.02 pA typ (20 femto-A!) and 4 pA max bias current, although it’ll be an order of magnitude higher when the temperature is 75°C.

Somewhere in between, in terms of bias current, lies the OPA277: 0.5 nA typ, 1 nA max.

Input Offset Voltage - TAoE p304

Ideally, shorting both input pins should generate a 0V output. In reality, the input stage is never 100% perfectly matched (although some chips are laser-trimmed to get them really close).

This input offset gets amplified and ends up affecting the output. This becomes much more of an issue when the gain of the op-amp is high.

For the LM324, the input offset voltage is 2 mV typ and 5 mV max. A more high-end “precision” op-amp such as the OPA277 has 20 µV typ, 50 µV max as specs.

Input offset voltage can be “trimmed away” with a small trimmer potentiometer and some extra resistors, if needed. What remains is a small temperature-dependent “input offset voltage drift”.

Common Mode Rejection Ratio - TAoE p305

Ideally, all an op-amp cares about is the difference between V+ and V-. In reality, the behaviour changes depending on whether both are near Vs+ or near Vs-. This change is the common mode, i.e. the difference stays the same, but both inputs are changing their voltage levels in common.

The CMRR is the rejection ratio. If a 1V change in both inputs leads to an unwanted offset of 1 mV, then we have a 1000-fold relationship between absolute changes and differential error.

For relative values, especially large ones, it’s more convenient to use the decibel as unit: +20 dB is ten times as much voltage. A 1 mV error per 1 V change is equivalent to a CMMR of 60 dB.

For the LM324, the CMRR is 80 dB typ (i.e. 0.1 mV/V), and 65 dB min. The high-end OPA277 has a better CMRR of 140 dB up to 10 Hz, dropping to ≈ 95 dB at 1 KHz and ≈ 35 dB at 1 MHz.

Power Suppy Rejection Ratio - TAoE p305

Similarly, there is some effect from power supply voltage variations. This can matter when the power supplies are not highly regulated, as some of that ripple might show up on the output pin.

For the LM324, the PSRR is 100 dB typ, and 65 dB min. For the OPA277, these values are again more accurately specified in the datasheet with a graph, going from 130 dB up to 100 Hz, and then dropping off at that same 20 dB per frequency decade as the CMRR.

Something to watch out for is that the PSRR is not necessarily the same for Vs+ and Vs-.

Slew Rate - TAoE p307

The slew rate of op-amps is often intentionally limited. This is the rate at which the output pin voltage can change. A high slew rate means that the op-amp can quickly track changes, but can also cause problems with accuracy, unwanted feedback, “ringing” and even oscillation.

For the LM324, the slew rate is 0.5 V/µs. For the LMC6482 it is 1.3 V/µs, and for the OPA277 it is 0.8 V/µs - all specified as their typical values. Some op-amps have a slew rate over 1000 V/µs.

Bandwidth - TAoE 247

All the common internally compensated op-amps have a fairly low frequency where they start to reduce their gain at a fixed 6 dB per octave (or equivalently: 20 dB per frequency decade). This is essential for op-amp stability - it acts like an RC low-pass filter, built right into the op-amp.

With such a downward slope, at some point the gain plot will drop below the 0 dB line. This is the frequency where the gain of the op-amp becomes less than 1.

For the LM324, this is typically at around 1 MHz, for the LMC6482 it’s 1.5 MHz, and for the OPA277 it’s also 1 MHz. Some high-end op-amps can go well into the 100’s of MHz.

Output Current

Most op-amps can only drive a fairly small amount of current - although there are exceptions.

The LM324 can sink or source at least 20 mA, the LMC6482 at least 11 mA, and the OPA277 also some 20 mA. Although with these larger currents, the output voltage range will be reduced.

Large output currents can lead to internal heat-up, which in turn may affect some of the op-amp’s parameters, so generally it’s not a good idea to load an op-amp with less than say 1 kΩ. For higher current, there are numerous ways to add an output driver, transistor, MOSFET, etc.

Input / Output Voltage Range

Not all op-amps can handle inputs swinging all the way from Vs+ down to Vs-. When they do, they are called “rail-to-rail in” (RRI).

Likewise for the output: it’s relatively difficult for an op-amp to drive its output pin all the way up to Vs+ or down to Vs-. Those that can are called - wait for it…“rail-to-rail out” (RRO).

Some op-amps are champions in this league and can do both - these are designated as “RRIO”.

The LM324 can go from Vs- to about Vs+ - 1.5V, which makes it very suitable for “single-rail” operation, i.e. running off a single power supply, tied to Vs+, with Vs- tied to ground. It also means that with a 5V supply, it can’t handle (or generate) signal levels above 3.5V.

The LMC6482 is RRIO, its output can swing to within 0.1V of both voltage rails. Interestingly, and this is often the case with RRI, it can in fact handle input signals levels slightly beyond its two supply voltage levels.

The OPA277 isn’t happy with input voltages less than 2V away from its supply rails, and its output also stays within 0.5V above and 1.2V below its power supply voltages, respectively.

Winding down…

This concludes our little tour of op-amp parameters for now. In many cases, you can get a lot of mileage by just picking a few op-amps to work with, with somewhat different trade-offs and properties, and sticking to them where possible. The LM324 (quad) and LMC6482 (dual) or LMC6484 (quad) units are low-cost and capable. And these are fairly arbitrary picks, really.

For more information than you’ll ever need, with all the background, detailed comparisons, and a huge variety of example circuits, see the Art of Electronics book by Horowitz and Hill. The 3rd edition came out in 2015, but earlier editions are also still an impressive and useful resource.