Voltage Follower - Operational Amplifier

The op-amp voltage follower is about as simple as you can get when it comes to setting up an op-amp. But despite the ease with which you can set up a voltage follower, this doesn’t make this configuration any less valuable or common.

When an op-amp is set up in the voltage follower configuration, the output voltage is the same as the input voltage. In an ideal world, it’s the exact same - no amplification, distortions, variations, phase shifts, anything. Which, when you hear that, you may wonder, what’s the point?

It all goes back to the first of the two main rules of op-amps, as discussed in the intro to op-amps. Op-amp inputs don’t sink or source any current - they’re extremely high impedance. An op-amp output, however, is able to sink or source quite a bit of current - they’re extremely low impedance. So, a voltage follower can be set up as a buffer, changing a low impedance output to a high impedance output.

As an example, we can think of many passive devices, such as certain thermocouples or pH sensors. Their output is a voltage differential but they can’t source very much current, being very high-impedance, before the load causes a voltage droop (similar to the issues we talked about with voltage dividers), which makes it very difficult to get an accurate measurement of the voltage. To overcome the problem of changing the value while reading it, we put that high-impedance output into the op-amp’s high-impedance input, and then measure the op-amp's low-impedance output without fear of affecting the device under test.

If there’s one thing you’ll learn as an engineer, is that you never get something for nothing. So, what are the drawbacks?

• Higher part and pin count. To use an op-amp in the circuit, you need to buy the part, allocate space for it, provide any supporting components, and make sure you have the appropriate power provided. This may not seem like such a big deal when you’re breadboarding or only making 5-10 parts, but if you’re making a design for 10,000 or 10,000,000 parts, those pennies add up very, very quickly.
• More complexity. This is related to the previous point but I consider it a standalone issue. Where there is greater complexity, there is greater chance of failure. Either failure in the design stage to catch something important, failure during the manufacturing stage when it’s being built, or failure when being used.
• Real-life restrictions. Op-amps are not perfect and their limitations may be enough to not work properly in your applications. Everything that we assumed was ideal - no amplification, distortions, variations, phase shifts, and infinite input and zero output impedance - none of these are actually true. There are minute amounts of all of these. There are many different op-amps out there and while a general op-amp is fine for many applications, if there are certain features that are more important than others, you can buy specialty op-amps that will trade-off certain performance characteristics for others.

Even with these drawbacks (applicable to any op-amp circuit, not just voltage followers), the issues are fairly minimal and in many cases, easily ignored or accepted. When a voltage follower is needed, it’s almost always worth it.