# GBW of an op-amp

I know that GBW is defined as the product of the open-loop voltage gain and the frequency at which it is measured but I'm still not too sure what it does and how it affects the performance of an op-amp perhaps? If I am given 2 op-amps with differing GBWs, do I pick the higher or lower value?

• You pick an op-amp that is suitable for the circuit it is intended to be used in. Commented Jul 10, 2020 at 16:35
• What do you mean. What if its 2 identical op-amps, only difference is the GBW Commented Jul 10, 2020 at 17:31
• Depending on the application circuit you might pick the one with lower GBW or, you might pick the one with higher GBW. And no, you can't say that an op-amp is the different in only one parameter - GBW knocks on to other parameters so pick the one that is most suited to your application (still not mentioned in your question). Commented Jul 10, 2020 at 17:40
• What are some applications where u need a high/low GBW? Commented Jul 11, 2020 at 7:27

GBW is useful to characterize the performance of an opamp, because most opamps have a "dominant pole" that causes the open-loop gain to decrease as frequency increases. This frequency response shows as a straight line on a log-log graph, and it can be defined by the frequency at which it crosses unity gain. It turns out that all other points on this line, when you multiply the gain by the frequency, give you the same number, and we call this the gain-bandwidth product. The only place this doesn't hold is at DC, where other factors limit the total available gain.

When you "close the loop" (apply negative feedback), the gain of the circuit cannot ever be better than the open-loop gain at any given frequency, but as long as the closed-loop gain is significantly less than the open-loop gain, the opamp will meet all of the normal expectations, and behave more or less "ideally".

So, for example, if you need gain of 100 (+40 dB) at up to 20 kHz for an audio application, you need to pick an opamp that has a GBW product that is significantly greater than 2 MHz.

Opamps with high GBW products tend to be more expensive, and also can be harder to stabilize, so you really don't want to go overboard and use way more GBW than your application actually needs.

• So does this mean that a GBW of 1Mhz is not suitable for a gain of 50 at 50kHz? Commented Jul 10, 2020 at 17:28
• Do the math! What do you think? Commented Jul 10, 2020 at 19:17

GBW is not a precise measurement so it must be greater than the needs of your design for Av* f-3dB << GBW. This is especially true on Filters where the Av*f is also multiplied by Q² to determine the minimum GBW required. ( little know fact)

If you study the datasheet and have detailed design expectations or "Specs", you will realize GBW is only 1 of 6 major specs or over 20 total specs.

Please learn to define your expectations using all environmental and component variations with price and availability. Then make an overall design "spec' list before you choose parts for inputs, outputs and process variables or functions.

Since Op Amps are unstable with negative feedback unless they have internal integrator compensation ( often near 10Hz ) the product of gain(G) and -3dB bandwidth (BW) is a constant. (GBW)

"Video" Amps on the other hand are designed for high gain without this compensation, so are defined differently.