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The reason I wanted to ask this question is because I was designing a project for an electronics course I was taking last semester. The idea was simple. Take input from a microphone and amplify it sufficiently to turn an LED on/off depending on the frequency that was passed. Now, in my research I came across an article that stated OP Amps (unless very expensive) are a source of tremendous electrical noise in their high-gain applications. So, I resolved to a two-stage design with a BJT amplifier (~50 gain) and an OP Amp (~gain 2) for an overall gain of 100 to reduce the alleged noise issues.

So, in short, why do OP Amps have electrical noise issues (if at all), and what is the source? Why are Op Amps more noisy than other types of amplifiers (BJT and MOSFET, for example)?

It seems that any application of amplifiers in general would potentially be a source of 'amplified' noise if there exists such noise in the input signal. Perhaps Op Amps lack sufficient filtering capabilities that are inherent to BJT and MOSFET design? Op Amps are, internally, transistors anyway, so why the noise difference?

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    \$\begingroup\$ In short, physics. \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 2 '13 at 16:37
  • \$\begingroup\$ Very insightful. \$\endgroup\$ – sherrellbc Aug 2 '13 at 16:51
  • \$\begingroup\$ Honestly, the wikipedia entry gives a very concise answer. Most of what you see in the opamp internally is going to be thermal noise that is amplified and also noise relating from the fact that electrons aren't flowing in a continuous state. There are interactions at the quantum level that cause noise. \$\endgroup\$ – scld Aug 2 '13 at 17:28
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    \$\begingroup\$ "What is the root cause of noise?" and "Are op-amps more noisy than other kinds of amplifiers, and why?" would each be a very big topic address in a single answer. Either one borders on "too broad" for this site. Could you at least split this into two different questions so we don't have to write a complete textbook to answer? \$\endgroup\$ – The Photon Aug 2 '13 at 17:29
  • \$\begingroup\$ @ThePhoton Chris is right, the Wiki article addresses the concept of 'electrical noise' nicely. I meant for this question to be geared more towards your second suggestion: "Are op-amps more noisy than other kinds of amplifiers, and why?" I made appropriate edits to reflect this. \$\endgroup\$ – sherrellbc Aug 2 '13 at 17:32
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Why are Op Amps more noisy than other types of amplifiers (BJT and MOSFET, for example)?

I doubt this is true, especially if you consider the noise factor for the same circuit gain (but another answer might give more concrete information).

So, in short, why do OP Amps have electrical noise issues (if at all), and what is the source?

TI has a couple of good app notes on this:

Chapter 10 of Op-amps for Everyone

Noise Analysis for High-speed Op-amps

In short, the feedback resistors and the op-amp's input stage transistors both contribute to noise.

Perhaps Op Amps lack sufficient filtering capabilities that are inherent to BJT and MOSFET design?

Op-amp circuits can be filtered to whatever bandwidth you like. It only requires adding an extra capacitor or two to the op-amp circuit, or for higher-order filtering adding an additional filter stage after the high-gain stage. Which would be equally required for a discrete circuit.

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There will always be noise in a circuit. No amount of filtering will get rid of thermal noise. As to why an "op-amp" is more than a BJT or FET, it's because the internal circuitry of an op-amp is actually just more than one transistor. Each of those stages will introduce their own noise.

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