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?
Best Answer
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).
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.
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.