What is ACTUALLY meant by op amp frequency range?

Question

Love all the videos... I have one nagging question that I can't seem to fully understand. When we speak of am op amps bandwidth, what area we actually talking about? I understand "frequency range" but are we saying this is the value of the input signal frequency? Isn't the input signal usually a fixed voltage - say in an inverting op amp circuit? What is actually changing at the frequency? If I'm using an op amp to design a synthesizer vco, does this mean my frequency range is the audible range? Why do I need to be concerned about frequency range and roll-off if I'm just applying a voltage to one is the op amp inputs? Please explain - thank you!

Jan Cumps · Answer

tmdoom said: I understand "frequency range" but are we saying this is the value of the input signal frequency? 
 It is the frequency range where the opamp has "enough" gain. Often represented by the -3dB drop-off point 
 
 tmdoom said: Isn't the input signal usually a fixed voltage 
 No. Ually i is an analog, changing signal 
 
 tmdoom said: What is actually changing at the frequency? 
 The gain, and the phase shift 
 
 tmdoom said: does this mean my frequency range is the audible range? 
 An opamp by itself can't be a VCO. You 'll need an oscillator circuit that can be influenced by a voltage. This could be made with opamps. 
 tmdoom said: Why do I need to be concerned about frequency range and roll-off if I'm just applying a voltage to one is the op amp inputs? 
 Show your circuit. If you apply a voltage to an opamp, that's DC. It will not become a VCO.

michaelkellett · Answer

The thing is - it's complicated. 
 And we can't explain it fully in an E14 reply. 
 But I'll try to give some pointers. 
 An op-amp is a theoretical thing, with infinite gain at all frequencies. 
 Of course real op amps are not that good - but at steady state (often called DC) or low frequencies the infinite gain and ignore-the -frequency approximation holds true. 
 The usual single figure approximation of a real op-amp's frequency sensitivity is its "Gain BW product" and assumes that the gain reduces as the frequency increases. 
 If GBW = 1,000,000 then the amplifier will have a gain of 1 at 1MHz, 10 at 100kHz, 100 at 10kHz and so on. It's really a high frequency approximation so can't be used to predict the gain at low frequencies. High and low frequencies in this context is relate to the GBW of the op amp. Generally a high frequency is anything above GBW/10 and low is anything below GBW/1000. 
 Generally (big approximation coming) for a ciruit to work the way you expect then the working frequency should be less than GBW / (10 x working gain). If its a filter it should be (10 x working gain x Q). (Q is the sharpness of the filter - any more explanation is outside the scope of this reply ). 
 So an audio amplifier with a gain of 10 and a required bandwidth of 20kHz needs a GBW of at least 20,000 x 10 x 10 = 2MHz. 
 There are other very important aspects of real op amps that usually need to be considered like: 
 steady state things: 
 input offset voltage 
 input offset current 
 input bias current 
 
 dynamic things: 
 slew rate 
 noise 
 tricky things: 
 input common mode range 
 output voltage range 
 offset and bias change with temperature 
 distortion/linearity 
 output drive current capability 
 
 Like I said - its complicated - but if you read a lot and look at a lot of circuits, build a lot of circuits, measure aloc, simulate aloc then it will become clearer ! 
 MK

What is ACTUALLY meant by op amp frequency range?

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