What is ACTUALLY meant by op amp frequency range?

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.

Thank you for being kind and for trying to answer my question.  I'm just not understanding what is "varying" re the frequency? Is it my input signal going from one frequency to another? What if I'm just applying a fixed voltage at the input of an op amp? No frequency variation. Just a voltage in and a voltage out with the op amp's gain. Is the bandwidth related to a "varying" input signal where the variation is occuring at a certain frequency? In a general sense if I'm building a circuit that's just going to gain up the output voltage compared to a fixed input voltage do i even need to consider bandwidth? If so why? I'm not varying the input signal at any frequency so why should I consider the op amp's frequency range. 

If I've built a vco with a frequency range of about five octaves and I'm going to have its output feed a filter circuit with this varying frequency vco waveform, I'm guessing i need to know if the op amp can provide a stable signal across that particular frequency range. 

It's funny because I've asked several engineering professors why I need to consider the frequency range of an op amp and all they explain is "how" to plot it / the value poles of cutoff etc... but literally none of them can explain what is varying at 10MHz or 100GHz or whatever for an op amp being used in a differentiator or integrator circuit which is being fed a fixed voltage signal - or one that I'm manually varying it's input voltage with a potentiometer - at a very slow change of resistance.

Maybe I'm over-thinking the concern of frequency range for the op amp and the type of circuit I'm using it in.  Maybe if i was using the op amp in a high frequency circuit where the input is changing rapidly THEN I'd need to be concerned about it's frequency range... it's these kinds of questions that no professor really addresses - they just slam you with theory and equations lol!! Thanks for any practical help.  

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

Thank you MK... I think this is making sense now... If I'm using an op amp as an audio amplifier then I need to consider an op amp that can operate over the audio range (20Hz to 20kHz) - meaning if I'm sending it a signal from my VCO then that signal has to work over that range.  If I'm using an op amp as a buffer or comparator but only varying the input voltage manually with a potentiometer then i really don't need to be concerned with the frequency range of the op amp because its input signal is only varying by the changes I'm making with a potentiometer - very low Hz - like as fast as i can twist the pot knob - so I'm not worried about the op amp frequency response.  Seems to me that frequency range and response for an op amp is really only important for very high speed input frequency circuit needs - as most op amps seem very stable over the relatively low frequency range of 20Hz to 20kHz. If I'm designing a microwave receiver or transmitter then I might need to consult the op amp spec sheets for it's frequency response.  If I'm just designing in the audio realm then there seems to be no need to worry about frequency response. I can't think of a situation that I need to be concerned with frequency response and gain drop-off when I'm using the op amp in audio circuits as most common op amps I've been experimenting with seem to be very stable and work just fine over the audible range.  If I'm wrong please correct me otherwise thank you for taking your time in answering my initial questions.  Happy upcoming holidays to you!!! 

You always need to think about the suitability of an op amp for a particular purpose.

You are correct in that most well known general purpose cheap op amps are OK for audio (ie will work but maybe not very well).

But very low power op amps may well have GBW so low that they can't readily cope with 20kHz.

A common low cost op amp is the LM324 - it has GBW = 1.2MHz and a slew rate of only 0.5V/us - both much too slow for most audio purposes.

A favourite general purpose op amp for audio is the TL072 (dual, or TL071 single). It has GBW  = 3MHz and slew rate is 13V/us. But you won't find a TL072 in modern high performance audio.

I use OPA192 family op amps a lot -

https://www.ti.com/product/OPA192

they have very good DC performance and reasonable GBW and slew rate.

The downsides are that they need at least a 4.5V supply and are quite expensive (and only come in surface mount packages.).

Probably quite a good choice in a synthesiser VCO (low offset and drift and quite fast enough).

But you should get into the habit of studying the data sheets of any parts that you use very carefully because it is such a good source of information about ciruit design in general.

MK

Yes you're absolutely right - thank you for offering your knowledge and experience.  Most appreciated!! 

Yes you're absolutely right - thank you for offering your knowledge and experience.  Most appreciated!!