Using opamp with GBP of 500kHz - 1Mhz for amplifying 20mV p-p signal to 5V p-p

[KamalS]

The output of my sensor is a sinosoidal (40KHz) voltage of 20mV p-p and I need to amplify this signal WITHOUT distortion and/or clipping to 5V p-p.
I have opamps available with me that have GBP ranging from 500Khz to 1Mhz.

Assuming I keep a 20% margin on the GBP specs, the 1MHz opamp reduces to 800KHz GBP.

Keeping this at mind, would it be possible to achieve the reauied amplification by:

1. Using single opamp
2. Using cascaded opamps from the SAME IC?

 

[AdrianN]

logical inverter op-amp 741

5Vpp output for an input signal of 20mVpp makes a gain of 250. You have 1MHz Op Amps, so the gain bandwidth product is 1MHz. If you divide 1MHz at a gain of 250, the result is 4kHz, which is 10 times lower than your 40kHz signal.

You need to split the gain in two, say 2 Op Amps with the gain of 16. Use a low noise Op Amp for the first stage.

Keep in mind that the amplitude starts dropping before the corner frequency. The corner frequency amplitude is at 3dB lower than the amplitude in band. Therefore, your bandwidth needs to be higher than the 40kHz signal.

 

[millwood]

741 op amp gbp

AdrianN's analysis is correct, for voltage feedback opamps. highly likely that's what you have.

you can use two (slow) opamps, or one fast opamp (ths4012 for example), or you can go with current feedback opamps.

 

[Audioguru]

op-amp gain gbp

Don't forget that a slow opamp usually has a poor slew rate. A lousy old LM358 or LM324 "low power" opamp barely has full output at 1kHz.
A half-decent opamp has full output at 100kHz.

 

[millwood]

gbp op amp

Don't forget that a slow opamp usually has a poor slew rate.

a slow opamp necessarily has a poor slew rate. bandwidth and slew rates are essentially two different measurements of the same thing.

 

[Old Nick]

opa637 corner frequecy

Don't forget that a slow opamp usually has a poor slew rate.

a slow opamp necessarily has a poor slew rate. bandwidth and slew rates are essentially two different measurements of the same thing.

that's not really correct.

The rated bandwidth of an amp is a measure of the small signal response, and doesn't take slew into consideration.

Slew is a large signal property.

 

[Audioguru]

group delay in op amp and bandwidth

The lousy old 741 opamp has such a poor slew rate that its max full output frequency is only 9kHz. Motorola made a 741S that had the same GBP frequency of 1MHz but its slew rate was much higher.

 

[millwood]

opamp stable slow feedback

fast amps will have higher slew rates, in general.

Of course, you can find a slow amp with a higher slew rate than a faster amp. no doubt about that.

But in general, you will find faster amps with higher slew rates.

the definition of slew rate, dv/dt, is a small signal definition. that's how you get the minimum slew rate requirement for an amp output a sinus signal.

in reality, most slew rate measurements are large signal: it is practically more difficult to measure small signal properties than large signal properties.

 

[LvW]

cfb opa627

fast amps will have higher slew rates, in general.
Of course, you can find a slow amp with a higher slew rate than a faster amp. no doubt about that.
But in general, you will find faster amps with higher slew rates.
the definition of slew rate, dv/dt, is a small signal definition. that's how you get the minimum slew rate requirement for an amp output a sinus signal.
in reality, most slew rate measurements are large signal: it is practically more difficult to measure small signal properties than large signal properties.

I am not willing to search for counter examples now - that means slow opamps with high slew rates and vice versa; however, one point requires a correction:

The slew rate is NOT a small signal parameter.
In contrary - it is measured with 100% negative feedback and with a voltage excitement which will bring the first opamp stage into saturation. Only then the opamp can "slew". It is a typical large signal parameter.

 

[millwood]

gbp opam

The slew rate is NOT a small signal parameter.

again, it depends. there are two ways to define slew rates: SR=dv/dt. that by definition, is a small signal measurements.

in practice, most slew rates are measured with the time the output swings to 90% of its full output (aka when the amp settles), and the input is typically a pulse / step function.

as to what constitute a "large signal", there is no generally accepted consensus: should an amp swing to 90/80/50/20% of its maximum output? with what kind of load? etc.

In contrary - it is measured with 100% negative feedback and with a voltage excitement which will bring the first opamp stage into saturation. Only then the opamp can "slew". It is a typical large signal parameter.

I am not sure what "100%" negative feedback means: gain=+/-1? it will be very hard to saturate a typical (VFB) opamp with a differential pair because of the negative feedback. the only way you will saturate it is for it to go into clipping. it would be quite unusual to measure SR, or any analog performance, with the amp goes into clipping.

also, keep in mind that many factors impact slew rates so they are just "generally" comparable from amp to amp, and even from application to application of the same amp. manufacturers sometimes give out different slew rates under different test conditions precisely because of that.

 

[LvW]

tpa6120 feedback

I´m sorry but most statements of your reply are definitely false.

Quote: again, it depends. there are two ways to define slew rates: SR=dv/dt. that by definition, is a small signal measurements.

No, this is not true. The formula is correct, but it has to measured under large signal conditiones. Otherwise it is a measure of the small signal rise time.

Quote: in practice, most slew rates are measured with the time the output swings to 90% of its full output (aka when the amp settles), and the input is typically a pulse / step function.

According to the formula above dv/dt is the slope of the opamp response - independent on the 90% point.

Quote: as to what constitute a "large signal", there is no generally accepted consensus: should an amp swing to 90/80/50/20% of its maximum output? with what kind of load? etc.

I have mentioned already what "large signal" means. I repeat: It does NOT depend on any output level, but instead on the input stage condition. The first stage must go for a short time into saturation - until the feedback works and brings it back to the linear region. This effect determines the slew rate,

Quote: I am not sure what "100%" negative feedback means: gain=+/-1?

This question touches basics of the feedback theory. 100% feedback means that 100% of the output are fed back. It´s quite logical. For an inverter with a gain of "-1" only 50% are fed back (opamp basics).

Quote: also, keep in mind that many factors impact slew rates so they are just "generally" comparable from amp to amp, and even from application to application of the same amp. manufacturers sometimes give out different slew rates under different test conditions precisely because of that.

Partly, this is correct. Slew rate is specified sometimes for different test conditions (for example upwards/downwards). But always with feedback and large input signals (see above).

 

[millwood]

Re: Opamp GBP

I´m sorry but most statements of your reply are definitely false.

Quote: again, it depends. there are two ways to define slew rates: SR=dv/dt. that by definition, is a small signal measurements.

No, this is not true. The formula is correct, but it has to measured under large signal conditiones. Otherwise it is a measure of the small signal rise time.

dv/dt is by definition an infinitesimal (read: infinitely small) change of voltage over an infinitesimal change in time.

if that doesn't constitute a "small" signal definition, what else does?

what you described is delta v / delta t: a finite change in voltage over a finite period of time.

this is just basic calculus. so if you agree that SR=dv/dt is correct (which you did), you have to disagree with yourself about whether SR is a small signal or a large signal definition.

Quote: in practice, most slew rates are measured with the time the output swings to 90% of its full output (aka when the amp settles), and the input is typically a pulse / step function.

According to the formula above dv/dt is the slope of the opamp response - independent on the 90% point.

a "slope", sir, is a small signal definition because it was talking about dv/dt.

the threshold is needed because almost all circuits require some settlement time.

Quote: as to what constitute a "large signal", there is no generally accepted consensus: should an amp swing to 90/80/50/20% of its maximum output? with what kind of load? etc.

I have mentioned already what "large signal" means. I repeat: It does NOT depend on any output level, but instead on the input stage condition. The first stage must go for a short time into saturation - until the feedback works and brings it back to the linear region. This effect determines the slew rate,

your are talking about group delay. read into TIM-D will help you in that regard.

Quote: I am not sure what "100%" negative feedback means: gain=+/-1?

This question touches basics of the feedback theory. 100% feedback means that 100% of the output are fed back. It´s quite logical. For an inverter with a gain of "-1" only 50% are fed back (opamp basics).

first of all, there are plenty of amps whose SRs are measured at anything over than gain of 1, +/-.

and 2ndly, there are plenty of amps who are not unity stable and cannot possibly work at unity gain.

your approach would have yielded no-meaningful SRs for those amps.

Partly, this is correct. Slew rate is specified sometimes for different test conditions (for example upwards/downwards).

very rarely SRs are specified on two directions.

typically, they are specified at different supply voltage, into different load, or under different gains, etc.

But always with feedback and large input signals (see above).

it is done with feedback because few amps, especially opamps, are used open loop.

on the 2nd point, you have to be able to separate how something is defined in theory and how that same thing is measured in real life. I said very early on that SRs are indeed measured in with large signal because of the difficulties in measuring it small signal - you would be measuring the slope of the waveform.

don't think so? take our your college calculus book and work out the formula for the minimum SR to reproduce a sine wave.

you can get some help from "Opamp application handbook" by Walter Jung. this very issue was talked early in the chapters.

OK, I cannot resist. the max slew rate for a sine signal V=Vp*sin(2pi()ft) is 2pi()fVp, which happens when t=n*/(2f), where n=0, 1, ......, pi()=3.14......

that signal will reach Vp, its max voltage, at t=1/4f. so the "large signal" SR would be 4fVp<2pi()fVp.

in other words, if you have an amp that can reproduce Vp in time, it wouldn't be fast enough to reproduce the sine wave.

hope this helps.

 

[Audioguru]

Re: Opamp GBP

I have mentioned already what "large signal" means. I repeat: It does NOT depend on any output level, but instead on the input stage condition. The first stage must go for a short time into saturation - until the feedback works and brings it back to the linear region. This effect determines the slew rate.

No.
Most opamp datasheets show the Power Bandwidth (maximum peak output voltage per frequency) at variable output amplitudes and at variable frequencies. The slew rate affects the output amplitude and frequency where a sine-wave distorts and its p-p level drops as it begins to convert to a triangle-wave.

Most half-decent opamps can have a sine-wave at the output that is 28V p-p at 100kHz without distortion. The lousy old 741 opamp reaches only 9kHz. At 100kHz a 741 opamp has a max output of only 3V p-p.

 

[millwood]

Re: Opamp GBP

a good opamp that is NOT unity gain stable is the opa637 (its sister, opa627, IS unity gain stable).

so it doesn't hurt to take a look at the datasheet and see how TI specified its SR, and whether it is at "100% negative feedback".

after that, you can try tpa6120 / ths6102 and see whether their SR is specified at "100% negative feedback" too.

Added after 1 minutes:

Most half-decent opamps can have a sine-wave at the output that is 28V p-p at 100kHz without distortion.

again, that depends. a lot of great opamps now run off 3.3v or 5v rails and it is impossible for them to output anything more than that.

those there are CMOS opamps with built-in switched capacitor power supplies.

 

[Old Nick]

Re: Opamp GBP

I´m sorry but most statements of your reply are definitely false.

Quote: again, it depends. there are two ways to define slew rates: SR=dv/dt. that by definition, is a small signal measurements.

No, this is not true. The formula is correct, but it has to measured under large signal conditiones. Otherwise it is a measure of the small signal rise time.

dv/dt is by definition an infinitesimal (read: infinitely small) change of voltage over an infinitesimal change in time.

if that doesn't constitute a "small" signal definition, what else does?

what you described is delta v / delta t: a finite change in voltage over a finite period of time.

this is just basic calculus. so if you agree that SR=dv/dt is correct (which you did), you have to disagree with yourself about whether SR is a small signal or a large signal definition.

Quote: in practice, most slew rates are measured with the time the output swings to 90% of its full output (aka when the amp settles), and the input is typically a pulse / step function.

According to the formula above dv/dt is the slope of the opamp response - independent on the 90% point.

a "slope", sir, is a small signal definition because it was talking about dv/dt.

the threshold is needed because almost all circuits require some settlement time.

Quote: as to what constitute a "large signal", there is no generally accepted consensus: should an amp swing to 90/80/50/20% of its maximum output? with what kind of load? etc.

I have mentioned already what "large signal" means. I repeat: It does NOT depend on any output level, but instead on the input stage condition. The first stage must go for a short time into saturation - until the feedback works and brings it back to the linear region. This effect determines the slew rate,

your are talking about group delay. read into TIM-D will help you in that regard.

Quote: I am not sure what "100%" negative feedback means: gain=+/-1?

This question touches basics of the feedback theory. 100% feedback means that 100% of the output are fed back. It´s quite logical. For an inverter with a gain of "-1" only 50% are fed back (opamp basics).

first of all, there are plenty of amps whose SRs are measured at anything over than gain of 1, +/-.

and 2ndly, there are plenty of amps who are not unity stable and cannot possibly work at unity gain.

your approach would have yielded no-meaningful SRs for those amps.

Partly, this is correct. Slew rate is specified sometimes for different test conditions (for example upwards/downwards).

very rarely SRs are specified on two directions.

typically, they are specified at different supply voltage, into different load, or under different gains, etc.

But always with feedback and large input signals (see above).

it is done with feedback because few amps, especially opamps, are used open loop.

on the 2nd point, you have to be able to separate how something is defined in theory and how that same thing is measured in real life. I said very early on that SRs are indeed measured in with large signal because of the difficulties in measuring it small signal - you would be measuring the slope of the waveform.

don't think so? take our your college calculus book and work out the formula for the minimum SR to reproduce a sine wave.

you can get some help from "Opamp application handbook" by Walter Jung. this very issue was talked early in the chapters.

OK, I cannot resist. the max slew rate for a sine signal V=Vp*sin(2pi()ft) is 2pi()fVp, which happens when t=n*/(2f), where n=0, 1, ......, pi()=3.14......

that signal will reach Vp, its max voltage, at t=1/4f. so the "large signal" SR would be 4fVp<2pi()fVp.

in other words, if you have an amp that can reproduce Vp in time, it wouldn't be fast enough to reproduce the sine wave.

hope this helps.

I've been on this board for a few years and I can assure you that LvW knows what he's talking about, (as does audioguru).

Slew IS a large signal property.

I have designed amps (I design CMOS cameras -which include op-amps - for a living), with bandwidths in the 10's of GHz, but are limited by slew to 10's of MHz. Any basic book should clear up the confusion you're having.

 

[millwood]

Re: Opamp GBP

I've been on this board for a few years and I can assure you that LvW knows what he's talking about, (as does audioguru).

i am sure as well.

Slew IS a large signal property.

i agree with that. what we are discussing here is that how SR is defined and how SR is measured diverge.

I have designed amps (I design CMOS cameras -which include op-amps - for a living), with bandwidths in the 10's of GHz, but are limited by slew to 10's of MHz. Any basic book should clear up the confusion you're having.

well, that depends on how you defined bandwidth.

 

[Old Nick]

Re: Opamp GBP

what we are discussing here is that how SR is defined and how SR is measured diverge.

dv/dt is by definition an infinitesimal (read: infinitely small) change of voltage over an infinitesimal change in time.

if that doesn't constitute a "small" signal definition, what else does?

erm...

well, that depends on how you defined bandwidth.

I define bandwidth the way the rest of the worlds electronics community do, and you'd do well to do the same.

 

[millwood]

Re: Opamp GBP

what we are discussing here is that how SR is defined and how SR is measured diverge.

dv/dt is by definition an infinitesimal (read: infinitely small) change of voltage over an infinitesimal change in time.

if that doesn't constitute a "small" signal definition, what else does?

erm...

that looks damn consistent. if you have questions, please let me know.

well, that depends on how you defined bandwidth.

I define bandwidth the way the rest of the worlds electronics community do, and you'd do well to do the same.

well, I am curious about this amp of yours that is capable of 10ghz but cannot process 10mhz signals.

can you tell us a little bit more about this "beast"?

 

[Old Nick]

Re: Opamp GBP

what we are discussing here is that how SR is defined and how SR is measured diverge.

dv/dt is by definition an infinitesimal (read: infinitely small) change of voltage over an infinitesimal change in time.

if that doesn't constitute a "small" signal definition, what else does?

erm...

that looks damn consistent. if you have questions, please let me know.

well, that depends on how you defined bandwidth.

I define bandwidth the way the rest of the worlds electronics community do, and you'd do well to do the same.

well, I am curious about this amp of yours that is capable of 10ghz but cannot process 10mhz signals.

can you tell us a little bit more about this "beast"?

It can 'process' signals of 10 MHz.

It is slew limited to 10's MHz across the rails.

It is quite clear your understanding of this is a bit limited.

Also your aggressive attitude is rather unbecoming.

 

[millwood]

Re: Opamp GBP

It can 'process' signals of 10 MHz.

It is slew limited to 10's MHz across the rails.

so this opamp is slew limited to 10mhz but how did it get rated 10ghz?

It is quite clear your understanding of this is a bit limited.

sure, that's why I am here to learn from you.

Also your aggressive attitude is rather unbecoming.

no reason to take it personal. if you know your stuff, lay it out and others will be convinced.