Odd observations (Q about opamps, GBW, and Slew Rate)

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Speakerguy

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OK, I just tried making a little triangle generator out on a solderless breadboard. I used the simple circuit found in the AN-31 app note (a pos feedback op amp tied into an inverting RC opamp thing).

Anyway, I tried it first with the bipolar NE5532 10MHz 9V/uS op amp. The peaks were noticeably rounded. I also tried the FET input OPA2132 8MHz 20V/uS op amp, and the peaks were much better.

Q: Why does the slower (8 vs 10MHz) opamp make better triangle waves, and why does the slower opamp have a higher slew rate?
 
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The slew rate was more than adequate in both cases, I was only making a ~30KHz triangle wave. It was the peaks that were rounded off, and I was well within the supply bounds (+/-12V supplies, ~4Vpk-pk output).
 
If you are just using a single op-amp oscillator then that is the normal RC time constant waveform you'll see. For a triangle wave, something like this is needed:
 

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That is the exact ckt I have rigged up. I will take a scope shot to show the difference between the NE5532 and the OPA2132.
 
There are those that just seem to have a way with words, then there are those that...um, no have way. I seem to fit the latter as I just can't think of a way to articulate why slew rate is an issue, but it is... Perhaps someone will approach the podium with a good explanaition...
 
The antique NE5532 has diodes across the inputs to keep them from blowing up. Maybe these diodes are messing up the comparator part of the circuit.
 

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speakerguy79 said:
Q: Why does the slower (8 vs 10MHz) opamp make better triangle waves, and why does the slower opamp have a higher slew rate?
Click to expand...
The slower op amp may actually have a greater bandwidth under the operating conditions. Or perhaps the slower amp is more conservatively rated.

Slew rate is actually somewhat independent of op amp frequency response. The slew rate is related to how fast the circuit can charge internal capacitances (primarily the compensation capacitance) for a large signal. It's determined by the current available to charge and discharge the capacitance which is not directly related to the frequency response.
 
Slew rate is actually somewhat independent of op amp frequency response.
Click to expand...

With all respect I don't quite agree. Since slew rate is a function of Δv/Δt
there is a relationship with BW and slew rate since slew rate is a function of time and amplitude where bw spec deals with unity gain.

Quote from article cited above.


What I am thinking is that these two specs are interrelated, but I just can't find a way to voice my argument. I guess I do not really understand it myself
 
Slew rate limits dv/dt, because dv/dt=I/C, where C is the compensation cap, and I is the current available to charge and discharge it. However, this is the slew rate of an internal node. If there is a gain stage with gain Av following the compensation node, then output slew rate = Av*I/C. It is true that higher bandwidth amps tend to have higher slew rates, because it would (generally) make little sense to have a wideband amplifier with a lousy slew rate. However, the relationship is very loose, and if you graphed available wideband IC amplifier gains vs their respective slew rates, the resulting line would not be monotonic by a long shot.
 
I kinda agree with both points. Max GBW product is dependent on the slew rate, but slew rate is not necessarily dependent on the max GBW. Both OpAmps meet the requirement for the "large signal frequency response' spec. I suspect the issue is something like Audioguru mentioned or simply that the power supply pins on the OpAmps aren't properly bypassed with caps.
 
As promised, the data. I guess I may be running kind of close to the slew rate limit; it appears the NE5532 isn't quite keeping up with the baseline triangle wave frequency of the OPA2132.
 

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  • OPA2132.GIF
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speakerguy79 said:
As promised, the data. I guess I may be running kind of close to the slew rate limit; it appears the NE5532 isn't quite keeping up with the baseline triangle wave frequency of the OPA2132.
Click to expand...
The rounded peaks are due entirely to the finite rise and fall times of the Schmitt trigger stage. These ramps cause the integrator to turn around slowly. Since the 5532 has a slower slew rate, its Schmitt transition times are about twice that of the 2132.

I ran a simple sim for your enjoyment.
The current sources represent the currents into the respective integrators. Their transition times represent the rise and fall times of the respective Schmitt trigger op amps. Note that the 5532 current source has transition times of 2us, while the 2132 is at 0.9us. I arbitrarily chose ±1mA for the currents, and the chose a capacitor value that would yield about 6V p-p at 25kHz. You can see that the triangle waveforms are very similar to speakerguy's scope photos.
 

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  • slew rate effect on triangle gen waves.PNG
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