Need explanation for problem with oscillator when swapping opamp

[ottarh]

Hi,

I try to build this simple square wave oscillator which is supposed to
use opamp TL062;

http://i25.photobucket.com/albums/c80/narma/bf651416.jpg

According to formulas this circuit should swing at about 55 kHz and
give several volts AC out.
Now I didn't have this opamp at hand so I tried LM324 which also is
supposed to be able to operate with a low voltage single supply.
But when measuring with my Fluke 79 I get swing at about 19 kHz at 0,85
Volt AC.
Can you please explain to me why these figures does not match with the
calculated ones?
Thank you for your help!

ottar

 

[Jenalee K.]

ottarh a écrit :

Hi,

I try to build this simple square wave oscillator which is supposed to
use opamp TL062;

http://i25.photobucket.com/albums/c80/narma/bf651416.jpg

According to formulas this circuit should swing at about 55 kHz and
give several volts AC out.
Now I didn't have this opamp at hand so I tried LM324 which also is
supposed to be able to operate with a low voltage single supply.
But when measuring with my Fluke 79 I get swing at about 19 kHz at 0,85
Volt AC.
Can you please explain to me why these figures does not match with the
calculated ones?
Thank you for your help!

ottar

Probably because the TL062 is a FET opamp while the LM324 is a bipolar
opamp.

Interestingly your oscillator circuit is in the LM324 datasheet, the
only difference is the value of R3 (100k vs 8k2). I couldn't find a
LM324 so I've setup your oscillator with a TL084, which is a close
relative of the TL062. I measured with an oscilloscope a frequency of
almost 55kHz and a squarish output of 2.9Vtt superimposed on 1.45V. My
multimeter said 1.2VAC and 54.7kHz.

Thanks,
Jenalee K.

 

[Tim Wescott]

Hi,

I try to build this simple square wave oscillator which is supposed to
use opamp TL062;

http://i25.photobucket.com/albums/c80/narma/bf651416.jpg

According to formulas this circuit should swing at about 55 kHz and
give several volts AC out.
Now I didn't have this opamp at hand so I tried LM324 which also is
supposed to be able to operate with a low voltage single supply.
But when measuring with my Fluke 79 I get swing at about 19 kHz at 0,85
Volt AC.
Can you please explain to me why these figures does not match with the
calculated ones?
Thank you for your help!

ottar

Sounds like a slew rate limitation, but if I'm reading my data sheet
correctly (never a sure thing) it should be plenty fast. Either the
thing isn't happy at low supply levels or you're loading it down
somehow. If you can, take a look at it with an oscilloscope. If you
can't experiment with the feedback resistor or cap to see if it'll work
more cleanly at way lower frequencies (like 10x lower).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Roger Hamlett]

Hi,

I try to build this simple square wave oscillator which is supposed to
use opamp TL062;

http://i25.photobucket.com/albums/c80/narma/bf651416.jpg

According to formulas this circuit should swing at about 55 kHz and
give several volts AC out.
Now I didn't have this opamp at hand so I tried LM324 which also is
supposed to be able to operate with a low voltage single supply.
But when measuring with my Fluke 79 I get swing at about 19 kHz at 0,85
Volt AC.
Can you please explain to me why these figures does not match with the
calculated ones?
Thank you for your help!

ottar

If you look at the 'large signal output swing', versus frequency graph,
for the 324, with a 5v supply, it only shows it managing about 15KHz, for
the full swing. Your example, sounds to be manging slightly better than
this. The same graph for the TL062, manages about 400KHz.
I'm afraid at this low voltage, with an output that swings this far, the
frequency performance of the LM324, is just not good enough.

Best Wishes

 

[John O'Flaherty]

Sounds like a slew rate limitation, but if I'm reading my data sheet
correctly (never a sure thing) it should be plenty fast. Either the
thing isn't happy at low supply levels or you're loading it down
somehow. If you can, take a look at it with an oscilloscope. If you
can't experiment with the feedback resistor or cap to see if it'll work
more cleanly at way lower frequencies (like 10x lower).

I think it is slew rate. An LM324 data sheet shows a voltage follower
pulse response that would imply 0.5 V/us, while the TL062 claims 3.5
V/us, or 7 times faster. The 324 is actually happier on a low supply,
since it goes closer to the rails, but the voltage swing should cancel
out with this circuit.