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Old 2nd May 2012, 01:57 PM   #1
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Default Help with push pull opamps

Hello, I could use some help with this op amp with LSK170 inputs. I'm simulating this little op amp and it seems to be fine when used alone but when I parallel them for a push pull arrangement, it oscillates. The gain is the same for both circuits. I don't understand why the single version would be stable at low gain (6dB) whereas the double version is not. The double version is stable at higher gains. I've tried by pass caps around the feedback resistors, as well as other kinds of bypass caps and nothing is helping. Any input is appreciated.
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File Type: jpg LSK170 op amp pulse.JPG (39.8 KB, 136 views)
File Type: jpg LSK170 op amp.JPG (66.4 KB, 138 views)
File Type: jpg LSK170 op amp push pull pulse.JPG (52.8 KB, 130 views)
File Type: jpg LSK170 op amp push pull.JPG (108.8 KB, 135 views)
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Old 2nd May 2012, 02:19 PM   #2
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This arrangement needs unity gain stable amplifiers. The reason is rather subtle, and in the hurry I don't think I'll be able to explain it well. Anyone?

Samuel
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Old 2nd May 2012, 02:26 PM   #3
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Why try discrete designs when there are opamps available that does unity gain just fine ?

The NE5532 is an excellent choice, its cheap and has very low THD and noise.
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Old 2nd May 2012, 02:53 PM   #4
PChi is offline PChi  United Kingdom
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My guess that the problem is due to the closed loop output impedance of the amplifiers increasing with frequency as the open loop gain falls.

The open loop output impedance of the amplifier is 27 ohms / 2 plus emitter resistances.
Any transient on the one of the outputs will be only partly attenuated by the open loop output impedance of the other amplifier which will then appear at it's input and be amplified. It must be just enough to sustain oscillation.

At higher gains the attenuation of the feedback is greater so no oscillation.

The single version is not very stable with some ringing at about 5 MHz at the input transition. It might be worth looking at the Bode plot of the open loop because there isn't much in the way of frequency compensation.
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Old 2nd May 2012, 02:54 PM   #5
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I see that you are driving a 1k load on the single amp and a 150 ohm load on the double amp. Increase the load resistance on the double amp to 2k and measure the performance.
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Old 2nd May 2012, 02:55 PM   #6
jcx is offline jcx  United States
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why the lack of compensation C - how did you determine the amps are stable, with what margins, loads?
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Old 2nd May 2012, 03:55 PM   #7
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Quote:
Originally Posted by Frank Berry View Post
I see that you are driving a 1k load on the single amp and a 150 ohm load on the double amp. Increase the load resistance on the double amp to 2k and measure the performance.
Thanks for the suggestion. I just now tried that but it doesn't make a difference. I think it made it worse actually.
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File Type: jpg LSK170 op amp push pull pulse 1.1.JPG (59.0 KB, 111 views)
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Old 2nd May 2012, 03:55 PM   #8
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Quote:
Originally Posted by Samuel Groner View Post
This arrangement needs unity gain stable amplifiers. The reason is rather subtle, and in the hurry I don't think I'll be able to explain it well. Anyone?

Samuel
I figured that much, thanks. I guess the real question I'm asking is how to make a discrete op amp unity gain stable. It's too subtle for me to figure out on my own.
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Old 2nd May 2012, 03:56 PM   #9
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Quote:
Originally Posted by Tekko View Post
Why try discrete designs when there are opamps available that does unity gain just fine ?

The NE5532 is an excellent choice, its cheap and has very low THD and noise.
Why sail a 40 foot boat across the Atlantic by yourself when you can fly there in 7 hours?
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Old 2nd May 2012, 04:01 PM   #10
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Quote:
Originally Posted by PChi View Post
My guess that the problem is due to the closed loop output impedance of the amplifiers increasing with frequency as the open loop gain falls.

The open loop output impedance of the amplifier is 27 ohms / 2 plus emitter resistances.
Any transient on the one of the outputs will be only partly attenuated by the open loop output impedance of the other amplifier which will then appear at it's input and be amplified. It must be just enough to sustain oscillation.

At higher gains the attenuation of the feedback is greater so no oscillation.

The single version is not very stable with some ringing at about 5 MHz at the input transition. It might be worth looking at the Bode plot of the open loop because there isn't much in the way of frequency compensation.
Thank you! Now I remember! The open loop gain has to cross the 0 dB point before the phase shift reaches -180 degrees. As we can see in this plot, I don't have that in this circuit.
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