Op Amps: Simulating a Two-Phase Oscillator

RE: Op Amps: Simulating a Two-Phase Oscillator

jw0752 — Sun, 12 Aug 2018 05:07:57 GMT

Hi Jon,

I followed up with a couple experiments using an LM324 and an LM741 to see how well the OP Amps could be used to provide a low impedance output of a resistive divider's voltage. It seemed to work quite well in the 10 to 20 mA current levels. One thing that I found interesting was the difference in the ability of the two different IC to sink and source current. For example in my experiment with the 741 I set it up to produce a 9V output from an 18V supply. I used a small red LED with a 330R resistor in series to provide a load on the output. With the LED on the HIGH side the 741 was very able to sink the 22mA current with no change in voltage. However when I put the LED on the LOW side the output voltage dropped to 7.8 V. Apparently the 741 is better at sinking current than it is at sourcing it. When I ran the same experiment with the LM324 I found just the opposite. When the LED was on the LOW side the voltage held solid at 9 volts but when I put the LED on the HIGH side the voltage of the LM324's output was pulled up to 9.7 V. Apparently the LM 324 is better at sourcing current than it is at sinking it. I am not sure how I will use this information but it was fun to discover it.

Thanks John

RE: Op Amps: Simulating a Two-Phase Oscillator

three-phase — Fri, 10 Aug 2018 20:42:01 GMT

Interesting blog, thanks for posting.

The audio engineers may not like the output generated, but us power engineers will take 2% THD every day of the week.

Kind regards

RE: Op Amps: Simulating a Two-Phase Oscillator

jw0752 — Fri, 10 Aug 2018 06:11:36 GMT

Hi Jon,

Thanks again for an interesting presentation. The one idea that clicked with my less advanced knowledge was that I can take a simple resistive voltage divider and use an op amp to produce a low impedance output of that voltage like you did for your artificial ground. One of your blogs is like a buffet where there is something of interest for everyone.

John

RE: Op Amps: Simulating a Two-Phase Oscillator

shabaz — Fri, 10 Aug 2018 03:41:54 GMT

Hi Jon,

Excellent and really valuable information : )

That is a neat trick to break the circuit for AC but not for DC, it is new to me, so I'm happy you discovered during your reading!

According to one of Randy Rhea's books (he doesn't discuss such op amp based oscillators, but the general theory should be the same) he suggests that a good characteristic is to have the maximum phase change slope (i.e. gradient) near the oscillation frequency i.e. 360 deg phase shift location, and your bode plot shows that, so this explains why you're getting an excellent sine wave. (Just for completion, the other characteristic is a gain peak near the frequency too).

RE: Op Amps: Simulating a Two-Phase Oscillator

genebren — Fri, 10 Aug 2018 03:03:09 GMT

Another excellent blog! The output really does look quite clean.

Well done.

Gene