Hello All,
New to the forum. Trying to get some advice on configuration of an OPA541. Got some mental cobwebs. Had to dust off the breadboard just to start this.
The short term goal is to achieve a chip amp AC supply that may need to be variable in the future with a max output of around 24 vacrms @ about 2 amps and a fixed 60hz.
It will ultimately drive an inductive load with motor type characteristics. But for now im just trying to get a basic setup with proper voltage gain.
I started with scraps leftover in drawers from my audio days of long ago TDA2030/40. Then I bought a couple of TDA7293s. Read some posts here about OPA541 being more robust and less likely to have a meltdown.
I have the OPA541 setup so far very minimally. .1 caps, supply caps, and .1 ohms sensing resistor for Pin8. supply is a rectified 24 0 24 / 200VA xfmr producing +/-35V rails.
I have a variable input coming from an LM324 ckt. OPA541 Output is very touchy. I cant achieve much more than a decent looking sine wave of 12 vacrms. If I push the input any further the output slams to a pretty decent looking rail to rail square wave of same period exactly in phase compared to input. Bear in mind I have no other resistors anywhere in the mix yet. I just have inverting pin grounded with signal coming in to noninverting input.
Hope I have given enough info above. Here are the questions I think I have due to the mental cobwebs😀
1. Is this too minimal? What other parts are most important to throw at it?
2. Is this a gain issue?
3. If so what are some suggestions on gain resistor calculations? And from which pin to which? Why does the main example in datasheet use inverting input?
There is not much to look at in datasheet regarding gain or resistors. The main emphasis is put on calculating the current limit resistor.
Any hints to point me in the right direction would be greatly appreciated.
Thanks in Advance
Ray
New to the forum. Trying to get some advice on configuration of an OPA541. Got some mental cobwebs. Had to dust off the breadboard just to start this.
The short term goal is to achieve a chip amp AC supply that may need to be variable in the future with a max output of around 24 vacrms @ about 2 amps and a fixed 60hz.
It will ultimately drive an inductive load with motor type characteristics. But for now im just trying to get a basic setup with proper voltage gain.
I started with scraps leftover in drawers from my audio days of long ago TDA2030/40. Then I bought a couple of TDA7293s. Read some posts here about OPA541 being more robust and less likely to have a meltdown.
I have the OPA541 setup so far very minimally. .1 caps, supply caps, and .1 ohms sensing resistor for Pin8. supply is a rectified 24 0 24 / 200VA xfmr producing +/-35V rails.
I have a variable input coming from an LM324 ckt. OPA541 Output is very touchy. I cant achieve much more than a decent looking sine wave of 12 vacrms. If I push the input any further the output slams to a pretty decent looking rail to rail square wave of same period exactly in phase compared to input. Bear in mind I have no other resistors anywhere in the mix yet. I just have inverting pin grounded with signal coming in to noninverting input.
Hope I have given enough info above. Here are the questions I think I have due to the mental cobwebs😀
1. Is this too minimal? What other parts are most important to throw at it?
2. Is this a gain issue?
3. If so what are some suggestions on gain resistor calculations? And from which pin to which? Why does the main example in datasheet use inverting input?
There is not much to look at in datasheet regarding gain or resistors. The main emphasis is put on calculating the current limit resistor.
Any hints to point me in the right direction would be greatly appreciated.
Thanks in Advance
Ray
I've never used these chips but would imagine they should be configured to normal rules which would mean setting a gain with a feedback network... like an opamp.
The inverting design possibly gives best performance on paper.
I think you need to configure this conventionally for starters, set a gain of say 20 or 30 using a 22k and 1k feedback network. I would AC couple the network return, just like most opamp designs. Use a 22k resistor to ground from the + input and AC couple your input signal.
The inverting design possibly gives best performance on paper.
I think you need to configure this conventionally for starters, set a gain of say 20 or 30 using a 22k and 1k feedback network. I would AC couple the network return, just like most opamp designs. Use a 22k resistor to ground from the + input and AC couple your input signal.
Thank you very much.
That helped shove me in the right direction. Not sure what fixed it most because I wasnt in a scientific, one thing at a time mood. I threw the kitchen sink at it.
It is ultimately gonna be used for an inductive load so I went ahead and added the diodes to the rails. Based on your suggestion I took a hard look at the gain. I sucked it up and used the inverting input. Set up a 100k/10k gain structure. I did find one sample circuit with a this feedback resistor network with a gain formula to the side in about a size 6 font.
The other thing I did and I dont know if I picked a good value or not was to add 2 equal voltage divider resistors rail to input to rail. 1Meg each. I was having a little bit of an issue with DC drift from the baseline.
Thanks again
That helped shove me in the right direction. Not sure what fixed it most because I wasnt in a scientific, one thing at a time mood. I threw the kitchen sink at it.
It is ultimately gonna be used for an inductive load so I went ahead and added the diodes to the rails. Based on your suggestion I took a hard look at the gain. I sucked it up and used the inverting input. Set up a 100k/10k gain structure. I did find one sample circuit with a this feedback resistor network with a gain formula to the side in about a size 6 font.
The other thing I did and I dont know if I picked a good value or not was to add 2 equal voltage divider resistors rail to input to rail. 1Meg each. I was having a little bit of an issue with DC drift from the baseline.
Thanks again
The 1 meg resistors sound a bit odd tbh. It would maybe make more sense to see it drawn but if you have the resistors connected as it sounds you have then any rail variations will couple to the input (noise, hum etc) and any slight change in one rail over the other will alter the offset.
The DC offset should be very low and very stable. I wonder if there is some high frequency instability somewhere... and breadboard layouts can be tricky in that regard. Perhaps try a good old Zobel type network on the output, say a 10 ohm and 0.1uF to ground.
Did you AC couple the input so there is no possibility of anything there causing DC offset ?
The gain in the inverting configuration is simply -(Rf/Rin) where your 100k should be Rf (connected from output to to - input) and Rin the input resistor. The - denotes phase inversion.
So you have a gain of -10 and with an input impedance set by the 10k
The DC offset should be very low and very stable. I wonder if there is some high frequency instability somewhere... and breadboard layouts can be tricky in that regard. Perhaps try a good old Zobel type network on the output, say a 10 ohm and 0.1uF to ground.
Did you AC couple the input so there is no possibility of anything there causing DC offset ?
The gain in the inverting configuration is simply -(Rf/Rin) where your 100k should be Rf (connected from output to to - input) and Rin the input resistor. The - denotes phase inversion.
So you have a gain of -10 and with an input impedance set by the 10k
I dreampt the 1megs up because I read about that voltage divider on an op amp page somewhere. But their example had used 100k resistors. But they also had their gain set with 10k/1k. So since the datasheet had 100k/10k for gain I jumped the voltage divider up by a factor of 10 likewise.
I did try a cap but it was skewing my input to output wave forms a bit. No big deal but I felt warmer and fuzzier when they crossed the zero line together. I did consider rail anomalies causing new problems. The cap will ultimately be the answer.
Yes the 100k is Rf but it actually connects based on their drawing to the other side of the .1 ohm current limit resistor coming off the output. And 10k from signal input to inverting input.
Yes thanks I will try that 10ohm / .1uf combo. I got some little hf fuzzies on a portion of my sine wave output.
I did try a cap but it was skewing my input to output wave forms a bit. No big deal but I felt warmer and fuzzier when they crossed the zero line together. I did consider rail anomalies causing new problems. The cap will ultimately be the answer.
Yes the 100k is Rf but it actually connects based on their drawing to the other side of the .1 ohm current limit resistor coming off the output. And 10k from signal input to inverting input.
Yes thanks I will try that 10ohm / .1uf combo. I got some little hf fuzzies on a portion of my sine wave output.
As you are on a breadboard make sure you return the load current back to the power supply itself and not a track on the breadboard.
What you are describing are exactly the kind of issues caused by layout and electrical routing.
The dividers you have seen on opamps are probably because they are running on a single rail and so need to bias the opamp to mid rail... you already have the perfect 0V reference with the dual power supply.
A small cap across the feedback resistor can sometime be useful but with 100k you would be looking at values of just a few picofarad.
Connecting Rf to the 'output' or load side of the limiter resistor includes that resistor in the feedback loop and so the output voltage is not altered under load by the presence of the 0.1 ohm.
Connect it to the other side and you have a 0.1 ohm in series with the load. Changes in output current loading would then reduce the output voltage.
What you are describing are exactly the kind of issues caused by layout and electrical routing.
The dividers you have seen on opamps are probably because they are running on a single rail and so need to bias the opamp to mid rail... you already have the perfect 0V reference with the dual power supply.
A small cap across the feedback resistor can sometime be useful but with 100k you would be looking at values of just a few picofarad.
Connecting Rf to the 'output' or load side of the limiter resistor includes that resistor in the feedback loop and so the output voltage is not altered under load by the presence of the 0.1 ohm.
Connect it to the other side and you have a 0.1 ohm in series with the load. Changes in output current loading would then reduce the output voltage.