I picked up one these: opa2544 datasheet

I assembled the circuit on page 8, figure 4:

enter image description here

I also placed freewheeling diodes on the outputs and bypass capacitors. The power supplies powering the opamps can source 9A each.

For testing, under no load (other than the meter), I placed a DC input to Vin of +10V, about +60V came out. I then placed a DC input of -10V, about -60V came out. Awesome, everything looks okay so far.

I then placed a +/-1 V sine wave @ 1kHz to Vin, the opamp began to over heat like nuts. Then my frequency generator said "high voltage detected" and began to go into protection. I did have my meter connected but I could not get a reliable voltage reading.

There is definitely something I am inherently doing wrong. I have designed simple, low power opamp circuits before, but not many of them. First time trying to make a high power amplifier circuit work.

Any advice on why I experienced this type of behavior?

Note: The opamp did have a heatsink attached to it with a screw and thermal tape.

2: https://i.stack.imgur.com/Y5Xew.png![enter image description here](https://i.stack.imgur.com/4wj3T.jpg)

Edit: I tried the 100pF across the feedback resistors and have loaded each opamp with 6kohm to ground. I re-tested and I was able drive the opamps to my required frequency of 1kHz and achieve the 120Vpp I was after. It is not clear if the suggestions given fixed the issue as I did not try to break it by removing these additions.

enter image description here

  • \$\begingroup\$ Did you also test the circuit when Vin = 0 V, then there should be zero volts across the load and each opamp should output almost 0 V. The fact that your signal generator complained about overvoltage worries me, there should be no high voltage at Vin when the opamps are functioning properly. Maybe the opamps have suffered damage, Maybe there is an oscillation going on (check with an oscilloscope). \$\endgroup\$ Oct 13, 2020 at 14:13
  • \$\begingroup\$ When I placed a DC input of +10V, I ramped it from 0 to +10V with a benchtop supply. At zero volts, the differential voltage was most nearly zero. I did the same kind of testing going from -10V to 0V. I also did try testing it with nothing attached to Vin. The expected result of sporadic voltage occurred from environment noise and then it started to lean towards the positive rail and offered a differential voltage of 65V. I must say I was quite surprised when the signal generator complained as well. \$\endgroup\$
    – joe
    Oct 13, 2020 at 15:18
  • \$\begingroup\$ What exactly is your load? \$\endgroup\$ Oct 13, 2020 at 16:55
  • \$\begingroup\$ Everything was tested at no load. I suppose the only "load" was the multimeter. \$\endgroup\$
    – joe
    Oct 13, 2020 at 19:33
  • 1
    \$\begingroup\$ When you say no load, you mean that the 30 ohms isn't always there? \$\endgroup\$
    – Mattman944
    Oct 14, 2020 at 13:22

1 Answer 1

  1. If it is overheating without a load, it may be oscillating. Long wires to the IC can create parasitic issues. Try a 100 pF cap across the feedback resistors (C2 and C3 in my schematic). Put it very close to the IC. This will limit the bandwidth, but it should still be sufficient for your application.

  2. If your actual load is anywhere near 30 ohms, I believe that you have underestimated the opamp power dissipation.

The dissipated power can be calculated, but it is easier for me to simulate. The power dissipated in each opamp is equal to the voltage drop from the rail to output, multiplied by the resistor current. This is only valid for the positive cycle, but since everything is symmetrical, that is all you need to simulate.

The opamp drop is 35V minus 1/2 of the output voltage. Since the circuit is symmetrical, there is a virtual ground half way through the resistor.

I ran the simulation using your original input parameters, +/- 10V sine.

Since you have 2 opamps in the package, the total package power will be twice what is shown in the bottom graph blue plot. My initial rough calculation (in comments) is too low, your heatsink needs to be even larger than I initially said.


simulate this circuit – Schematic created using CircuitLab

enter image description here

  • \$\begingroup\$ Thanks for your help! I repeated the same circuit for my other circuit and it seems to work great. I picked yours as the answer. Also, if you are interested in a bounty for my other question, have at it: electronics.stackexchange.com/questions/529115/… \$\endgroup\$
    – joe
    Oct 27, 2020 at 22:21

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