When the power supply is ±5V, the chip heats seriously, help to see what is the reason
Hi user6350528,
My wild guess is that you have instability issue, which is caused by C68 or 47nf capacitor. You may either remove the C68 or increase R71 from 100Ω to 1kΩ or slightly larger. Please let me know what is going on after the change.
When op amp is driving a large capacitive load, the op amp will become unstable, and I think that this is what is going on. I assumed that U12A and U12B are the OPA2727 op amp.
I performed a quick AC stability analysis, it is shown that the first stage's loop gain has only 35 degrees of phase margin, which is unstable.
You need add 4.7pf capacitor in parallel with 20kΩ feedback resistor, this will give you 74 degrees of phase margin. Increasing R71 does not work in this configuration.
OPA727 AC Analysis 06272023.TSC
If you have additional questions, please let me know.
Best,
Raymond
What is the voltage at pin 5 supposed to be? According to the schematic, it is floating. If the actual board does not have a DC bias, you should add it; see, for example, R3 in figure 1-9 of AN-31 amplifier circuit collection:
Hi user6350528,
Clemens, Thanks for pointing it out! I overlooked the input front end.
If the pin5 is capacitive coupled to the non-inverting node, you would need to have place a high pass filter instead, or connect the input stage posted by Clemens. Otherwise, no DC biasing is present at the Vin+ or pin5 at U12B, which is floating. The op amp should be stable now as shown in the step transient response below.
OPA727 Step Transient 06272023.TSC
Best,
Raymond
