PA92 OPAMP phase compensation
1.&nbsp; Answer from Apex Microtechnology

I mailed the question directly to Apex Microtechnology and got an answer from Mr. Eric J. Boere (Apex Field Application Engineer, responsible for European support). With his permission, I share his answer with you:


  I have checked out the link provided in your e-mail and all answers in there are kind of right, especially the one by @sstobbe; "The PA92 is unlike the jellybean opamps commonly used, this amplifier is not unity gain stable and requires compensation when employed as a unity gain buffer"
  
  If you look at the small signal response graph, you will see that instability is spelled by the open loop gain sloping down at -60dB per decade (octave) by the time it’s 1 (0dB)
  
  
  
  A unity gain -stable opamp, would have showed a small signal frequency response sloping down at -20dB/decade all the way down to and below 0dB, after which there could be other pole/zero clusters to bend the graph to -40db/decade or even beyond that.
  
  In order to stabilize the opamp, the external phase compensation can be used. The phase compensation component(s) RC and CC slow the opamp down. Just picture yourself a capacitor slowing down the response of transistor Q3 in the intermediate stage of the equivalent schematic of PA92:
  
  
  
  To get rid of the oscillations, use the phase compensation components recommended by the datasheet:
  
  
  
  So, for your unity gain PA92, Cc should be 150pF, and Rc be 100Ω. Please adhere to the recommendation to use a Cc that is rated for the full supply voltage, in your case +100 – (-100) = 200V (if you check out the equivalent schematic above you’ll understand why; this capacitor can ‘see’ almost the full supply voltage as the differential voltage between gate and drain can change that much while the output is driven from (almost!) rail to rail). It would be even better to use a higher rated part, for instance a 500V capacitor.
  By the way: there would be nobody holding you back from using 220pF + 100Ω to ensure the part doesn’t oscillate.
  
  I highly recommend you to read our Application Note #01 as it can save you a lot of time using our power opamps.
  Just to call out a few things to pay attention to:
  
  
  Power supply bypassing (not doing so can cause the part to oscillate, too!)
  Output protection by means of (ultra-)fast reverse recovery diodes (if you have reactive loads)
  Input protection by means of (small signal) diodes, rather than Zener diodes (because the latter exhibit higher capacitance)
  Heat sinking (Iq=14mA, VSS=200V, Pq=2.8W; the part is dissipating 2.8W doing nothing!)
  Etc.
  
  
  Last, but not least I would like to draw your attention to our Power Design Tool, which can be downloaded for free here: https://www.apexanalog.com/support/power_design_tool.html
  
  Using this tool can also save you a lot of time!


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Mr. Eric J. Boere sent me another mail to answer my question about the table in the datasheet:


  


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Thank you Eric for helping me out!



2.&nbsp; Practical fix

2.1&nbsp; TEST 1: RC = 100Ω and CC = 147pF

I have soldered RC = 100Ω and CC = 147pF onto the compensation pins of the PA92. This corresponds to the first line of the table from the datasheet:



The oscillations are gone! At least on the PA92 OPAMP used as mere voltage follower. The other two PA92 OPAMPs used in the more complicated gyrator circuit (simulating an inductance, known as the "Antoniou circuit") are still oscillating:



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2.2&nbsp; TEST2: RC = 100Ω and CC = 247pF

I soldered an extra capacitor of 100pF, raising CC up to 247pF. I still got oscillations in the Antoniou circuit (about 10V peak-to-peak), although the DC-components of the signals are behaving as expected:



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2.3&nbsp; TEST3: RC = 100Ω and CC = 347pF

I soldered again an extra capacitor, raising CC to 347pF. The oscillations in the Antoniou circuit are weakened to 5V peak-to-peak (compared to 10V peak-to-peak before).

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2.4&nbsp; TEST4: RC = 100Ω and CC = 447pF

After adding yet again 100pF (CC is now 447pF), all oscillations in the Antoniou circuit are gone:



Note: The three oscilloscope screenshots I pasted here from the Antoniou circuit, cannot be compared indiscriminately to each other. They all have different scales and are measured under different input conditions. Unfortunately, I didn't make consistent scope screenshots accross all 4 tests ( C=147pF, C=247pF, C=347pF, C=447pF). These scope screenshots merely illustrate the fact that oscillations die out with greater capacitance applied on the compensation pin of the PA92 OPAMP.

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2.5 Test conclusions

The conclusions I make from these test results are:


The PA92 OPAMP used as simple voltage follower works fine with RC = 100Ω and CC = 147pF.
The PA92 OPAMPs used in the Antoniou circuit (gyrator to simulate an inductance) need RC = 100Ω and CC = 447pF to eliminate oscillations.