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pstb analysis shows different results at different iprobe position

Irenexox

Hi all, I tried to do pstb analysis for a voltage regulator with ping-pong autozero amplifier. However, I got different loop gain results when I place the iprobe at different node 1 and 2. the schematic and loop gain is as below. the pink and red lines are for iprobe at node1 while green and blue are results using iprobe at node2. Although the pahse margin and gain margin agrees for both iprobe, the loop gain magitude and phase are different at low frequency. Can anyone explain this result and at which node should I insert the iprobe, please?

The amplifier is using ping-pong autozero architecture and the schematic is as below. aux1 and aux2 are operating in different phase. In phase 1, aux1 is autozeroing its own offset and aux2 is connecting in loop to cancel the offset of main amp. In phase2, vice versa.

  • Dear Irenexox,

    I apologize for my slow response, but I am just able to respond now  - sorry!

    A couple of comments come to mind.

    1. Are you sure the DC operating points are identical for the two past analyses you performed? I don't know the details of your "ping-pong autozero" architecture, but I am concerned that it might result in multiple stable DC operating points. If there are multiple stable DC operating points, the open loop gain and phase computations at each DC operating point will differ - especially at low frequency which is what you are observing. Have you had any time to check that the two DC operating points are identical? 

    2. The high frequency open loop gain and phase responses may still match in the presence of multiple stable operating points as the poles and zeroes that form the bandlimiting are likely less dependent on the DC operating point and more dependent on resistances and capacitances within the loop

    3. As a side note, in the first graphic schematic, you have labeled a signal as "Vout". However, node "Vout" is outside the feedback loop and hence is not under feedback control. The open loop gain and analyses you are performing will only include the feedback loop and hence not reflect the gain associated with signal Vout. This is likely obvious to you, but I thought I should at least mention it.

    Shawn

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