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SLVA662 – July 2014 Demystifying Type II and Type III Compensators Using OpAmp and OTA for DC/DC Converters

Hi Officer,

I have a question on Type II compensator shown in this document SLVA662 page 4.

it mentions 'the transfer function is obtained by calculating the impedance offered by the network placed in the Op-Amp feedback path (Zf) and dividing it by the upper resistor (R1)'.

R4 is not part of this transfer function,. Is this because in AC small signal mode, the node highlight in red is equivalent to shorted to ground and R4 shorted out ?

thanks for your help in advance,


  • Hello Weilong,

    You are correct in you understanding.  If you examine the paragraph immediately following Figure 1 on page 2, you will see a phrase mentioning:

    "However, though both resistors of the voltage divider affect the DC level of the converter’s output, from the AC point of view, only the upper resistor enters into the AC analysis. So the lower resistor (R4) is considered just a DC-biasing resistor, and therefore we usually ignore it in control loop (AC) analysis."

    While the OTA case in Figure 2 is forbidden from observing the presence of a virtual GND during AC loop analysis, the two cases in Figures 1 & 3 are able to observe the inverting and noninverting amplifier inputs being held at the same voltage level via a virtual short.

    Please let me know if you have any further questions.



  • Thank you Alec. This is very helpful.