Electronic – Miller compensation capacitor sizing

amplifieraudiobjtcapacitorcommon-emitter

How can I size the circled capacitor in order to get rid of the peaking?
Is there some formula?
Cheers

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Best Answer

Given;

  • uncompensated discrete amplifier with closed loop gain of 20 dB , (R1+R2)/R1= 10x
  • high Q resonance at 5MHz,
  • ~20dB/decade rolloff >10MHz
  • 0dB gain at 50 MHz which becomes the GBW product.

We could compute the open loop gain

  • and determine the ω/RC breakpoint for some circuit Req and added Miller C to achieve 0 dB gain

    • and then expect 45 deg phase shift at unity gain at GBW/G = 50MHz for phase margin.

  • example

    • if \$a_v=60dB\$ and 1st order = -20dB/decade above 50MHz the breakpoint "at least" must be less than 3 decades lower or 50kHz. ( if 80 dB then 4 decades lower)
    • the Req=R29//hFE*R24 + R25
    • your C1 is then C1=1/(2π f*Req)

There will be tolerances for each transistor GBW in this design which determines the extra margin to reduce breakpoint. In typical BJT Op Amps , the breakpoint is 10Hz. For video Amps not unity stable it may be > 10kHz +

e.g. The goal is to make the slope 1st order or phase shift <=90 deg at unity gain and eliminate the cascade stage higher order effects at high gain where negative feedback almost shifts into positive feedback "low phase margin"

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