I have designed an 2 stage opamp with all MOSFETs in subthreshold region as shown in the figure below,

enter image description here I have gain of 48 dB, GBW of 60KHz, Slew rate of 0.014 V/us and Power dissipation of just 0.6 micro Watts.

Here is the Frequency response , enter image description here

I have a appreciable Phase margin of 60 degrees from the frequency response. I am a little down on noise and harmonic distortion analysis in LT spice.. But i do know that FFT of the output gives an idea of the THD.But how to calculate the percentage harmonic distortion ?

Here is a picture of FFT of the output, (Note :i am operating at 400Hz sine input)

enter image description here

What does this values in db mean and i do see peak at 800Hz which is an harmonic of fundamental frequency 400Hz , what should i conclude from it and how is the THD evaluated?

UPDATED FFT PLOT with more cycles as requested by andyaka I would also like to know about the stability analysis of the same.

  • \$\begingroup\$ You have bigger problems with that FFT ... possibly a DC component that is being mistaken as an impulse? That is hiding the other harmonics. You could calculate the 2nd harmonic distortion though. \$\endgroup\$ Jul 16, 2016 at 15:36
  • \$\begingroup\$ Sir, could you please explain on that. \$\endgroup\$ Jul 16, 2016 at 15:38
  • 1
    \$\begingroup\$ Try using a much finer resolution FFT over many cycles of the input signal. Also, why are you injecting a signal that produces an output that is -20 dBV? \$\endgroup\$
    – Andy aka
    Jul 16, 2016 at 15:59
  • \$\begingroup\$ @Andyaka the circuit is operating at low supply voltage, so maximum positive swing is only around 0.43V and the gain is 48dB. I have updated the FFT with more cycles. please do check \$\endgroup\$ Jul 16, 2016 at 16:49

1 Answer 1


Well, using your much improved FFT you could approximate THD to the amplitude of the 2nd harmonic - it appears to be about -42 dBV and your main signal is at -15 dBV. The 2nd harmonic is therefore 27 dB lower.

As a ratio that is 22.4 times lower or about 4.5 % THD.

If you included the 3rd harmonic at about -58 dBV you have to convert to a real voltage and take the square root of the sum of squares hence, -42 dBV becomes 7.94 mV RMS and -58 dBV becomes 1.26 mV RMS. Now square and add them then take the square root to get 8.04 mV RMS.

Convert this back to dBV to get -41.9 dBV i.e. virtually no difference between using the 2nd harmonic on its own (-42 dBV) but, just for the record the difference is 26.9 dB or a ratio of 22.1 or about 4.52% THD.

In conclusion, unless you have a compelling reason to go any higher I would just approximate THD to the ratio of the 2nd harmonic to the fundamental.

Regards "stability analysis" I'm not sure what you require to demonstrate.

  • \$\begingroup\$ Thank you. is the phase margin good enough please check the frequency response .Also i would like to hear your suggestions on my above work. \$\endgroup\$ Jul 16, 2016 at 17:24
  • \$\begingroup\$ Phase margin of 60 degrees is good. Pretty normal for standard op-amps. \$\endgroup\$
    – Andy aka
    Jul 16, 2016 at 17:26
  • \$\begingroup\$ Thank you, any suggestions on improving my work ? \$\endgroup\$ Jul 16, 2016 at 17:28
  • \$\begingroup\$ It's not an area I have skills in - I use op-amps and don't design them!! \$\endgroup\$
    – Andy aka
    Jul 16, 2016 at 17:30

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