I'm exploring the .MEAS command with the following simple BJT Class-AB amplifier:

straightforward BJT Class-AB amplifier

And am getting the following results from my .MEAS commands in the error log:

vout_max: MAX(mag(v(vout)))=(27.4778dB,0°) FROM 10 TO 1e+010
vout_3db: vout_max/sqrt(2)=(24.4675dB,0°)
f_3db: freq=(142.323dB,0°) at 1.30662e+007

The results for vout_max and vout_3db seem correct, they agree with the plot to at least 2 decimal places.

I'm very puzzled by the results for f_3db however. It appears to call V(vout) crossing the -3dB point at 13.06MHz, where the plot (screenshot below) has it at 19.41MHz.

screenshot of the plot window showing cursors measuring the -3dB point at 19.405 megahertz

  • Why are these numbers so different?
  • Have I written these .MEAS commands wrong?

I expected these to agree to at least several decimal places given they're both calculated from the same simulation run.

  • \$\begingroup\$ None of your results match the schematic. This is not a Class AB amplifier, rather it is a unity voltage gain common collector(aka emitter follower) bipolar amplifier with an overall current gain < 2 due to 1000/500 ratio. \$\endgroup\$ Commented Nov 21, 2019 at 1:00

1 Answer 1


AC analysis produces complex numbers with 'real' and 'imaginary' parts that carry the magnitude and phase information in a vector. The function MAG() is used to extract the magnitude.

You have done this for vout_max, but not for V(vout), so the comparison will give unexpected results. Change V(vout) to MAG(V(vout)) in the last .MEAS command and it should find the correct frequency.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.