Why? Physics or Maths depending how you view it

Posted by 13th Duke of Wymbourne (A) on August 19, 2024 at 19:31:25

In Reply to: RE: Thank you DoW ... posted by andyr on August 19, 2024 at 16:49:40:

If you model a non-linear transfer function as power series there will be term for the fundamental that varies as K1*Vin (K1 is the gain), the second harmonic varies as K2^2*Vin, the third harmonic varies as K3^3*Vin. This is why the harmonics go up more quickly than the fundamental. If you plot these terms in dB (log axes) they have straight lines: the fundamental with a slope of 1dB out for 1dB in, second harmonic with a slope of 2dB out for 1dB in etc. Radio engineers use a metric called 'Intercept point' that is an extrapolation of those lines to a point where they meet. This is not physically possible to have any harmonic have the same level as the fundamental but it is a useful concept to predict the amount of any harmonic at lower voltage levels using just the intecept number.
I have some examples of intercept point applied to commercial audio power amps, PM if you'd like a copy.

This post is made possible by the generous support of people like you and our sponsors:

Follow Ups Full Thread

Topic - Can someone explain these THD measurements from LTspice? - andyr 23:16:45 08/18/24 (15)

RE: With a bit of assistance from here and elsewhere ... I believe I have the answer. ... - andyr 22:11:22 09/05/24 (0)

Where's all the noise and distortion coming from? - Geoffkait 08:07:36 08/21/24 (0)

RE: Can someone explain these THD measurements from LTspice? - 13th Duke of Wymbourne 10:27:23 08/19/24 (6)

RE: Thank you DoW ... - andyr 16:49:40 08/19/24 (1)

Why? Physics or Maths depending how you view it - 13th Duke of Wymbourne 19:31:25 08/19/24 (0)

And do what Tom suggests, plot the drain current to be sure the swing is symmetrical - 13th Duke of Wymbourne 10:35:38 08/19/24 (3)

RE: Sorry - can't quite see how to plot the Drain current ... - andyr 17:05:06 08/19/24 (2)

RE: Sorry - can't quite see how to plot the Drain current ... - 13th Duke of Wymbourne 19:52:36 08/19/24 (1)

RE: Yes, that plot *is* the voltage across R1... - andyr 20:00:55 08/19/24 (0)

RE: Can someone explain these THD measurements from LTspice? - tomservo 10:20:49 08/19/24 (5)

RE: Thank you, Tom ... - andyr 16:58:42 08/19/24 (4)

RE: Thank you, Tom ... - tomservo 09:59:12 08/20/24 (3)

RE: "when you change the resistor values" ... - andyr 15:57:11 08/20/24 (2)

RE: "when you change the resistor values" ... - tomservo 05:37:21 08/21/24 (1)

RE: "what do you want to build?" ... - andyr 15:30:30 08/21/24 (0)

Follow Ups

Why? Physics or Maths depending how you view it - 13th Duke of Wymbourne 19:31:25 08/19/24 (0)

Why? Physics or Maths depending how you view it

Post a Message!