My attempts at a design of a 3 stage amplifier
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 Today, 06:40 AM #51 edbarx   diyAudio Member   Join Date: May 2018 Online, I read in this same forum, that LTSpice is unreliable for distortion analysis, especially, when it comes to calculating the distortion percentage. Yesterday, I did a distortion analysis for a perfectly looking sine wave and got 700%! This is worse than ridiculous. For anyone who took interest to study mathematics beyond secondary level, a distorted sinewave looks like a dented mud-guard and is horrible to look at. The Fourier Analysis results looked more realistic indicating distortion components starting from -20dB downwards. I estimate this as a figure of around 1% of distortion although this is plain guesswork. I cannot imagine why the fundamental frequency is included in the Fourier distortion analysis. The fundamental is not a distortion but the signal component! A better way would be an analysis of the signal with a pure sinewave removed, so that, only distortion components are left. Plotting that difference would give a very clear idea of the actual distortion signal waveform. if LTSpice can give a list of (t, V), time, voltage, points for a waveform output, I should be able to process that information to get a clear visual idea of the distortion waveform. I assume LTSpice can be directed to draw custom graphs like that.
 Today, 06:52 AM #52 suzyj   diyAudio Member     Join Date: Jan 2006 Location: Western Australia There's a little bit you need to do with LTspice to do good distortion analysis. Straightforward things like ensuring you're simulating an integer number of cycles of your harmonic, turning off compression etc. Lots of us make really extensive use of LTspice for distortion. It's a very accurate tool if driven intelligently. __________________ http://www.suzyj.net
edbarx
diyAudio Member

Join Date: May 2018
Here are two attachments with one showing output voltage VS input voltage and the other is the edited circuit with more power transistors.

The output VS input chart shows some hysteresis, that is, distortion as this is a deptarture from the theoretical line form. As can be seen, the distortion figure should be very low.
Attached Images
 circuit-with-added-power-transistors.png (49.0 KB, 19 views) output-vs-input-voltage.png (32.6 KB, 19 views)

Mooly
diyAudio Moderator

Join Date: Sep 2007
Have a look at this, a 20kHz voltage source and resistor (resistor isn't even needed actually).

We have added the commands to calculate distortion, and added the plotwinsize=0 to turn off compression.

Now I'll add a timestep. The simulation is set to run for 4ms and the FFT sample is at the default of 262144. Dividing 4ms by 262144 gives us 0.0000000152587890625 seconds. Now we'll paste that into the timestep window and run the simulation.

Finally another trick up our sleeve. We can add the command .options nundgt=7 which gives double precision to the calculation of variables. Again we run the sim. This is as good as it gets. The fundamental and nothing else. Distortion is 0.000000000000...…
Attached Images
 DS1.JPG (295.6 KB, 15 views) DS2.JPG (218.3 KB, 15 views) DS5.JPG (196.5 KB, 15 views)

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