Distortion simulation LTSpice

[Uros11]

Hello,

I have a project, which is to make audio amplifier, and for now, I have everything except distortion analysis. I have simulated the circuit, plotted the FFT in both decibel and linear representation. I also used .four directive to get distortion percentage. You can see everything from pictures. It doesn't look very good, and I don't know if it is because of the settings or my circuit?

 

[Ylli]

100 pF from Q4 collector to Q4 base. Does that help?

 

[Uros11]

Oh yes, I also put that cap before, but I removed it (don't know why honestly). Now I put it again I get similar results as you can see.

 

[Krisfr]

Please post your .four directives for checks on correctness. Thanks

 

[Uros11]

Hello, you can find it in the first picture on initial post

 

[Baldin]

Do some .tran sims to make sure you actually get a sinus on output .... seems from the plot that you are not at the moment ... probably not biased corectly

 

[Uros11]

Hello, I saw that my mistake was instead of putting .option i put .options, so everytime I ran it, it was wrong. Thank you all for your time.

 

[Ylli]

"SINE(0, 800m, 50)"
.four 1KHz 10 V(output)"

Do you see any issues there?

 

[Uros11]

Yes, I saw that, and fixed it. Thank you all very much for your time, especially you Ylli, because you were there in every thread, and helped me a lot with this project.

 

[Ludus Tonalis]

You are asking for a fourier analysis at 1kHz, but you have set your stimulus at the input to 50Hz. They need to be the same frequency.

.option and .options are the same thing, incidentally.

 

[Uros11]

Hey,

Yes, I know it should be the same, and when it was the same it didn't work. When I changed .options to .option everything was good.

 

[danny_66]

Hello,

Here are the options that I use,
they adapt the tran sim parameters depending on the frequency.

input voltage:
SINE(0 1 {freq} 0 0)

.tran 0 {simtime} {dlytime} {timestep}
.options plotwinsize=0
.options method=gear
.options numdgt=7
.param Freq=1k
.param numcyc=50
.param dlycyc=5
.param FFT=2**16
.param simtime=numcyc/Freq+dlytime
.param dlytime=dlycyc/Freq
.param timestep=(simtime-dlytime)/FFT
.four {freq}15 8 V(Vout)
.four {freq}15 8 V(Vvas)
.four {freq} 4 8 V(Vout)

Regards,
Danny

 

[Ludus Tonalis]

I don't know if it is because of the settings or my circuit?

In the end it's the circuit. Working .asc file attached. The only change to your circuit is that I added a miller capacitor to the VAS - your circuit was unstable and oscillating.


View attachment Uros11.asc