Here's the circuit from post #14 with some local emitter negative feedback in the last stage, which significantly reduces the distortion.
To view the harmonic distortion percentage, use the .four command as shown, and look in the View/Spice error log after doing the simulation.
Such feedback in the first stage should give a further reduction in distortion (with a reduction in gain that negative feedback always gives, of course).

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Thanks crutschow this is very helpful. With this circuit the THD is 1.67%, is that a lot of distortion?

I then modified the circuit by adding some negative emitter feedback to stage 1. The THD dropped to 0.61%.

Finally, I added two-stage feedback and that reduced the THD to 0.16%.

2019

 

Many people can hear 0.1% distortion. People who are tone deaf and do not like music cannot hear 50% distortion.
In your circuit the amount of distortion increases a lot if the input level increases so all the peak levels of music and speech from your amplifier will be terribly distorted.

You never answered what is your tiny input signal and why is its level so low. A microphone level is at least 20 times higher.
You also never answered why are all the resistors values so low and the capacitor values so high.

Your circuit came from a book. Many (all?) books, professors and teachers make mistakes you know.






many books

 

With this two-stage feedback circuit, the overall voltage gain should be Av = RF / RE1 = 18KΩ / 50Ω = 360.
In reality, the overall gain turns out to be 240. Vin = 500uV and Vout = 103.56mV. I have Rser = 0Ω so I
don't understand what is causing this difference.

2019

 

the overall voltage gain should be Av = RF / RE1 = 18KΩ / 50Ω = 360.

That's only true if the amp open-loop gain is infinite (or very high).
That's not true in this case, so you have to include the finite open-loop gain in your closed-loop gain calculation.

 

That's only true if the amp open-loop gain is infinite (or very high).
That's not true in this case, so you have to include the finite open-loop gain in your closed-loop gain calculation.

Thanks. I will do some research regarding open-loop gain and closed-loop gain.

 

Thanks. I will do some research regarding open-loop gain.

Here is my calculation.

Av(no feedback) = 556

β = RE1 / RE1 + RF = 50Ω / 50Ω + 18KΩ = 0.00277

Av(with feedback) = A(no feedback) / 1 + [A(no feedback) * β] = 556 / 1 + [556 * 0.00277] = 219

The gain per the sim is 207. Have I done the calculation correctly?

 

Av(no feedback) = 410

How did you come up with this number?

 

My simulation shows that with an input of 0.5mV peak then its output is a little higher than 100mV peak so it gain is a little more than 200.

 

My simulation shows that with an input of 0.5mV peak then its output is a little higher than 100mV peak so it gain is a little more than 200.

Sounds good. Thanks.

 

How did you come up with this number?

Hi Jony. I think the gain should be 556 w/o feedback. And then I tried to calculate the
gain with feedback in post #26.

 

Your two-stage amplifier made with transistors was used 50 years ago or is used in school today. Now we use an opamp.