Cannot Run A Transient Analysis On This Two-Stage Amplifier

ronv

Joined Nov 12, 2008
3,770
All of a sudden I can no longer run a Transient Analysis on this Two-Stage Amplifier
even though it ran okay before. I am interested in graphing Vout. I didn't knowingly change anything since the other day. Could someone please tell me what I have done to mess it up?

Thanks
Works for me.
 

eetech00

Joined Jun 8, 2013
1,968
All of a sudden I can no longer run a Transient Analysis on this Two-Stage Amplifier
even though it ran okay before. I am interested in graphing Vout. I didn't knowingly change anything since the other day. Could someone please tell me what I have done to mess it up?

Thanks
Hi

There's already a built-in model for the 2N3904. Comment out the 2N3904 model line then try the sim.

eT
 

Jony130

Joined Feb 17, 2009
5,180
It is supposed to have
a voltage gain of 4,037
But this 4,037 is a gain of the amplifier itself without including Rg effect.
The if we include the Rg the gain will drop to around 4037*1/1.6 ≈ 2500 (2443 in LTspice).

And after if I change the simulation temperature to 17°C the sim shows 4010.
 

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Audioguru

Joined Dec 20, 2007
11,249
Why are all the resistor values so low? Why are the capacitor values so high? Guessing instead of calculations?

What is "A Transient Analysis"?
Don't you simply want to show a few cycles of the output (or any other place) sinewave to see its amplitude and distortion?

Then why clutter up the commands with .model that has minimum hFE, the .op, the .ac and the .four commands?
When I delete those extra commands I get lots of gain like this:
 

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Thread Starter

newbie2019

Joined Apr 5, 2019
95
Why are all the resistor values so low? Why are the capacitor values so high? Guessing instead of calculations?

What is "A Transient Analysis"?
Don't you simply want to show a few cycles of the output (or any other place) sinewave to see its amplitude and distortion?

Then why clutter up the commands with .model that has minimum hFE, the .op, the .ac and the .four commands?
When I delete those extra commands I get lots of gain like this:
Ask Professor Malvino it is his design in his textbook. His design with negative feedback doesn't work right either. How can removing those commands change
the operation of the circuit?
 

Thread Starter

newbie2019

Joined Apr 5, 2019
95
But this 4,037 is a gain of the amplifier itself without including Rg effect.
The if we include the Rg the gain will drop to around 4037*1/1.6 ≈ 2500 (2443 in LTspice).

And after if I change the simulation temperature to 17°C the sim shows 4010.
If I set Rg resistance to zero, the overall voltage gain is vout / vin = 36mV / 0.5mV = 72.
 

Thread Starter

newbie2019

Joined Apr 5, 2019
95
The circuit does not have an "Rg". Maybe you mean "Rser"?
When I made Rser zero and deleted the clutter of commands, the output is 67 times more than yours like this:
I hear you but I still get the same result. The only thing that I can think of is if the 2N3904 model
differs from yours?
Try this circuit
The positive peak is 373mV and the negative peak is 427mV looking at the
graph but Vout is 391mV per the ac analysis at Temp = 25C. Is that THD?
 

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Jony130

Joined Feb 17, 2009
5,180
The positive peak is 373mV and the negative peak is 427mV. Is that THD?
Yes, but they are better visible if you increase the input signal amplitude. And this distortion are caused by transistor himself. To be more specifice because of the fact that re resistance is not constant but it changes with input signal re = Vt/Ic. This is why you see this distortion.
And the voltage gain You or Professor Malvino calculated are based on a linearized transistor model (AC analysis in LTspice).
 

Thread Starter

newbie2019

Joined Apr 5, 2019
95
Yes, but they are better visible if you increase the input signal amplitude. And this distortion are caused by transistor himself. To be more specifice because of the fact that re resistance is not constant but it changes with input signal re = Vt/Ic. This is why you see this distortion.
And the voltage gain You or Professor Malvino calculated are based on a linearized transistor model (AC analysis in LTspice).
Okay. I increased the input signal to 500uV. The graph now indicates a positive peak of about 1.5V and a negative peak of about
2.4V. The ac analysis says that the output is 1.96 volts. So the distortion is terrible and this amplifier would sound like garbage if
I built it?
 

crutschow

Joined Mar 14, 2008
25,247
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).

upload_2019-5-17_14-3-29.png
 

Audioguru

Joined Dec 20, 2007
11,249
What is your signal source that has a peak level of 0.5mV (500uV) which is almost nothing? If you turn up the volume a little then the output will be clipping badly.
Nobody mentioned noise (rumble and hiss). With such a high gain then the output will produce noticeable noise.
 
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