Hi:

I don't understand why the output voltage has a maximum positive peak of
600 mV and a maximum negative peak of -700mV.

I have experimented by varying the Q-point but no luck.

Any help would be appreciated.

David

 

hi 55,
What is the circuit designed to do.?
It's a very poor design.
E

 

hi 55,
What is the circuit designed to do.?
It's a very poor design.
E

Its supposed to amplify an audio signal with a voltage gain of 50. This is taken
from a Heathkit course. Its hard to learn this stuff when the course material is
garbage.

 

hi,
OK.
Look very carefully at the form factor of the output sine wave, what do you observe.?
E
BTW: the Form Factor is the 'purity' of sine wave.

 

hi,
Right click in the Plot area.
On the Pop up menu, select View
Select FFT [Fast Fourier Transform]
Use the Default settings
Click OK.
Look at the Harmonics in the FFT plot.
E

 

hi,
OK.
Look very carefully at the form factor of the output sine wave, what do you observe.?
E
BTW: the Form Factor is the 'purity' of sine wave.

Hi Eric:

The upper and lower peaks have a different magnitude and the width of the upper and lower portion of the cycle is different. Is that what you mean by Form Factor?

 

You didn't properly set the DC quiescent current (q-point). You base DC voltage should be in the range of about 1V to 2V (Vb = Ve + 0.7V).

But in your circuit you have 10.7V at the base terminal. And the collector voltage should be set around 0.5Vcc ( around 7.5V in your circuit).

Also, your circuit has a very nonlinear voltage gain.

The voltage gain is a signal level dependent.

Av = gm * Rc||RL ≈ 40*Ic * Rc||RL.

When the input signal swings positive so that the collector current increases, the voltage gain also increase his value. And for negative input signal swings the collector current drops. So the gain decrease his value. As you can see the voltage gain of this CE amplifier was changing when the output signal has swinging. This represents a high level of distortion.

Try this circuit

Maybe this post help you set the proper operation point
https://forum.allaboutcircuits.com/threads/common-emitter-max-current-ic.71069/#post-494633

 

Hi Eric:

The upper and lower peaks have a different magnitude. Is that what you
mean by Form Factor?

See Post #5

 

The transistor is being driven into a non-linear region of its voltage-gain characteristic.

 

I cannot open a .asc file. I presume it is a file from a simulator program.
Is the output AC coupled? If it is, then the average value must be 0V.
If the peak positive value is not the same as the peak negative value, then the wave shape is not symmetrical about 0V. This would indicate distortion in a sine wave signal.

 

See
To ensure that nonlinear distortion is acceptable for an amplifier with a common emitter, it is necessary to apply a signal with an amplitude of less than 1 mV.

 

Your transistor has no negative feedback to reduce its extreme distortion. A transistor without negative feedback is non-linear (reduced voltage gain) when approaching cutoff (the top of the output waveform).

I biased the base better, reduced the value of the emitter resistor, and increased the values of the collector and load resistors.
Then I added another emitter resistor to add negative feedback that reduces the gain and reduces the distortion.

 

Your transistor has no negative feedback to reduce its extreme distortion. A transistor without negative feedback is non-linear (reduced voltage gain) when approaching cutoff (the top of the output waveform).

I biased the base better, reduced the value of the emitter resistor, and increased the values of the collector and load resistors.
Then I added another emitter resistor to add negative feedback that reduces the gain and reduces the distortion.

Hi.
I do not mind what you wrote, but I do not like that there are different load resistance in your three amplifiers.
Why do not you use Fourier analysis - ".Four".If you did, your remark about distortions would not be just words.

 

The OP asked why is the positive peak much less than the negative peak and I explained and showed that the severe distortion from his transistor that has no negative feedback caused it. In my second simulation I showed that correcting the biasing and lightening the load did not eliminate the distortion but my third simulation did eliminate the distortion. The OP did not want distortion percentages.

I made a typo with the load on my third simulation. Here it is with the same 5.6k ohms load as he used:

 

Thanks everyone. I need to digest all of this great information. Much
appreciated.

 

I cannot open a .asc file. I presume it is a file from a simulator program.
Is the output AC coupled? If it is, then the average value must be 0V.
If the peak positive value is not the same as the peak negative value, then the wave shape is not symmetrical about 0V. This would indicate distortion in a sine wave signal.

Hi Mr. Chips

Yes its from LTspice