Clippping in the Output of a BJT Amplifier

Thread Starter

Murdock39

Joined Apr 21, 2021
11
Can anybody exaplain why I am getting this clippping in the output waveform? Any help in this regard will be much appreciated.


1618990216186.png
 

ericgibbs

Joined Jan 29, 2010
13,670
hi M39,
Welcome to AAC.
I would check the Base biassing , the Collector voltage is limited close to saturation.
Measure the Base voltage and Collector DC level without an input signal, what do you measure.?

E
 

Thread Starter

Murdock39

Joined Apr 21, 2021
11
hi M39,
Welcome to AAC.
I would check the Base biassing , the Collector voltage is limited close to saturation.
Measure the Base voltage and Collector DC level without an input signal, what do you measure.?

E
Thanks ericgibbs, You are right. I checked the biasing and I found the mistake.

I have another question if you could answer. The peak values of negative and positive peaks are different, positive peak is 95 mv while negaitve peak is -98mv. What could be the possible reason for this difference in magnitude ?

1618991592676.png
 

Thread Starter

Murdock39

Joined Apr 21, 2021
11
Hello,

The bias is not correct.
Have a look here for more info on the calculations:
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/biasing-calculations/

Bertus
Thanks Bertus, the link you suggested helped. I checked the biasing and I found the mistake.

I have another question if you could answer. The peak values of negative and positive peaks are different, positive peak is 95 mv while negaitve peak is -98mv. What could be the possible reason for this difference in magnitude ?

View attachment 236112
 

Thread Starter

Murdock39

Joined Apr 21, 2021
11
hi M39,
Check this LTS of your circuit note the Base to 0V resistor, and the DC level of the Collector voltage.
E
Can you explain why the absence of base to ground resistance is producing a sin wave which is not perfect. What can be the possible explainination?
 

ericgibbs

Joined Jan 29, 2010
13,670
Can you explain why the absence of base to ground resistance is producing a sin wave which is not perfect. What can be the possible explainination?
Hi,
Look at this plot, no 12K to Base,
What is the typical turn on Base voltage of a NPN transistor.???
E

Update,
The Quiescent Base Bias voltage/current is too high a value and the transistor is driven almost into the Collector voltage saturation mode.
The 12k reduces the Quiescent Base drive, so the collector is at approx Vcc/2

Do you follow.?
 

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Thread Starter

Murdock39

Joined Apr 21, 2021
11
Hi,
Look at this plot, no 12K to Base,
What is the typical turn on Base voltage of a NPN transistor.???
E

Update,
The Quiescent Base Bias voltage/current is too high a value and the transistor is driven almost into the Collector voltage saturation mode.
The 12k reduces the Quiescent Base drive, so the collector is at approx Vcc/2

Do you follow.?
Yes I understand it now! Thanks!
 

Audioguru again

Joined Oct 21, 2019
3,550
The 2N3904 has a typical hFE of 230 then the base current in the 150k base resistor causes clipping.
The OP discovered that it does not clip when the base resistor value is increased to 1M.
I simulated that using a transistor with less hFE also prevents the clipping.
A high ambient temperature also will probably cause clipping with the 1M base resistor.

The OP needs to see why negative feedback is used so that any passing Vbe or hFE in a transistor will prevent clipping and also prevent the distortion that was mentioned. A voltage divider for the base voltage as shown and an emitter resistor are usually used.
 

bertus

Joined Apr 5, 2008
21,559
Hello,

Here is a comparison between bias equations from the page I gave in post #3:
Transistor bias equations summary.png

The clipping can occur when the transistor is in saturation (to high collector current) or in cut-off (to low collector current).

Bertus
 

crutschow

Joined Mar 14, 2008
27,761
The peak values of negative and positive peaks are different, positive peak is 95 mv while negaitve peak is -98mv. What could be the possible reason for this difference in magnitude ?
Distortion of the waveform.
A small emitter resistor will reduce the distortion (the tradeoff is a reduction in gain).
 

Thread Starter

Murdock39

Joined Apr 21, 2021
11
The 2N3904 has a typical hFE of 230 then the base current in the 150k base resistor causes clipping.
The OP discovered that it does not clip when the base resistor value is increased to 1M.
I simulated that using a transistor with less hFE also prevents the clipping.
A high ambient temperature also will probably cause clipping with the 1M base resistor.

The OP needs to see why negative feedback is used so that any passing Vbe or hFE in a transistor will prevent clipping and also prevent the distortion that was mentioned. A voltage divider for the base voltage as shown and an emitter resistor are usually used.
Thanks Audioguru again! Your given information is helpful.
 
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