# Clippping in the Output of a BJT Amplifier

#### 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.

#### ericgibbs

Joined Jan 29, 2010
17,987
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

#### ericgibbs

Joined Jan 29, 2010
17,987
hi M39,
Check this LTS of your circuit note the Base to 0V resistor, and the DC level of the Collector voltage.
E

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#### 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 ?

#### Murdock39

Joined Apr 21, 2021
11
Hello,

The bias is not correct.

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

#### 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
17,987
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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#### 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
6,152
Your simulation software does not show the range of base voltage and range of hFE of a 2N3904 transistor. Also, the temperature affect also is not shown.

#### ericgibbs

Joined Jan 29, 2010
17,987
Can anybody exaplain why I am getting this clipping in the output waveform?
The TS asked simply why its clipping, nothing about hFE or Temperature effect.

#### Audioguru again

Joined Oct 21, 2019
6,152
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
22,215
Hello,

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

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
32,852
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).

#### 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.