# Common Emitter Amplifier design & Simulation LTspice - help needed

#### mishra87

Joined Jan 17, 2016
1,040
Hello Guys,
I started designing the common emitter amplifier with below specification.
Design Specification :
VCC = 12V
IC = 4mA
AVOL = 70

I have done some calculation to find all values needed in below circuit. When i run the simulation i found the gain of amplifier is 4.
I have expected the output voltage is 50mV*70 = 3.5Vpeak.
Could anyone figure the reason why the gain is only 4 ?

How do i can find close loop gain ?

Many thanks

#### Attachments

• 1.7 KB Views: 45

#### dl324

Joined Mar 30, 2015
16,993
Why do you think 4 is wrong?

#### ericgibbs

Joined Jan 29, 2010
18,990
hi 87,
Note you should set the AC =1 for AC analysis, this gives a true reading for the response

E

#### Attachments

• 1.7 KB Views: 8

#### Audioguru again

Joined Oct 21, 2019
6,758
Your value for C5 causes it to have a reactance of 107 ohms at 1kHz.
Do it like this:

#### Attachments

• 26.3 KB Views: 38
Last edited:

#### ericgibbs

Joined Jan 29, 2010
18,990
hi 87,
This is another option.
E

#### Attachments

• 39.6 KB Views: 31
• 38.1 KB Views: 31

#### mishra87

Joined Jan 17, 2016
1,040
Why do you think 4 is wrong?
Hi,
I do not know so that is why i asked.

Regards,

#### mishra87

Joined Jan 17, 2016
1,040
Your value for C5 causes it to have a reactance of 107 ohms at 1kHz.
Do it like this:
Hello Thanks,

Your simulation result is not distorting.

Regards,

#### mishra87

Joined Jan 17, 2016
1,040
hi 87,
Note you should set the AC =1 for AC analysis, this gives a true reading for the response

EView attachment 252621
Hi,
Gain at 1KHz is 22db.
I still do not understand.

How this relates to my questions in #1

Thanks

#### mishra87

Joined Jan 17, 2016
1,040
Hello All,

Please forgive if i am missing something here in my understanding...

1. Input = 50mVpeak, output = 200mV
How do i calculate the gain ?

2. All component value calculated for below..
VCC = 12V
IC = 4mA
AVOL = 70

3. I expect the output = AVOL * Input = 70*50mV = 3.5Vpeak.

#### Audioguru again

Joined Oct 21, 2019
6,758
Your simulation result is not distorting.
A transistor with no negative feedback and with a fairly high output level is not linear so it produces distortion.
My simulation of your circuit shows severe distortion. Here it is again with distortion and with very low distortion.
EDIT: I corrected the scale of the distorted signal:

#### Attachments

• 19 KB Views: 16
Last edited:

#### Jony130

Joined Feb 17, 2009
5,514
You did not provide enough room for the voltage to swing at the collector. Your DC voltage collector is Vc = 12V - 4mA * 437.5Ω = 10.2V.
Therefore 10.2 +/- 3.5V = 13.7V....6.7V how can the voltage at the collector be large than Vcc? And this is why you are getting distorted waveform.
Also, the C5 capacitor value is wrong (too small).

#### Audioguru again

Joined Oct 21, 2019
6,758
Not only the collector voltage is too high but the emitter resistor value is so high that it throws away 1/3rd of the 12V supply voltage.
I agree that the wrong base biasing causes the collector DC voltage to be too high.
The collector voltage is far from clipping, the distortion is caused by having no negative feedback to reduce it, but negative feedback also reduces the gain.

#### mishra87

Joined Jan 17, 2016
1,040
A transistor with no negative feedback and with a fairly high output level is not linear so it produces distortion.
My simulation of your circuit shows severe distortion. Here it is again with distortion and with very low distortion.
EDIT: I corrected the scale of the distorted signal:
Hello ,
Thanks.

This time your signal is looking good.
Could you please share LTspice to see the changes you have made.
You have attached only output waveform.

Cheers

#### ericgibbs

Joined Jan 29, 2010
18,990
Gain at 1KHz is 22db.
I still do not understand.
How this relates to my questions in #1
How can you read the Gain be 22dB.???
It relates to your question Why is my Gain low.

#### Attachments

• 139.2 KB Views: 6

#### Jony130

Joined Feb 17, 2009
5,514
Could you please share LTspice to see the changes you have made.
Look here

#### Attachments

• 1.8 KB Views: 7

#### Audioguru again

Joined Oct 21, 2019
6,758
Jony and I reduced the values of R1, R2 and R3 to get more output voltage swing.
I removed C5 so that distortion is very low but then the gain is low.
Jony added R6 and increased the value of C6 for a gain of 70 but with fairly high distortion.

#### Attachments

• 33.3 KB Views: 17
• 22 KB Views: 16
• 1.4 KB Views: 10