# Common-Emitter Amplifier Design.

Joined Mar 4, 2023
8
I have an analog circuits project. For the given circuit , RL is 20k ohms. Vout should be Vpp = 22-24 V (peak-to-peak) and gain should be greater than 10. Capacitor corner frequencies should be less than 10 Hz and power consumption for each resistor should be less than 0.2 W. I can not adjust the required peak to peak voltage. What should I do?

#### Attachments

• 36.1 KB Views: 29

#### LvW

Joined Jun 13, 2013
1,766
At first - you should convince yourself if you really understand how such a transistor amplifier works.
Then - as a second step - you should remember and collect all relevant formulas and expressions (voltage gain, bias parameters, 1st-order highpass properties, negative feedback alternatives,...)

Joined Mar 4, 2023
8
At first - you should convince yourself if you really understand how such a transistor amplifier works.
Then - as a second step - you should remember and collect all relevant formulas and expressions (voltage gain, bias parameters, 1st-order highpass properties, negative feedback alternatives,...)
I started with assuming Icq = 1.6 mA. After some calculations I plotted the AC load line and picked my Rc and Re values but when I simulate it using ltspice Vout is always clipped.

#### ericgibbs

Joined Jan 29, 2010
19,024
hi uu,
Welcome to AAC,
E

Joined Mar 4, 2023
8
hi uu,
Welcome to AAC,
E
Thank you! All given component values are on the attached image and I wrote the ones given in the text. All the missing ones should be designed.

#### ericgibbs

Joined Jan 29, 2010
19,024
hi uu,
Is it possible that you could post a diagram of the circuit showing all the component values you have calculated.?

E

Joined Mar 4, 2023
8
hi uu,
Is it possible that you could post a diagram of the circuit showing all the component values you have calculated.?

E
I dont know why it is clipped in below

#### Attachments

• 270.4 KB Views: 20

#### ericgibbs

Joined Jan 29, 2010
19,024
hi uu,
Thanks for the diagram, I see you are using LTSpice, if you post your asc file I will give it a run in LTSpice.
E

#### LvW

Joined Jun 13, 2013
1,766
I dont know why it is clipped in below
What is the DC working point (Vce/Ic) ?
You have mentioned a dc quiescent current Ic=1.6mA.
Can you compute the DC voltage drop across the collector resistance of 12 kOhms?
Can you compare this value with the available supply voltage?

Last edited:

Joined Mar 4, 2023
8
hi uu,
Thanks for the diagram, I see you are using LTSpice, if you post your asc file I will give it a run in LTSpice.
E
OMG, Thank you so much! I improved it right now but still swing is not symmetrical.

#### Attachments

• 2.1 KB Views: 8

#### ericgibbs

Joined Jan 29, 2010
19,024
hi uu,
I have redrawn it in lTS.

Look at the power line Polarity for starters, there are other problems

Update: Ignore that polarity, query I can just see the Minus sign, !

Last edited:

#### ericgibbs

Joined Jan 29, 2010
19,024
hi uu,
This is ac AC analysis plot, also check the signal and biassing on the transistor Base.!
E

Joined Mar 4, 2023
8
hi uu,
This is ac AC analysis plot, also check the signal and biassing on the transistor Base.!
E
View attachment 288870View attachment 288869
Thank you! I did not adjust the capacitors yet first I wanted to work on gain and now symmetry is the problem but not much of a problem I guess.

#### ericgibbs

Joined Jan 29, 2010
19,024
hi uu,
Let us know how it goes, cannot help you too much as this is a College assignment.

E

#### MrChips

Joined Oct 2, 2009
30,986
CE changes the gain-frequency response. Remove CE for now and check the frequency response.
Base voltage bias will affect clipping. Remove the sine wave input and adjust DC base bias resistors R1 and R2 until you get collector voltage equal to ½Vs.

#### LvW

Joined Jun 13, 2013
1,766
I think. the last line is - more or less - a "trial and error" method without a deeper understanding of whats going on.
Dont you think a more systematic design with some calculations would be a better help?

#### MrChips

Joined Oct 2, 2009
30,986
I think. the last line is - more or less - a "trial and error" method without a deeper understanding of whats going on.
Dont you think a more systematic design with some calculations would be a better help?
Ah! If that is what TS wants, sure.
Calculate voltage at emitter.
Determine base voltage to give the desired VBE.
Calculate R1-R2 values to give the desired base voltage.

#### LvW

Joined Jun 13, 2013
1,766
Ah! If that is what TS wants, sure.
Calculate voltage at emitter.
Determine base voltage to give the desired VBE.
Calculate R1-R2 values to give the desired base voltage.
Yes - and regarding the "desired base voltage": It is not too important if we assume (and this is ALWAYS an estimate) a base-emitter voltage of 0.65 or 0.7 volts. This is due to the DC feedback effect (caused by the emitter resistors) which makes the whole circuit pretty insensitive to BJT parameters.

#### crutschow

Joined Mar 14, 2008
34,709
but still swing is not symmetrical.
Circuit design is not just plugging in formulas, you also need an understanding of the circuit operation.

Since the impedance from the transistor collector to the plus supply is is higher than the impedance from the emitter to the minus supply, the output clipping point at the collector cannot be symmetrical as the transistor goes from full on (saturated) to full off.
It is those two impedances that supply the current to the output load on alternate half cycles.
You would need to make those two impedances equal for symmetrical clipping.

#### MrAl

Joined Jun 17, 2014
11,587
I have an analog circuits project. For the given circuit , RL is 20k ohms. Vout should be Vpp = 22-24 V (peak-to-peak) and gain should be greater than 10. Capacitor corner frequencies should be less than 10 Hz and power consumption for each resistor should be less than 0.2 W. I can not adjust the required peak to peak voltage. What should I do?
Hi,

What you should do is learn to analyze circuits in general. That would not be that hard for this problem because you are given the Beta and the base emitter diode drop. That implies that you can use the current generator and diode transistor model to calculate your resistor values.
Once you learn to analyze this circuit, you can then calculate the values for the resistors, and also check your design using math rather than a simulator. This gives you a lot of insight into these circuits.

The transistor model that can be used seems to be the current controlled current source (CCCS) and a diode for the base emitter diode. This is a rather simple model but gives you a lot of insight as to how the circuit works without getting over complicated. If i can get the time (and energy ha ha) i'll post an example using different parameters. The degree of 'accuracy' you can get like this is astonishing, although it will be in theory only.

Once you learn this though you will never have to ask another question like this because you will be armed enough to do it yourself.