Common Emitter Amp Design help!!

Thread Starter

FlyingRat

Joined Jan 26, 2016
14
But in your amplifier without external load the voltage swing has almost nothing to do with Icq/Ibq. So this is why you can pick Ic valuer randomly.
And to get max.swing ensure that Vce = 0.5Vcc.
My teacher said that there will be a load, but that's not essential part to know when building amp. So I guess I SHOULD start from the output characteristics - because with that I can pick random Icq/Ibq and ensure that Q point 'meets' Vcc/2 for max.swing?
Is there a way I can stay in touch with you for further (faster) references, except forum?
 

Jony130

Joined Feb 17, 2009
5,488
You can do what you want. But the output characteristics is not needed. For example let say I pick Rc = 100Ω and Ve = 1V.
Ic = (0.5*(Vcc - Ve))/Rc = 6.5V/100Ω = 65mA and Re = 1V/65mA = 15Ω. So as you can see my Q point 'meets' for max.swing.
 

Thread Starter

FlyingRat

Joined Jan 26, 2016
14
You can do what you want. But the output characteristics is not needed. For example let say I pick Rc = 100Ω and Ve = 1V.
Ic = (0.5*(Vcc - Ve))/Rc = 6.5V/100Ω = 65mA and Re = 1V/65mA = 15Ω. So as you can see my Q point 'meets' for max.swing.
So, based on few of your posts on different topics, I tried some of my calculations. And here it goes:
Assuming:
Rc = 100ohm and Ve = 1V.

Ic = (0.5*(Vcc - Ve))/Rc = 7V/100Ω = 70mA
Ic = Ie
Re = Ve/Ie = 1/70mA = 14.29ohm
Vbe = 0.7V
Vb = Vbe + Ve = 0.7 + 1 = 1.7V
hfemin = 100
Ib = Ic/hfemin = 70mA/100 = 700uA <-- Why to choose hfemin insted hfeaverage??
R1 = (Vcc-Vb)/(11*Ib) = 13.3/11*700uA = 1727ohm
R2 = Vb/(10*Ib) = 1.7/(10*700uA) = 243ohm

So if we try to calculate total power dissipation we get: Ptmax = Vce*Ic = 7.5*70mA = 525mW. And this is 50% of max power dissipation for my transistor. That is good.
What do You think?
P.S. Rc is not my load, it's the Rt as you can check in my schematic I attached. I guess that's the same sh*t, execpt Rt is only for AC output.

IMAG1770.jpg
 

Jony130

Joined Feb 17, 2009
5,488

dannyf

Joined Sep 13, 2015
2,197
"Ptmax = Vce*Ic = 7.5*70mA = 525mW. And this is 50% of max power dissipation for my transistor. That is good."

This is a to92 device and its Rjta is probably around 200c/w. In your case, that means a temperature rise of 100c.

It is hot enough to melt solder.

It is nuts to run this type of services at 70ma Ic. 7ma Ic maybe. 70ma Ic, nfw.
 

Thread Starter

FlyingRat

Joined Jan 26, 2016
14
"Ptmax = Vce*Ic = 7.5*70mA = 525mW. And this is 50% of max power dissipation for my transistor. That is good."

This is a to92 device and its Rjta is probably around 200c/w. In your case, that means a temperature rise of 100c.

It is hot enough to melt solder.

It is nuts to run this type of services at 70ma Ic. 7ma Ic maybe. 70ma Ic, nfw.
Maximum collector current for my type of bjt is 2Amps though.
 

Thread Starter

FlyingRat

Joined Jan 26, 2016
14
Ok guys, I included my calculations for DC analysis, and I think I got pretty much good results, though Ucq is 7,94V instead 7,5V for max.swing, but I guess this is okay as well.
Next thing is that I need to pick my capacitor values for my estimated Q point, and I guess there is no way doing that if I don't know value of load Rl and which frequencies I need to 'boost up'.
I will include my SPICE schematics (green sine - Uce, blue sine - input) and analytic values so you can comment those.
Thanks!

P.S. Jony130,
I still don't know how You choose Rc-100ohm. I know that is assumption, but why 100ohms.

DC.JPG
IMAG1783.jpg
 
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