Here is a little gem that I found on line. A direct-coupled CE amp.
I calculated the voltage gain for each stage and for the amplifier overall.
The overall gain is 11,320 and yet LTS results in a gain of 350. 99% of
vs enters the first stage but I can't figure out why the gain is so low.

Any suggestions?

 

hi aac,
Do you have a link to the online circuit, RE2 looks a high value..
What did your maths say for the gain of the 1st stage.??

E

 

hi,
A LTS sim of the steady state voltages.
How do they compare with your calculations.?
E

 

Well, let me try.

From LTspice we know that Ic1 = 323μA and Ic2 = 423μA

The second stage gain is Av2 ≈ 38*Ic2 *Rc = 38*423μA*10kΩ ≈ 160 V/V

And the input impedance of a second stage is Zin2 = β/gm = 300/(38*423μA) = 18.66kΩ

The first stage gain is Av1 ≈ (Rc1||Zin2)/(RE1) = 2.1 V/V

Hence the overall voltage gian is Av = 2.1*160 = 336 V/V

 

hi aac,
Do you have a link to the online circuit, RE2 looks a high value..
What did your maths say for the gain of the 1st stage.??

E

My steady state voltages agree with the text article.

Yes, I played with RE2 and it seemed to make things worse.

Here is the link. Schematic is on page 300.
http://www.talkingelectronics.com/Download eBooks/Principles of electronics/CH-11.pdf
My Av1 = 65 (See attached gain analysis)

 

Here is the link. Schematic is on page 300.
http://www.talkingelectronics.com/Download%20eBooks/Principles%20of%20electronics/CH-11.pdf
My Av1 = 65 (See attached gain analysis)

Why did you omitted the Ce1 capacitor in your LTspcie simulation?

 

Why did you omitted the Ce1 capacitor in your LTspcie simulation?

The circuit doesn't work with that capacitor.

 

But lack of this capacitor is the reason why your calculation does not match the LTspice result.
Because without this capacitor (Ce1) the first stage gain is very low Av1 = (Rc1||Zin2)/( RE1+re1 ) compare to the situation when you include this capacitor Av1 = (Rc1||Zin2)/re1

 

But lack of this capacitor is the reason why your calculation does not match the LTspice result.
Because without this capacitor (Ce1) the first stage gain is very low Av1 = (Rc1||Zin2)/( RE1+re1 ) compare to the situation when you include this capacitor Av1 = (Rc1||Zin2)/re1

If you put the RE1 bypass cap into the circuit, the gain is nuts and the amplifier is useless.
Why would they even show such a circuit?

I have re-calculated my gain as attached.

 

If you put the RE1 bypass cap into the circuit, the gain is nuts and the amplifier is useless.
Why would they even show such a circuit?

I don't know. Ask the authors.

 

I don't know. Ask the authors.

The author probably wouldn't understand.

 

Well, let me try.

From LTspice we know that Ic1 = 323μA and Ic2 = 423μA

The second stage gain is Av2 ≈ 38*Ic2 *Rc = 38*423μA*10kΩ ≈ 160 V/V

And the input impedance of a second stage is Zin2 = β/gm = 300/(38*423μA) = 18.66kΩ

The first stage gain is Av1 ≈ (Rc1||Zin2)/(RE1) = 2.1 V/V

Hence the overall voltage gian is Av = 2.1*160 = 336 V/V

In your Av2 calc where did the "38" come from?

Why does Av2 = RC2 / r'e2 not work?

aac

 

In your Av2 calc where did the "38" come from?

From what I see you are using re = Vt/Ie but we could use a transconductance instead gm = dIc/dVbe = Ic/Vt ≈ 1/re
Therefore 1/Vt = 1/26mV ≈ 38 and gm = 38*Ic or gm = 40*Ic for Vt=25mV

 

From what I see you are using re = Vt/Ie but we could use a transconductance instead gm = dIc/dVbe = Ic/Vt ≈ 1/re
Therefore 1/Vt = 1/26mV ≈ 38 and gm = 38*Ic or gm = 40*Ic for Vt=25mV

I understand now. Thanks.