# common-source amplifier

Discussion in 'Homework Help' started by anhnha, Oct 2, 2013.

1. ### anhnha Thread Starter Well-Known Member

Apr 19, 2012
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I am simulating common-source in LTspice and I don't know why there is no amplification at all.
Can anyone explain why? It knows there is something obvious that I don't see.

In common-source Av = gm.Rd but in in my picture Vout and Vin are
same in magnitude.
Here is Vin:

And this is Vout:

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2. ### anhnha Thread Starter Well-Known Member

Apr 19, 2012
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Um, sorry, this is Vout. It is even smaller than Vin

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3. ### WBahn Moderator

Mar 31, 2012
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Which node is n002. Don't just say it's the output. Identify which node in your circuit it is.

My guess is that it is the node that connects the resistor to the 5V supply.

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Simply you don't proper bias a mosfet.
See my simulation file

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5. ### anhnha Thread Starter Well-Known Member

Apr 19, 2012
776
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Yes, it is at drain of mosfet.

6. ### anhnha Thread Starter Well-Known Member

Apr 19, 2012
776
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I wonder why my bias is incorrect. I use input signal 3V DC offset and AC amplitude is 1V.

7. ### Jony130 AAC Fanatic!

Feb 17, 2009
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The mosfet that you decided to choose for simulation is a POWER MOSFET.
And as you can see in datasheet for Vgs = 3V will be greater than 25A (LTspice show 113.8A)
http://aosmd.com/pdfs/datasheet/AO6408.pdf
So for this Vgs and Rd = 1K MOSFET will be Full ON (triode region).

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8. ### WBahn Moderator

Mar 31, 2012
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I'm not sure about this, either. What is the threshold voltage for the MOSFET you are simulating?

If you are seeing 5V at the drain of the MOSFET (connection between it and the resistor), then the MOSFET isn't turning on which implies that the threshold voltage is more than 4V.

One thing to do is to plot out the DC transfer characteristic by applying a slow ramp from 0V to 5V (say over 1s) and see what your active region is.

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9. ### WBahn Moderator

Mar 31, 2012
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But she is seeing 5V at the output (MOSFET drain to resistor connection), which indicates that the MOSFET is Full OFF.

10. ### Jony130 AAC Fanatic!

Feb 17, 2009
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But in my simulation mosfet is Full ON as expected and Vds = 100uV max and Id = 4.999mA
anhnha must have something wrong with his simulation MOSFET model.

I use this model

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Last edited: Oct 2, 2013
11. ### anhnha Thread Starter Well-Known Member

Apr 19, 2012
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Thanks, the threshold voltage is 1V. I was scared seeing too many parameters. I don't have much time and feel really worry now. I will look it carefully and come back again.

12. ### WBahn Moderator

Mar 31, 2012
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I hope I didn't offend the OP by saying "she". I see "anhnha" and my mind reads it as "annah".

Your sim makes more sense.

In the OP's sim, the node voltage is going slightly above 5V, which means some kind of capacitive push, most likely.

13. ### anhnha Thread Starter Well-Known Member

Apr 19, 2012
776
48
Hi Jony.
In your simulation Vin is 4mv peak to peak and Vout is 2.526V peak to peak, not exact number because I don't know how to measure it exactly.
From that voltage gain Av= Vout/Vin = 2.526/0.004= 631.5
Does it match with theory?
In common source Av = gm.Rd.
In this case gm = Kp=90, right? Then Av = gm.Rd = 90000. Where I am wrong?

WBahn:
Not at all!

14. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Do you know the theory?
For Vgs = 1.007V and Vp = 1V; Id = 2.2mA

Id = 0.5*Kp *(Vgs - Vth)^2

So
$gm = 2\sqrt{0.5Kp *Id} = 0.629S$

And the Voltage gain

Av = gm * Rd = 629V/V

LTspice shows 627.865V I use AC analysis to find a voltage gain.
See my simulation file.

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15. ### anhnha Thread Starter Well-Known Member

Apr 19, 2012
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Thanks, I was mistaken between Kp and gm; they are not the same.
Jony, I don't understand ac analysis. What does ac analysis do? It analyses parameters with frequency sweep, right?
And how can you plot voltage gain?

16. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Yes.

Do you open the LTspice file that i have attached in the previous post?

17. ### anhnha Thread Starter Well-Known Member

Apr 19, 2012
776
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Yes, I ran it but I see nowhere in ac analysis they say about voltage gain. And in your simulation, there are two lines, one is dotted and the other is bold. I think they are output voltage and voltage gain respectively but I can't figure out where I can choose to plot voltage gain. Is it automatically plotted in ac analysis?

18. ### Jony130 AAC Fanatic!

Feb 17, 2009
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I simply set AC input voltage to 1V, next I ran a sim and plot Output voltage.
And since Vin = 1V the Vout is a voltage gain already. Av = Vout/Vin = Vout/1 = Vout.
The bold trace show Voltage gain (in dB or linear axis) vs frequency and dotted trace show phase shift vs frequency.

No, I save a plot setting.

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19. ### anhnha Thread Starter Well-Known Member

Apr 19, 2012
776
48
Well, that means we can't directly plot Vout, right?
Ah, I didn't notice that.

20. ### anhnha Thread Starter Well-Known Member

Apr 19, 2012
776
48
Another question, mosfet in LTspice has a lot of parameters such as this one:
Do you usually use most of them? These parameters I don't know when we need to use?
Rg=3 Rd=4.8m Rs=3.6m, Cgdmax=.7n Cgdmin=.25n Cgs=1n Cjo=.36n