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Simulators >> Circuit Simulators >> Few questions
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Message started by kidman on May 20^th, 2009, 3:31am

Title: Few questions
Post by kidman on May 20^th, 2009, 3:31am

1- I noticed something odd happening. When i use AC analysis and plot the gain the gain given was 70 but when I used DC analysis and plot dvout/dvin the curve shown showed that the max gain you could get out of the circuit is 40. I want to know what is the reason for that.

2- When making AC analysis I usually make the AC magnitude be 1 volt. Does that value actually matter? NB i'm using Vdd=1.2

3- Is there a way to let the simulator calculate or plot the Dynamic Range of the circuit?

Thanks!

Title: Re: Few questions
Post by Geoffrey_Coram on May 20^th, 2009, 4:19am

1. Are you sure you took enough points to compute an accurate numerical derivative?

2. The value doesn't matter; the circuit is linearized for ac analysis -- using 1 means you don't have to divide out the value at the end. (For differential circuits, I suppose you might use half a volt on each input.)

Title: Re: Few questions
Post by kidman on May 20^th, 2009, 4:29am

1- I just used the calculator. When i plot Vout vs Vin I open the calculator, click on the button "wave" then click of the vout curve. Then there's a function in the calculator called "deriv" that computes the derivative of Vout w.r.t vin. The result is dvout/dvin vs vin

Title: Re: Few questions
Post by buddypoor on May 20^th, 2009, 5:36am

kidman wrote on May 20^th, 2009, 3:31am:

Of course, there are many circuits which exhibit a DC gain which differs from the ac gain. During computation of the dc gain (dVout/dVin) all capacitive effects are neglected (perhaps ac feedback ?) Check your circuit .

Title: Re: Few questions
Post by kidman on May 20^th, 2009, 6:11am

The circuit is a simple basic common source amplifier

Title: Re: Few questions
Post by subgold on May 20^th, 2009, 6:15am

kidman wrote on May 20^th, 2009, 4:29am:

it has nothing to do with the calculator. you need to set the step size of your dc simulation small enough to obtain sufficient resolution.

Title: Re: Few questions
Post by buddypoor on May 20^th, 2009, 6:33am

kidman wrote on May 20^th, 2009, 6:11am:

The circuit is a simple basic common source amplifier

Well, another source of error could be that the axis in the dc plot are not spaced linearly. When there are no caps in the circuit the dc gain must be identical to the ac gain. Try to measure the slope of the dc response curve by hand rather than let it compute by taking the derivative.

Title: Re: Few questions
Post by Geoffrey_Coram on May 20^th, 2009, 8:53am

The "dc gain" is vout/vin, and that certainly can be different from the ac gain, which is the derivative of vout with respect to vin.

In order to compute dvout/dvin, you would numerically compute
v(out2) - v(out1) / ( vin1 - vin2 )
where in1 = in2 + epsilon, and v(out1) is the output voltage when the input is vin1. The trick is to pick epsilon small enough that you really get the tangent to the curve, yet large enough that you're not dominated by numerical noise.

Title: Re: Few questions
Post by buddypoor on May 20^th, 2009, 12:48pm

Geoffrey_Coram wrote on May 20^th, 2009, 8:53am:

The "dc gain" is vout/vin, and that certainly can be different from the ac gain, which is the derivative of vout with respect to vin.

Yes, you are right. It was my fault to call the derivative "dc gain".
Nevertheless, it is not uncommon that the slope of the dc transfer curve differs from the ac gain. That was my point. Thanks.

Title: Re: Few questions
Post by kidman on May 20^th, 2009, 1:04pm

Which one should I beleive?

Title: Re: Few questions
Post by buddypoor on May 21^st, 2009, 1:46am

kidman wrote on May 20^th, 2009, 1:04pm:

Which one should I beleive?

Nothing ! You should not believe one of us but try to become convinced.
Therefore, my recommendation again:
- Perform a dc-analysis and evaluate the slope of the curve around the correct bias point. All axis, of course, scaled linearly.
- Then, perform an ac analysis. Check if the same bias point is used from the program !
- If there is no considerable capacitice effect in the circuit, in both cases the gain must be identical.

Title: Re: Few questions
Post by Geoffrey_Coram on May 21^st, 2009, 6:07am

buddypoor wrote on May 21^st, 2009, 1:46am:

- If there is no considerable capacitice effect in the circuit, in both cases the gain must be identical.

Oy! there's a good point I hadn't been thinking about.

Title: Re: Few questions
Post by raja.cedt on May 21^st, 2009, 8:51am

hi buddypoor,
thanks for very interesting discussion, my doubt is in both cases gain has to be same if he use enough resolution in dc sweep.why capacitive feedback will effect this because even ac simulation also while consider at low frequency how they will impcat?

I have another question,i found opamp output impedance by injecting ac current source and finding voltage bump but this is not equal to my small signal output impedance? Now which one is correct?

Title: Re: Few questions
Post by buddypoor on May 21^st, 2009, 11:26am

raja.cedt wrote on May 21^st, 2009, 8:51am:

Sorry, but I didn´t get the meaning of the first part referenced above.

As for the 2nd part, how did you arrive at the value for "my small signal output impedance" ? By calculation or by what procedures ?
The method using an ac current source is 100% correct.
Do you have in both cases the same operating point ?

Title: Re: Few questions
Post by raja.cedt on May 21^st, 2009, 12:19pm

hi buddy poor,
in the first what i mean to say for finding dc gain both methods has to give same answer, how capacitive coupling will effect? or could you please explain how to find dc gain?
for second question i derived expression for output impedance and substituted all small signal parameters (gm,gds) from .op result and i have same operation point in both cases?

Thanks,
Rajasekhar.

Title: Re: Few questions
Post by buddypoor on May 22^nd, 2009, 1:16am

raja.cedt wrote on May 21^st, 2009, 12:19pm:

During dc analysis all capacitors are neglected (set to zero Farad).
Therefore, if caps are there, results of dc and ac analyses are different.
Regarding the output impedance, I would trust the simulation results.
Probably your calculations using formulas are not correct.
Which formula did you use ? Is the circuit a simple common source amp without any feedback?

Title: Re: Few questions
Post by raja.cedt on May 22^nd, 2009, 2:49am

hi buddypoor,
it is a single stage differential op amp with unity gain feedback.I used
r=(rp//rn)/(1+A) where a=gmn*(rp//rn).

thanks,
rajsekhar.

Title: Re: Few questions
Post by buddypoor on May 22^nd, 2009, 5:15am

raja.cedt wrote on May 22^nd, 2009, 2:49am:

hi buddypoor,
it is a single stage differential op amp with unity gain feedback.I used
r=(rp//rn)/(1+A) where a=gmn*(rp//rn).

thanks,
rajsekhar.

On May 20th you spoke about a common source amplifier; now it is an opamp ?
Can you, please provide a circuit diagram ?
I cannot follow you, because an opamp with unity gain feedback
does not need any resistors. What is rp and rn and gmn ???