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Design >> Analog Design >> feed Forward compensation
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Message started by raja.cedt on Dec 22^nd, 2011, 2:55am

Title: feed Forward compensation
Post by raja.cedt on Dec 22^nd, 2011, 2:55am

hello,
i have little bit confusion about feed forward stage noise contribution to overall opamp input refereed noise.

Let us say i have two stage opamp and around this opamp i have connected FF stage, now i guess FFf stage will add some current noise at the o.p but when referring to input voltage, i guess it will divided by ovaral gain so it's contribution is small. Is any thing wrong. becaz i read some where saying that FF compensation add more noise.

Thanks,
Raj.

Title: Re: feed Forward compensation
Post by Vladislav D on Dec 29^th, 2011, 2:16pm

I guess, the output current noise due to FF amplifier should be divided by transconductance of the FF OTA. What do you mean by "overall gain"? There are two transfer functions voltage -voltage for the opamp and voltage-current for a FF amplifier. Thus, you have two independent input referred noise sources.....

Title: Re: feed Forward compensation
Post by raja.cedt on Dec 30^th, 2011, 2:06am

hello,
overal gain means sum of the two path gains, and could you explain why it has divide with only gmf?

Thanks,
Raj.

Title: Re: feed Forward compensation
Post by thechopper on Dec 30^th, 2011, 4:08am

Hi Raja,

If overall gain means sum of two path gains, that means you are adding the signal independently amplified by each path. Same applies to noise: one noise source is processed through one path, the other one through the other path, and the add at the output. so each one needs to be referred back with the corresponding gain of each path.

Best
Tosei

Title: Re: feed Forward compensation
Post by Vladislav D on Dec 30^th, 2011, 4:36am

raja.cedt wrote on Dec 30^th, 2011, 2:06am:

hello,
overal gain means sum of the two path gains, and could you explain why it has divide with only gmf?
Thanks,
Raj.

Input referred noise is a kind of fictional thing. The source of the current noise at the output of a circuit is an FF OTA, and not the opamp. So, we divide current noise by transconductance of the OTA.
Another look at the problem is that, opamp is a voltage-voltage amplifier and has no voltage-to-current gain. So, you cannot divide output current by opapm gain since it has units (V/V)

Title: Re: feed Forward compensation
Post by raja.cedt on Dec 30^th, 2011, 5:06am

hello sir,
please note that there is no voltage amplifier, every where you have current and some how you are making node impedance lower if you want to get voltage at that note. One more thing when you want to use feedforward stage o/p node should be high impedance.

Come to our point of discussion, FF stage inject some noise into o/p node, which gets converted into voltage now you can refer it to any where you want. Please check the attached file.

Thanks,
raj.

Title: Re: feed Forward compensation
Post by analogbeginer on Jan 1^st, 2012, 8:32pm

Hi raja,

I have little confusion. "every where you have current and some how you are making node impedance lower if you want to get voltage at that note" Isnt the other way. If the node impedance is lowered, we are devoid of Swing. So, to get voltage, impedance shld be made high. Please correct me if I'm wrong.

Title: Re: feed Forward compensation
Post by aaron_do on Jan 2^nd, 2012, 5:28pm

Hi Raj,

for the circuit you drew, it looks like you can simply take the output noise current * RL, and divide by the overall gain (as you said yourself).

Quote:

i have little bit confusion about feed forward stage noise contribution to overall opamp input refereed noise.

However, I'm a bit confused about where the op-amp is. I think if you included some resistive feedback path, I would need another look at the schematic before coming to such a simple conclusion about the noise performance.

cheers,
Aaron

Title: Re: feed Forward compensation
Post by Vladislav D on Jan 3^rd, 2012, 3:33am

Quote:

hello sir,
please note that there is no voltage amplifier, every where you have current and some how you are making node impedance lower if you want to get voltage at that note.

OK. Terminology "opamp" confuses me since opamp by definition has low output impedance and, consequently, is a voltage amplifier

Quote:

One more thing when you want to use feedforward stage o/p node should be high impedance.

Don't understand this statement. Impedance of the loading network is determined by the nature of a signal that you have at the output,i.e voltage or current.

Quote:

Come to our point of discussion, FF stage inject some noise into o/p node, which gets converted into voltage now you can refer it to any where you want. Please check the attached file.

The calculation you do are correct. Also, I believe that FF stage adds additional noise especially if FF coefficient is large.

Title: Re: feed Forward compensation
Post by raja.cedt on Jan 3^rd, 2012, 4:37am

hello all,
here my intention is to show how FF stage noise gets converted into voltage and how we can refer it back to input through the whole gain of the amplifier (i don't want to say opamp here, i kept resistance at the o/p to represent the conversion process rather real load).

@Vladislav: yes, no need to have high impedance there...

Thanks,
Raj.