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Design >> Analog Design >> feedback amplifier
https://designers-guide.org/forum/YaBB.pl?num=1368816253

Message started by aman gupta on May 17^th, 2013, 11:44am

Title: feedback amplifier
Post by aman gupta on May 17^th, 2013, 11:44am

I have to find the feedback polarity. I proceeded as follows :

1. As Iin = iF , increasing Iin will increase iF
2. Now drain current for M_F is increasing so V_gs of the transistor should increase
3. That means increasing Iin increase Vout
4. Now as Vout increases the current through R_D2 decreases
and all current through R_D2 will flow through M₂ , this means drain current through M₂ is decreasing
5. This will cause gate voltage of M₂ to decrease, this will cause current through R_D1 to increase which means drain current through M₁ increasing which further implies that gate of M₁ or drain of M_F increasing

Now in short i found that increasing Iin will cause drain voltage of M_F to increase

Now my question is how to proceed further ???

Title: Re: feedback amplifier
Post by raja.cedt on May 17^th, 2013, 1:31pm

hello,
Please don't assume Iin=If which is the final job of the loop, so don't assume this. Let us assume zero input current and zero input voltage.

Now increase gate of M2, drain of M2 decreases,Mf drain increases hence Drain of M1 decreases so -ve feedback.

Thanks,
Raj.

Title: Re: feedback amplifier
Post by aman gupta on May 17^th, 2013, 9:52pm

Can you tell me if Iin increases then why gate voltage for M1 rises ??

Title: Re: feedback amplifier
Post by raja.cedt on May 18^th, 2013, 12:14pm

Hello,
1. General way: Whenever some current pumped into a node, potential increases and if current pulled out of node potential decreases(For any stable network).

2. Intuitive way: If you increase Iin some how finally loop should increase current through Mf means Vout should increase, hence M2 gate should decrease so finally M1 gate potential should increase.

Btw did you refer razavi basic micro-electronics text book, i guess he described this kind of problems very clearly...

Good night,
Raj.

Title: Re: feedback amplifier
Post by aman gupta on May 18^th, 2013, 9:39pm

raja.cedt wrote on May 18^th, 2013, 12:14pm:

Thanks a lot ......... Now i got it
yes i am reading that book......... but i didn't try to apply the intuitive way :-[

Title: Re: feedback amplifier
Post by aman gupta on May 18^th, 2013, 10:42pm

raja i have 1 doubt
by looking intuitively if we see that as iF increases then gate of MF increases. Now MF is in common source config , so drain of MF should decrease i think . Wat do you say ??

or even if we assume Vout to be constant initially then increase in iF will also cause decrease in drain voltage in common source.

And the thing you told by analysing the effect of Vout on drain of MF through M1 and M2 is the feedback ( which is negative here) effect i think..... pls correct me if i am wrong..........

Title: Re: feedback amplifier
Post by raja.cedt on May 19^th, 2013, 12:48am

yes. Please read my previous post which also saying exactly same.

Title: Re: feedback amplifier
Post by ywguo on May 19^th, 2013, 3:19am

Hi aman,

Quote:

by looking intuitively if we see that as iF increases then gate of MF increases. Now MF is in common source config , so drain of MF should decrease i think . Wat do you say ??

or even if we assume Vout to be constant initially then increase in iF will also cause decrease in drain voltage in common source.

If the load of the MF is a resistor/transistor, the drain voltage of MF decreases as its gate voltage increases because it sinks more current. In that case, the relationship of gate voltage and drain voltage depends on the I/V curve of both the common source amplifier and its load, eg. an resistor load.

But if Iin increases, the drain voltage increases if the gate voltage is not changing, I mean assuming there is not feedback. And the drain voltage increases a lot because that is a high impedance node. Now the negative feedback makes the gate voltage of MF increases so that the drain voltage would not increases so much.

Best Regards,
Yawei

Title: Re: feedback amplifier
Post by aman gupta on May 19^th, 2013, 11:08am

ywguo wrote on May 19^th, 2013, 3:19am:

Hi aman,

Quote:

Is it a high impedance node because gate of M1 does not sink current ??? if yes then if the current has some other path through MF then why we still call it high impedance node ??

Title: Re: feedback amplifier
Post by raja.cedt on May 19^th, 2013, 2:30pm

low impedance node ~1/(Gmf1*Gm1*Gm2*R1*R2).
Imagine Mf is a diode connected transistor with high gain in the feedback path