Finding Feedback Polarity - Step By Step Guide

[amangupta1219]

Homework Statement

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

Homework Equations

The Attempt at a Solution

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 ?

 

[rude man]

The Attempt at a Solution

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

V_gs is not controlled by i_d. It's the other way around.

If I_in is increased, the gate of M1 will momentarily rise, doing what to the drain of M1, doing what to the drain of M2, doing what to the current in MF?

 

[amangupta1219]

I need to revise the concepts ...
can you tell me if Iin increase why gate of M1 will rise ??

 

[rude man]

I need to revise the concepts ...
can you tell me if Iin increase why gate of M1 will rise ??

Because, initially, there is insufficient gate voltage to accommodate the extra input current. The drain impedance at constant gate voltage is very high, so high impedance * extra current = sudden jump in gate voltage of M1. The balance is eventually restored by the output voltage rising to fully accommodate the extra input current.

 

[amangupta1219]

Can we say that if Iin increases then iF will have to increase because gate current is 0 for M1 (always).
Now if above statement is right then how can we relate it to Vds of MF ?
Edit/Delete Message

 

[rude man]

Can we say that if Iin increases then iF will have to increase because gate current is 0 for M1 (always).
Now if above statement is right then how can we relate it to Vds of MF ?
Edit/Delete Message

Remeber that an ideal current source such as Iin can generate infinitely high voltages if necessary! Yes, a step increase in Iin has to flow through MF's drain, so the voltage would climb as high as it had to to sink the increased Iin. In practical terms MF would probably break down and carry the current anyway, possibly damaging the MOSFET.

 

[rude man]

Can we say that if Iin increases then iF will have to increase because gate current is 0 for M1 (always).

Yes.

 

[anhnha]

Because, initially, there is insufficient gate voltage to accommodate the extra input current. The drain impedance at constant gate voltage is very high, so high impedance * extra current = sudden jump in gate voltage of M1. The balance is eventually restored by the output voltage rising to fully accommodate the extra input current.

Please explain it in simple English? Or simple terms in electronics.