# Homework Help: Feedback amplifier

1. May 17, 2013

### amangupta1219

1. The problem statement, all variables and given/known data

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

2. Relevant equations

3. The attempt at a solution

1. As Iin = iF , increasing Iin will increase iF
2. Now drain current for MF is increasing so Vgs of the transistor should increase
3. That means increasing Iin increase Vout
4. Now as Vout increases the current through RD2 decreases
and all current through RD2 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 RD1 to increase which means drain current through M1 increasing which further implies that gate of M1 or drain of MF increasing

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

Now my question is how to proceed further ???

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2. May 17, 2013

### rude man

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?

3. May 17, 2013

### amangupta1219

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

4. May 18, 2013

### rude man

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.

5. May 18, 2013

### 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 ???
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6. May 18, 2013

### rude man

Remeber that an ideal curent 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.

7. May 18, 2013

Yes.

8. Jun 2, 2013

### anhnha

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