# How to 'feel' the difference in impedence looking through Drain or Source?

#### Suyash Shandilya

Joined Sep 14, 2016
11
I understand basic analog electronics and network thery concepts. Have been doing a few questions lately. I can derive, using the small signal model the exact formula to compute the impedance seen while looking into a MOSFET or JFET from its drain or source terminal. I can mathematically understand that it is higher when looking through drain than when looking through source. BUT HOW?

I want to understand this qualitatively. Maybe it is the general case while looking into any current source from its either end but I want to seek a verbal description.

Thanks in advance to all those who will be replying. I really love this forum.

#### GopherT

Joined Nov 23, 2012
8,012
I understand basic analog electronics and network thery concepts. Have been doing a few questions lately. I can derive, using the small signal model the exact formula to compute the impedance seen while looking into a MOSFET or JFET from its drain or source terminal. I can mathematically understand that it is higher when looking through drain than when looking through source. BUT HOW?

I want to understand this qualitatively. Maybe it is the general case while looking into any current source from its either end but I want to seek a verbal description.

Thanks in advance to all those who will be replying. I really love this forum.
Are you referring to the “body diode” that allows reverse current to flow?

#### Suyash Shandilya

Joined Sep 14, 2016
11
No. Not at all. All I am saying is that impedance looking through source is a relation like [rDS + Rd]/[1 + GmRds] Which is a much smaller value as compared to Gm.Rds.Rs + Rds + Rs which is the relation for Drain impedance. I want to know how are these values so different in magnitude. How do I physically visuallise this? After all, a MOSFET is innately symmetric.

#### crutschow

Joined Mar 14, 2008
25,269
The difference is because the gate-source voltage controls the drain-source current whereas the gate-drain voltage has little effect on that current.
So since the gate-drain voltage has little effect on the current, the impedance looking into the drain is quite high.
That's why it can be considered a current source.

But a change in the source voltage (with the gate voltage fixed) changes the gate-source voltage such as to change the drain-source current in the same direction as the change in source voltage, making it appear that the source impedance is low.

Due to the inherent symmetry of a FET (as you noted) the terminal determined to be the drain is the one that is most positive for an N-FET, and most negative for a P-FET.

#### Suyash Shandilya

Joined Sep 14, 2016
11
Y
The difference is because the gate-source voltage controls the drain-source current whereas the gate-drain voltage has little effect on that current.
So since the gate-drain voltage has little effect on the current, the impedance looking into the drain is quite high.
That's why it can be considered a current source.

But a change in the source voltage (with the gate voltage fixed) changes the gate-source voltage such as to change the drain-source current in the same direction as the change in source voltage, making it appear that the source impedance is low.

Due to the inherent symmetry of a FET (as you noted) the terminal determined to be the drain is the one that is most positive for an N-FET, and most negative for a P-FET.
Yes... This is something I was looking for. I understand it now. Thanks a lot.