Calculating Resistance

Question

Can somebody tell me why R_o2 is RD Why dont we take Rs into consideration ?

Their answer is Ro2=vt/it = Rd

My question is why didnt they take in consideration Rs and the current source?

@LongPham Updated. Cant be more specific than this – Sijaan Hallak Jul 12 '18 at 17:03 — Sijaan Hallak, Jul 12 '18 at 17:03

score 2 · Answer 1 · answered Jul 12 '18 at 17:05

R_o2 is the impedance looking into the drain node from the perspective of \$V_t\$. That drain has two other connections: -

A current source that has infinite impedance (irrespective of what is connected to the source) i.e. it is the actual drain connection and,
Resistor \$R_D\$

Hence the signal impedance R_o2 is \$R_D\$.

score 1 · Accepted Answer · answered Jul 12 '18 at 17:11

1

In your diagram, we can see a voltage controlled current source.

The control voltage is \$V_{GS}\$. And \$V_{GS}\cdot g_m\$ current can only flow if \$V_{GS}\$ voltage is larger or smaller than \$0V\$. But in your circuit \$V_{GS} = 0V\$ therfore \$V_{GS}\cdot g_m\$ also must be zero.

The situation will be different if you add \$r_o\$ resistor between the drain and the source terminal. In this case \$V_{GS}\$ is no longer equal to \$0V\$. Because now \$r_o\$ provides a path for a current from \$V_t\$ to GND. So the voltage at \$V_S\$ is larger than \$0V\$

I/O Resistance of common source MOSFET with source degeneration

answered Jul 12 '18 at 17:11

G36

13,642
1
18
33

Wait, how did you get that Vgs = 0? I know that `Vg = 0`. But why `Vs=0` ? – Sijaan Hallak Jul 12 '18 at 17:14
@SijaanHallak To turn-on VCCS you need Vgs voltage to appears first. Is this possible in your circuit? – G36 Jul 12 '18 at 17:17
Doesnt Rs get some voltage from our test voltage? @g35 – Sijaan Hallak Jul 12 '18 at 17:22
@SijaanHallak How? – G36 Jul 12 '18 at 17:25
Vt is in parallel with the VCCS and Rs, so some of the voltage will drop on Rs ... correct me if I'm wrong which I feel I am – Sijaan Hallak Jul 12 '18 at 17:27
1

Do you agree with this Vs = Id*Rs and to Id current to flow you need Vgs first right? – G36 Jul 12 '18 at 17:30
I get it now! Saved me a lot of time. Thanks. Would be appreciated if you can give me some more questions and answers of yours about CS. – Sijaan Hallak Jul 12 '18 at 17:32
Which questions? – G36 Jul 12 '18 at 17:37

Calculating Resistance

2 Answers2