# How to make NMOS resistance emulate a normal resistor?

I am trying to see the behaviour of an NMOS' resistance with gate-source voltage.

This is my NMOS as voltage (Vgs) controlled resistor' schematic (I am doing DC sweep on V2 from 2V to 10V with step size as 1V, ignore the -10V in the sweep in image): The output (Vgs or V2 vs NMOS resistance [Vds/I(R1)]) I get: So at 3V Vgs, I get NMOS resistance of about 1 kilo ohms. I make a similar circuit on right by using a resistor with value of about 1 kilo omhs: Points to observe:

1. Vgs is set as 3V to get a resistance of 1001 ohms.
2. The left and right circuits are similar, they only differ how the 1001 resistor is used. R6 is same as R4, R3 as R5 and so on.
3. R5 is connected to ground and it shouldn't be, but then I could not make it connect in any other way because then it wouldn't make R5 parallel with the NMOS resistance as R3 is with R2

What is going unexpected: The current drawn/flowing the voltage sources V4 and V3 are not same. That means, the circuit on left is not doing a the job of a 1001 resistor. What went wrong? How should the left circuit be modified to emulate the 1001 ohm resistor on the right. I am feeling that I might have done something wrong with placing the V4 voltage source or maybe the NMOS resistance is somehow changing when other resistors are applied due to a new Vds (and not the same as the one in the first image).

However, I am not able to pinpoint the issue. So, how to make NMOS resistance behave like the normal resistor?

Current flowing in V4 and V3: This is the ALD1106 LTSpice code:

.MODEL ALD1106 NMOS (LEVEL=1 CBD=0.5p CBS=0.5p CGDO=0.1p CGSO=0.1p GAMMA=.85
+ KP=479u L=10E-6 LAMBDA=0.029 PHI=.9 VTO=0.8 W=20E-6)

• What is R6 supposed to do? Seems like you have R5 connected to ground so R6 wont do anything? – BeB00 Nov 8 '20 at 18:12
• @BeB00 I understand that its a mistake to connect R5 to ground whereas the analogous resistor in the right circuit is not connected to ground. But then that resistor is in parallel with 1001 ohm resistor and in the left I need a similar requirement. It is unfortunate that one of the NMOS' ends is connected to ground. So, could you tell the correct way then? – Prasanjit Rath Nov 8 '20 at 18:20
• Remove R1 and V1 and the ground of V1, connect the negative of V2 to the node of R5 and R6. – BeB00 Nov 8 '20 at 18:31
• You can only make the source-drain of a MOSFET behave as a resistor for small voltage variations. As soon as you apply too much voltage the MOSFET will go into saturation and no longer behave more like a current source. – Bimpelrekkie Nov 8 '20 at 18:32
• This is true also – BeB00 Nov 8 '20 at 18:45

## 1 Answer

Thanks to BeB00, Fredled and Bimpelrekkie's suggestions, I was able to figure out the issue.

The issue arose from a fact that I had embedded in my brain- drain in NMOS should be connected to a Vdd (high potential) and source in NMOS should be connected to ground (zero potential). Following this fact I forced the NMOS with an additional V1 (as in 3rd diagram).

The drain is not connected to higher potential because as such there is no drain/source because the terminals are symmetrical. Rather, the terminal that eventually is connected to the higher potential (not necessarily a forced battery) is called as drain and the terminal that stays at lower potential (not necessarily 0V) is called as source (for NMOS). However, care must be taken before considering interchanging of the terminals.

The second mistake I did was connecting the source to ground. The source had to be connected to the node of R6 and R5 and to the negative terminal of a battery. Now, you might think why negative terminal of battery, why not ground? Because, if you connect to ground your R6 resistor has ground on both ends making it obsolete (a resistor with same potential on both ends is similar to it not being there). It is then not the same as the circuit on the right.

Now, what does the negative terminal represent then? You can refer to difference between negative terminal and ground to understand the difference. Negative terminal doesn't have to be at 0 V unless ground. Only the difference between positive and negative terminal has to be the required volts you chose. Hence, that would make the R6 at some potential other than 0, thus not making it obsolete.

Correct schematic: Resistance(Vds/Id) vs Vgs: Same current in R3 and R5 at Vgs=3.5V: Hope this is useful for others!