# nmos pmos ro is too low for the Gain result

#### yef smith

Joined Aug 2, 2020
488
Hello,I have build a common source with 28.5dB gain which is 25.11 linear gain in LTSPICE.

From gm/id value:
Id = 50 μA gm/id = 12.6 => gm = 12.6×50×10-6 = 630×10-6
Id vg Vds in the Vgs conditions is shown below.
g_ds = d_Id/d_Vds plot is shown in the end.

For Vds = 0.9 I get rds_nmos = 1.9×10-5. These results are for NMOS. A = gm×R=(1.9×10-5)×(630×10-6) which is far away from 25.11. In the end i have added pmos d_I/d_Vds i get there g_ds=0.0001
from equation shown in the end i get 1/gds_n=52085 1/gds_p=4887 still A=gm*Rt=4.41 far away from 25
Where did I go wrong recreating the simulation result?
Thanks.

Last edited:

#### crutschow

Joined Mar 14, 2008
32,026
You are biasing the bottom P-MOSFET drain with a positive voltage.
It should be a negative voltage.

#### yef smith

Joined Aug 2, 2020
488
Hello crutschow,My biasing is great as you can see bellow.i get 50uA and my deisgn is working fine.
The problem is that my ro calculation is about 4K ohms
could you say where did i go wrong in this aspect?
Thanks.

#### crutschow

Joined Mar 14, 2008
32,026
Since I don't understand why you are biasing a P-MOFSET with a positive drain voltage, I can offer no further relevant comment.

#### yef smith

Joined Aug 2, 2020
488
because positive and negative is with respect to drain and source.

#### yef smith

Joined Aug 2, 2020
488
UPDATE:

Hello crustchow, from the p-channel mosfet laws.
Vs=1.8 Vg=0.891 Vd=0.9 Vt=-0.4
Vsg=1.8-0.891>|-0.41| so its ON

Vsd=1.8-0.9=0.9> Vsg-|Vt|=0.509
So its conducting and in saturation state.
I dont see any problem .
Its not what i have asked in the original post but 'just to explain you my logic which is also perfectly working in the simulations of the PMOS shown above.

#### crutschow

Joined Mar 14, 2008
32,026
because positive and negative is with respect to drain and source.
Of course.
And below you show the drain and gate voltage positive with respect to the source of a P-MOSFET, which is inverted from normal.

#### yef smith

Joined Aug 2, 2020
488
UPDATE:

Hello crustchow, from the p-channel mosfet laws.
Vs=1.8 Vg=0.891 Vd=0.9 Vt=-0.4
Vsg=1.8-0.891>|-0.41| so its ON

Vsd=1.8-0.9=0.9> Vsg-|Vt|=0.509
So its conducting and in saturation state.
I dont see any problem .
Its not what i have asked in the original post but 'just to explain you my logic which is also perfectly working in the simulations of the PMOS shown above.

#### crutschow

Joined Mar 14, 2008
32,026
from the p-channel mosfet laws.
Those are incorrect.
The drain and gate voltages should be negative with respect to the source for normal operation.

If they are working as shown, then something is wrong with the device.

#### yef smith

Joined Aug 2, 2020
488
Hello,i have attached a published article with similar implementation.
I dont think a university would publish an article with a biasing mistake
I also would like you to help me understand how to calculate r0 in LTSPICE.

#### Attachments

• 614.7 KB Views: 3

#### crutschow

Joined Mar 14, 2008
32,026
My confusion is that you are showing the bottom transistor with a P-MOSFET symbol, whereas the publication shows an N-MOSFET, so which are you using?
(Note that there are two different types of MOSFET symbols being used, which may be the source of the misunderstanding. Look at which terminals the arrows are connected).

#### yef smith

Joined Aug 2, 2020
488
could you to help me understand how to calculate r0 of a mosfet in LTSPICE?

#### Audioguru again

Joined Oct 21, 2019
5,813
Most Mosfets have part numbers and the r0 is listed in their datasheet. Are you making your own Mosfets??

#### Attachments

• 78.7 KB Views: 4

#### yef smith

Joined Aug 2, 2020
488
Hello AudioGuru, its a validation issue.
I want to match my gain result with theory.
A=gm*(ro_n||ro_p)

I am using a level 8 spice model shown bellow.
gm = 12.6×50×10-6 = 630×10-6 because
g_ds_pmos=0.0001845 => rds_pmos=5420
I get r_ds very small from regular derivative.
How can i get my ro from the spice model?

Spice model:

T58F SPICE BSIM3 VERSION 3.1 PARAMETERS
*
* SPICE 3f5 Level 8, Star-HSPICE Level 49, UTMOST Level 8

Spice model manual:

#### Audioguru again

Joined Oct 21, 2019
5,813
You are still wrongly showing a symbol of a P-channel Mosfet for the N-channel M8.