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Hello all! I want to simulate a diode connected t...
# analog-design
e
Hello all! I want to simulate a diode connected transistor with W/L = 10. That's the error message I got after I hit the simulate button. I really don't know where the issue is coming from. Also how do one specify an aspect ration of say hundred with say Wmax < 0.5u (I got this info form the 
sm141064.ngspice
)? This makes it difficult working with xschem. Please I need your assistance on this. Thank you. PS: I'm using the GF180mcuC transistors. I've attached the screenshot of the schematics and the error messages.
l
You should use 0.3u and 5u for W and L.
m
It is my understanding that 
nf=24
, 
W=0.3u
, 
L=5u
represents a physical layout of 
24
transistors of 
W=0.0125u L=5u
. Maybe you intended 
nf=24, W=7.2u, L=5u
? 
nf
and 
mult
are not interchangeable.
mult=1 nf=2 W=2u L=0.5u
(SGDGS) and 
mult=2 nf=1 W=1u L=0.5u
(2 x SGD) have the same number of physical gates and in LVS are equivalent to one 
mult=1 W=2u L=0.5u
. The difference is that 
nf
is used in the xschem model in the calculation of the source/drain area and perimeter.
Copy code
ad="'int((nf+1)/2) * W/nf * 0.29'" 
pd="'2*int((nf+1)/2) * (W/nf + 0.29)'"
as="'int((nf+2)/2) * W/nf * 0.29'" 
ps="'2*int((nf+2)/2) * (W/nf + 0.29)'"
e
@Jorge Marin
Thank you very much @Mitch Bailey. This explanation made me understand the nf and multiplicity terms better and what they look like from layout perspective. But after making the necessary changes to the netlist, I'm still getting the same 
unknown subckt error
. I don't know why. I see that from the source/drain area/perimeter calculation you pasted in the thread, the micrometer suffix 
u
is not attached to those constant numbers in the calculation. But in the netlist my schematics generated from here, I see the 
u
attached to those constant numbers. Could this be part of the issue?
m
Sorry, I copied the calculations from the sky130 models. Could you share a screen copy of your current schematic and the spice netlist generated from it?
e
Here they are.
m
So the size of each individual mosfet is 
W=0.25u L=0.5u
?
e
Yes.
m
Why do you have the model name as 
nfet_03v3.4
? Does it work with a model name of just 
nfet_03v3
? (I’m not too familiar with spice simulation.)
e
I noticed that this path 
~/share/pdk/gf180mcuC/libs.tech/ngspice
that ngspice is suppose to look into has this file 
sm141064.ngspice
with the spice definitions. When I defined the transistor as just nfet_03v3 the simulation brings up 
could not find valid model name
error. But when I define the transistor as one of the specific model name e.g 
nfet_03v3.4 or nfet_03v3.11 or nfet_03v3.0
it brings up just 
unknown subckt error
.
@Mitch Bailey for no reason just using 
nfet_03v3
seems to be working fine now. Before it wasn't. Thank you.
m
Maybe before the parameters were missing 
u
.
e
Please how do I extract parameters like transconductance, threshold voltage etc from the nfet device? Does something like this 
vthn = @m.xm1.msky130_fd_pr__nfet_01v8[vth]
work for gf180mcuC just like for skywater130? Because it seems not to be working here. Maybe I'm doing something wrong.
m
You might want to ask in a new thread because I’m not an expert. Unless you’ve changed your schematic, you still might have an extra 
.0
at the end of the model name in the control statement.
e
I'm sorry it seems I have a lot of questions. During circuit design let's say one of the transistors needs a sizing of 400 i.e 
W/L = 400
. Please how would enter the sizing in spice with 
L = 0.5u
. Would use 
W = 200u
. Would the 
wmax
not affect this? Is that where 
nf
come in handy? This is just an arbitrary question I'm asking, so something maybe wrong with the question.
Okay @Mitch Bailey I'd check it again.
@Mitch Bailey and @Luis Henrique Rodovalho thank you so much for you response. It all working fine.
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