I need help with using the If-Else loop containing scalar and nonscalar

Create a plot of iD as a function of vDS from 0  ≤ vDS ≤ 8 V, for three values of vGS =2, 3, and 4 V.  

Constants:  Kn=0.2 mA/V2; VTN =1.2 V; λ=0.01 V-1.

Here is my work I have so far:

Vds = 0:0.1:8;            
Vgs = 2:1:4;              
kn = 0.2;                 
Vtn = 1.2;                
lambda = 0.01;            
Vds_Sat = Vgs - Vtn;
if (Vds <= Vds_Sat)  
    ID = Kn*(2*(Vgs-Vtn)*Vds - (Vds^2));
else (Vds >= Vds_Sat)
    ID = Kn*((Vgs - Vtn)^2*(1 + lambda * Vds));
end
plot(Vds.,ID)

It is telling me I have an error on line 7 and that my matrix dimension must agree, could you please help me with this?

[Walter Roberson]

There is no such thing as an "If loop". There is an "if statement". Loops execute the body multiple times; statements execute the body (or not) exactly one time.

Answer 1

[Matt J]

This operation

Vds <= Vds_Sat

does not make sense since the left and right hand side are vectors of different sizes.

[Steven]

How can I make them equal so that I can plot them?

[Matt J]

Loop over Vgs and operate on one value at a time

 for Vgs=[2,3,4]

   Vds_Sat = Vgs - Vtn;

   etc...

 end

[Matt J]

Also, use logical indexing instead of IF...ELSE statements. Also, make sure you know the differences between Vds^2 and Vds.^2

idx = (Vds <= Vds_Sat)  ;

ID(idx) = Kn.*(2.*(Vgs-Vtn).*Vds(idx) - (Vds(idx).^2));

[Steven]

Is there a way to do it with and if statement? I'm also not really clear on how to do it with a for loop. We have not been taught indexing in class so I don't know how to do that.

[Matt J]

An if-statement when (Vds <= Vds_Sat) is a vector will not do what you want here. Some kind of indexing and/or a for-loop will be necessary. If you want to stay as close as possible to your original code, you could double loop over Vds and Vgs

 VDS=0:.1:8;

 for Vgs=[2,3,4]
  for ii=1:length(VDS)

       V_ds=VDS(ii);

       Vds_Sat = Vgs - Vtn;

        if (Vds <= Vds_Sat)  
          ID(ii) = Kn*(2*(Vgs-Vtn)*Vds - (Vds^2));
        else (Vds >= Vds_Sat)
          ID(ii) = Kn*((Vgs - Vtn)^2*(1 + lambda * Vds));
        end

   end

    plot(....)

 end

[Steven]

if true
  % code
end
Kn = 0.2;                 %Set value for Kn
Vtn = 1.2;                %Set value for Vtn
lambda = 0.01;            %Set value for lambda

 VDS=0:.1:8;

 for Vgs=[2,3,4]
  for k=1:length(VDS)

       V_ds=VDS(k);

       Vds_Sat = Vgs - Vtn;

        if (Vds(k) <= Vds_Sat(k))  
          ID(k) = Kn(k)*(2*(Vgs(k)-Vtn(k))*Vds(k) - (Vds(k)^2));
        else (Vds(k) >= Vds_Sat(k))
          ID(k) = Kn(k)*((Vgs(k) - Vtn(k))^2*(1 + lambda(k) * Vds(k)));
        end

   end

    plot(VDS,ID)

 end

Say if I had the for loop using k instead of ii like you showed me would all the other variable have to be in terms of k. i.e. variable(k)?

[Steven]

This didnt work for me

[Steven]

I got the plot to work but it only plots the case when Vgs = 4. How can I obtain the plots when Vgs = 2 & 3?

[Matt J]

Use the HOLD command. Or, plot to different figure windows

 figure; plot(....)