Forward Transfer Characteristic of Triode - Gray Searle P1.1

[TRAyres]

Homework Statement

This problem is from Gray and Searle's Electronic Principles:
P1.1 - The MOS transistor characteristics of Fig. 1.8 are a graphical presentation of the functional relationship
$i_{D} = i_{D}(v_{GS},v_{DS} )$
in which $v_{GS}$ is taken as a parameter and $i_{D}$ is plotted against $v_{DS}$. Alternatively, we could display this relationship by taking $v_{DS}$ as a parameter and plotting $i_{D}$ versus $v_{GS}$. The resulting family of curves is called the forward transfer characteristic of the triode.
(a) Use the data of Fig. 1.8 to generate this family of curves. Plot the three curves that correspond to ##v_{DS} = 2 volts, 4 volts, and 6 volts.
(b) Can a load-line construction analogous to that illustrated in Fig. 1.10 be used with this family of curves?

Homework Equations

Figure 1.8:

Figure 1.9:

Figure 1.10:

Application of KVL to the output mesh of Figure 1.9 yields:
$V_{B} = v_{O} + v_{DS}$ or, equivalently, $V_{B}=i_{D}R_{L}+v_{DS}$

The intercepts of the load line are:
When $v_{DS}=0, i_{D} = \frac{V_{B}}{R_{L}}$ and
When $i_{D}=0, v_{DS}=V_{B}$

The Attempt at a Solution

For part (a), I created the LTSpice circuit of an nmos and stepped it by VDS, while sweeping VGS - this gets me the forward transfer characteristic (admittedly there is a difference between the nmos model in LTspice and the output characteristics of the nmos under consideration, but that could be fixed. It's good enough, but more importantly it allows us to get very good values whereas reading values from the graph is not very accurate).

I'll affix the LTSpice circuit to this post later.

So the question is part (b) - is there a load-line we can construct for the forward transfer characteristic? It doesn't seem like it, because the swept parameter ($v_{GS}$ ) isn't in the output mesh (whereas $v_{DS}$ is). If we set $v_{DS}$ to some value, we get a horizontal line for $i_{D}$, because the current through RL is set.

I guess I'm looking for input to see if my thoughts on part (b) are way off?
Thanks all!

 

[KataKoniK]

Hello,

I would like to provide my thoughts on part (b) of this problem. From my understanding, a load-line construction is used to determine the operating point of a circuit by graphing the load line (which represents the load resistance) on the output characteristics graph of the device. However, in this case, we are dealing with the forward transfer characteristic, which plots the relationship between the input voltage and output current.

In this characteristic, the input voltage ($v_{GS}$) is taken as a parameter, while the output voltage ($v_{DS}$) is varied. Therefore, it does not seem possible to construct a load line on this graph, as the load resistance is not directly involved in the relationship between $v_{GS}$ and $v_{DS}$. Instead, the load resistance ($R_L$) is used to determine the slope of the output curve, but it is not directly plotted on the graph.

In conclusion, I do not believe that a load-line construction can be used with the family of curves obtained in part (a) of this problem. However, I would be interested to hear if anyone has a different perspective on this question.