Why Small-Signal Output Resistance is Independent of Device Width

mosfet

For a MOSFET in deep triode region, we can approximate it as a resistor with the following:

$$
\frac{1}{{\mu}_n C_{ox}(W/L)(V_{GS} – V_{Th})}
$$

However, in small-signal for a MOSFET in saturation, I know that
\$R_{out} = \frac{1}{\lambda \cdot I_d}\$, where lambda is channel-length modulation factor and that is proportional to inverse of length. So increasing length increases \$R_{out}\$. But why isn't it dependent on width? Surely, if I make the transistor very wide, resistance should drop.

Best Answer

The small signal output resistance depends on W because \$I_D\$ depends on it.

After all, you have:

$$R_o=\dfrac{1}{{\lambda}I_D} $$

With \$I_D\$ being the current in the saturation region for the MOSFET. In saturation, $$I_D=\dfrac{1}{2}\text{K}\frac{W}{L}(V_{GS}-V_T)^2$$

So yeah, if you increase W, the \$R_o\$ does decrease because \$I_D\$ is in its denominator.

$$R_o=\dfrac{1}{\dfrac{1}{2}{\lambda}\text{K}\frac{W}{L}(V_{GS}-V_T)^2} $$

Related Solutions

NMOS Transistor Voltage Range For Specification

You don't know V_DS or I_D. But your task is to obtain a resistance of 200 to 1000 Ohms.

That means, depending on how your "variable resistor" will be used, in your final solution you need to have either

\$200 < \dfrac{V_{DS}}{I_D} < 1000\$

\$ 200 < \dfrac{\mathrm{d}V_{DS}}{\mathrm{d}I_D} < 1000\$

Electronic – Input Characteristics of MOSFET in triode region

What do you mean by "input characteristics"?

Textbooks and datasheets describe the behavior of MOSFETs using two graphs:

Output characteristics: \$I_D\$ versus \$V_{DS}\$ with \$V_{GS}\$ as parameter.
Transfer characteristic: \$I_{D}\$ versus \$V_{GS}\$ at a given fixed \$V_{DS}\$ value (this latter is chosen so that the MOSFET is in saturation region).

There is no "input characteristic" (such as the \$I_B\$ versus \$V_{BE}\$ curve of a BJT) because the other input quantity besides \$V_{GS}\$, namely \$I_G\$, is virtually zero at DC (and all these curves assume DC operations). Therefore it wouldn't make much sense to plot \$I_G\$ versus \$V_{GS}\$, unless you wanted to analyze leakage gate current, but I assume you are not interested in that.

So it is clear (also by a comment of yours) that by input characteristic you mean the transfer characteristic (TC). Note that the TC is plotted with a fixed drain-source voltage that guarantees that the MOSFET is in saturation for each \$V_{GS}\$ value on the horizontal axis. This is done because the TC is useful when the MOSFET is in saturation, i.e. when the output current depends solely on the input voltage (not considering "Early effect"), for example when you want to use the MOSFET as an amplifier and you need to draw a load line to design its bias circuit.

If you plot the TC for different values of \$V_{DS}\$ you get a family of TC curves. For example consider this circuit simulation with LTspice:

enter image description here

Plotting the TC for different \$V_{DS}\$ values you get:

enter image description here

As you can see, the more you increase \$V_{DS}\$ the more the curve resembles a parabola, as you would expect for the TC in saturation. Notice that this part shows a threshold voltage \$V_{th} \approx 4V\$.

Let's consider what happens if \$V_{DS}\$ is not big enough to drive the MOSFET in saturation for every \$V_{GS}\$ value, like in the lowest blue curve (Note: to present a more revealing plot I selected the curve corresponding to \$V_{DS} = 2V\$, whereas the lowest blue curve above corresponds to \$V_{DS} = 1V\$):

enter image description here

As you can see, in saturation region you get a quadratic curve, whereas in triode region you get a linear curve. Everything as expected, except that real devices don't have an abrupt change between the two regions and that the linearity of the triode region is not perfect because of the device not being ideal (SPICE models usually take into account these effects).

If you see in your simulation an abrupt departure from this behavior it could be that you tried plotting the curves outside the range of the voltages/currents admissible for your device. Notice that I limited the first plot to max 14A/20V which are the absolute maximum ratings for the device I chose. If you don't keep this in mind you will destroy the device (in real life) or get odd results (in simulations).

EDIT (in response to a comment and a question edit)

You ask why the "perfectly" linear curve for \$I_D\$ versus \$V_{GS}\$ in ohmic region is not exploited. Here is some insight:

Why do you need a linear characteristic between input (\$V_{GS}\$) and output (\$I_D\$)? Usually to use the device as a (linear) amplifier. But what are the conditions that allows to have that linearity? \$V_{DS}\$ must be held constant. Therefore to make an amplifier this way you have to insert a load in the output circuit and still keep \$V_{DS}\$ constant. You can understand that such a load cannot be a simple resistor (which is the simplest kind of load). Therefore you need a much more complex circuit (with other active devices).

On the other side, you can use the same MOSFET biased in saturation and get a decent linear amplifier: even if the behavior of the device is not intrinsically linear, but quadratic, there are linearization techniques (e.g. employ simple feedback schemes, like a resistor in series with the source terminal) that allow the overall amplifier to become more linear.

Best Answer

Related Solutions

NMOS Transistor Voltage Range For Specification

Electronic – Input Characteristics of MOSFET in triode region

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