Question

An enhancement type NMOS transistor is connected as a load device in the circuit with another MOSFET as shown in the figure below:



The supply voltage $V_{DD}$ is fixed at 5 V and the input voltage $V_{in}$ is varied from 0 to 5 V. If both the transistors have negligible effect of channel length modulation and a threshold voltage $V_T=1V,$ then which one of the following figures represents the transfer characteristic of the circuit in a better way?

• A.
• B.
• C.
• D.

Answer 1

The correct option is C

Since the configuration represents an invetor MOSFET with an active load, the output will be high for low values of input signal and low for high values of input signal.
Now, since the load transistor $T_1$ has shorted drain and gate.
$V_{DS}>V_{GS}-V_T$
0 > - $V_T$
Which is always true hence the transistor $T_1$ will always work in saturation region.
When $V_{in}=DV,I_D=0$,
Hence, $I_D=K_a(V_{GS1}-V_T)$

$0=(V_{GS0}-V_T)$

$V_{GT}-V_{ST}-V_T=0$

For transistor $T_V{V}_{Th}=V_{0}and{V}_{GS}-V_{DD}=5V$

Thus, $V_{GS}-V_0-V_1=0$

$5-V_0-1=0$

$-V_0=-4V$

$V_0=4V$
Hence maximum value of output = 4 V.