Revision of Circuits and Electronics Part 1 - Inside The Digital Gate (lecture 5)

To build a digital gate, a switching device is needed.

The MOSFET device (Metal Oxide Semiconductor Field Effect Transistor) is a 3 terminal element that behaves like a switch with the gate being the control terminal.

If v_GS is less than a threshold voltage of say 1V then the switch is off. If v_GS is more than a threshold voltage  then the switch is on.

Looking at the simplified i_DS vs v_DS graph, it can be seen that when  i_DS flows (switch is on), v_DS=0, and when I_DS does not flow (switch is off), V_DS is recorded.

The MOSFET behaves like an inverter. The voltage transfer characteristic can be plotted - this is a graph of v_out vs v_in.

 There is a question about whether the inverter satisifies the static discipline for different thresholds. The lines for V_IL and V_IH need to be plotted first on the voltage transfer characteristic graph. The V_OL and V_OH lines need to be plotted next. Parts of the  transfer characteristic graph needs to lie within the green shaded areas to satisfy the static discipline.

The switch resistor  (SR) model of MOSFET is a more accurate MOSFET model.  When v_GS is more than a threshold voltage, the on state, there is some resistance between the drain and the source.

Under the more accurate model, the inverter satisfies the static discipline.


Video:- http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-002-circuits-and-electronics-spring-2007/video-lectures/lecture-5/


Lecture notes : - http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-002-circuits-and-electronics-spring-2007/lecture-notes/