mosfet-3 - THE MOSFET AS AN AMPLIFIER AND AS A SWITCH...

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THE MOSFET AS AN AMPLIFIER AND AS A SWITCH MOSFET AS A SWITCH Figure 1.15 (a) shows the basic structure of the most commonly used common-source circuit. Figure 1.15 (b) shows the voltage transfer characteristic of the CS circuit of Fig. 1.15 (a). Since v GS = v I , the MOSFET will be effectively cutoff for v I < V t . Thus, for v I < V t , i D will be negligibly small, and v O will be equal to the supply voltage V DD . When the v I V t , MOSFET is turned on, i D will be increases, and v O will be equal to the zero. Figure 1.15 (a) Basic structure of the common-source amplifier. (b) Transfer characteristic showing operation as an amplifier biased at point Q.
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When the MOSFET is used as a switch, it is operated at the extreme points of the transfer curve. Specifically, the device is turned off by keeping v I < V t resulting in v o = V DD . The switch is turned on by applying a voltage close to V DD , resulting in v o very small. From the transfer characteristic curve we observe that the common-source
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