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Basics of Transistors

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Neetha Patil

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Hiiii
What is Virtual Ground and Virtual VDD?what is Floating means ?explain with example plzzzzzzz

Thank u in advance
Regards
Neetha
 

errakeshpillai

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Virtual Ground or Virtual Vcc is a term used with OP-AMP.
This effect comes due to the infinite input resistance of the OP-AMP.

OPAMP_text_html_357a3efc.gif

Referring to the above figure, you can see that the Non-inverting terminal is directly grounded.
So due to the infinite input resistance of OP-AMP, the potential at the highlighted point will also be 0V.
This is the virtual ground property of OP-AMP.

Similarly if the same terminal is connected to some voltage source say X, the this voltage will directly appear at the highlighted point.
This is the virtual Vcc property.
 
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Neetha Patil

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Hiiiiiiii
i added gpdk 90nm tech library in the sys in which i worked in gpdk 180 nm tech, if we added like that then it will work or not ?and if i want to do at a time 180nm and 90nm in diif tabs in window it will work or not ? other wise i have to do different systems for diff libraries? because iam not getting results of simple inverter logic in MCML in both 180nm and 90nm tech with load as resitor and load as pmos in both cases . but if i check my nand/and logic its running and gives same result which i got before. i am attaching my schematic please tell me where is error.
this is 180nm tech with resistor as load in MCML and their waveforms .



this is 180nm tech with pmos as load in MCML and their waveforms .

 

Miguel Gaspar

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Virtual Ground is a cero reference but without current. That is the case of the op-amp with very high gain

the opamp equation is: Vo = A(V+ - V-)

V+ - V- = Vo/A but if A = ∞ . . . . . . . . V+ - V- = 0, V+ = V-, a short circuit and because Rin is very high there is no current. That's why is called virtual because there is no current.

Floating in a circuit means that there is no conection to ground, for example the car electronics circuits. Another example is when you declare that your load is a resistor and none of his pins are conected to ground.
 

zorro

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Hi,

Let me fix a mistaken term in Errakeshpillai's post (#2).
Where you read "infinite input resistance" it should be "infinite voltage gain" (two instances).
(Of course, "infinite" is the ideal condition.)
Regards

Z
 

mrarslanahmed

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virtual ground is most oftenly used for opamp here voltage at the positive terminal is virtually equivalent to negative terminal due to ideally infinite gain...
 

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