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Forum: Analog Circuits URGENT. Help on Power of Inverter.


Author: Karthiga G. (karthiga05)
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Hi everyone. there is usually no power consumed in an inverter. Bt im 
required to find out a condition that allows an inverter to consume 
power. does anyone of u knw the condition? Thanks in advance!

if logic '1'is being applied, a logic '0' is what comes out and vice 
versa. power is not consumed when both of this conditions are applied. i 
would like to knw what is the condition required for power to consume.

Author: Lothar Miller (lkmiller) (Moderator)
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> i would like to knw what is the condition required for power to consume.
First try this:
Switch both of the fets half way on...
So instead of applying a '1' or a '0', choose a '1/2' (with 5V supply 
this is about 2.5V). A little hint here: have a look for the threshold 
voltage of the fets.

And then step two:
What happens, when you switch from 0 to 1 and back very very fast?
Each time you have to cross the 2.5V.
And additionally: you have to charge and dirscharge parasitic capacitors 
at the output (not drawn in the picture...)
Both effetcs consume power.

Author: Karthiga G. (karthiga05)
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erm. im just learning theory. im only required to find out the 
condition, is it this ---> significant power is only drawn whn the 
transistors in the CMOS device are switching between on and of states?

Author: Lothar Miller (lkmiller) (Moderator)
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Yes.

Author: Karthiga G. (karthiga05)
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Lothar Miller wrote:
> Yes.

okay. great, thanks. :)

Author: Karthiga G. (karthiga05)
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Lothar Miller wrote:
> Yes.

on and off meaning switching from 1 to 0 right?

Author: Lothar Miller (lkmiller) (Moderator)
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> on and off meaning switching from 1 to 0 right?
Hmmm... You got something wrong... :-/
To keep it short: excessive power is consumed if you don't apply to the 
input a steady 1 or a solid 0, but "something in between". This I meant 
with the phrase "1/2"...

And this "something in between" you can achieve by simply leaving the 
input unconnected. Then both of the gate will charge up to a 
intermediate voltage, and both of the transistors will conduct corrent 
from Vcc to GND. This case is widely known as a short circuit.

Author: Karthiga G. (karthiga05)
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Lothar Miller wrote:
>> on and off meaning switching from 1 to 0 right?
> Hmmm... You got something wrong... :-/
> To keep it short: excessive power is consumed if you don't apply to the
> input a steady 1 or a solid 0, but "something in between". This I meant
> with the phrase "1/2"...
>
> And this "something in between" you can achieve by simply leaving the
> input unconnected. Then both of the gate will charge up to a
> intermediate voltage, and both of the transistors will conduct corrent
> from Vcc to GND. This case is widely known as a short circuit.

okay! got it. thanks so much! :)

Author: Karthiga G. (karthiga05)
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Lothar Miller wrote:
>> on and off meaning switching from 1 to 0 right?
> Hmmm... You got something wrong... :-/
> To keep it short: excessive power is consumed if you don't apply to the
> input a steady 1 or a solid 0, but "something in between". This I meant
> with the phrase "1/2"...
>
> And this "something in between" you can achieve by simply leaving the
> input unconnected. Then both of the gate will charge up to a
> intermediate voltage, and both of the transistors will conduct corrent
> from Vcc to GND. This case is widely known as a short circuit.

actually what do you mean by 'simply leaving the input *unconnected*'

Author: Lothar Miller (lkmiller) (Moderator)
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> actually what do you mean by 'simply leaving the input *unconnected*'
- leave it floating
- do not connect a external signal to it
- do not apply GND or Vcc/5V to it

Author: Karthiga G. (karthiga05)
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Lothar Miller wrote:
>> actually what do you mean by 'simply leaving the input *unconnected*'
> - leave it /floating/
> - do not connect a external signal to it
> - do not apply GND or Vcc/5V to it

okay! thanks again!

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