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Help With an Equation

Discussion in 'Electronics Homework Help' started by ashesv14, Apr 21, 2014.

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  1. ashesv14


    Feb 10, 2014
    The equation is to find the resistor value for the inverter
    Vout is 0.1V and Vdd is 5V.

    ratio of the resistance are:

    Vout/Vdd = 0.1/5 = (Rds/(RL+Rds))

    or RL = 49Rds

    Can I have a explanation how RL=49Rds, where Vout=0.1V and Vdd=5V
  2. LvW


    Apr 12, 2014
    I am afraid, there is no help possible without some additional information.
    *What kind of inverter (opamp or transistor)?
    *What is RL and Rds ?
    *What kind of "equation"? Gain?
  3. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    You take the equation(0.1/5) = 0.02 = (Rds/(RL+Rds)) and use algebra to solve for RL = 49Rds. There are usually limits (a range of suitable values) for Rds. Plug one in and multiply by 49 to get RL.
  4. Merlin3189


    Aug 4, 2011
    How about using ratios?:
    0.1/5 = 1/50 (making both numbers 10x bigger keeps the ratio the same)
    So RDS / (RDS + RL) = 1 / 50
    So whatever RDS is; 1 Ohm, 1 kOhm, 1 lot of 270 Ohm, etc.
    then (RDS + RL) is 50 of them; 50 Ohm, 50 kOhm, 50 lots of 270 Ohm (ie. 13.5kOhm), etc.
    The RDS is one of them, so RL must be 49 of them: in math language, RL is 49 x RDS

    As Hevans says, you can use algebra to show that
    RL is RDS multiplied by (one less than ( Vdd divided by Vout) )
    or rearrange to find any other one of the values.
    You can put all the known numbers into a formula which is any way round, then the algebra becomes easier.
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