# Help With an Equation

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

1. ### ashesv14

5
0
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

604
146
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. ### hevans1944Hop - AC8NS

4,536
2,112
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

250
69
Aug 4, 2011
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.