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'[EE]: opamp info'
2002\03\28@150128
by
Richard Sloan
Is it possible to make an opamp design with one variable 50K resistor and have its gain variable from 1/10x to 10x?
Where on the web is good opamp info?
Richard.

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2002\03\28@155553
by
Thomas McGahee

Yes.
If you are using an inverting configuration, you would need three
resistances. For example, choose a 5k resistor for the input
resistor. The feedback resistance would consist of a fixed 500 ohm
resistor in series with a 50k pot wired as a rheostat.
This would give a gain variable from 1/10x to 10.1x
If you absolutely want a gain range from 1/10x to 10x, then you
have to solve the set of simultaneous equations:
.1=Rf/Rin and
10=(Rf+50k)/Rin
Rearranging the first equation we get:
Rf=.1*Rin
Substituting into the second equation we get:
10=(.1*Rin+50,000)/Rin
1=(.1*Rin+50,000)/(10*Rin)
1=(.01*Rin+5,000)/Rin
Rin=.01*Rin+5,000
Rin.01*Rin=5,000
.99*Rin=5,000
Rin=5,000/.99
Rin=5050.5050
So, Rin would be 5,050.505 ohms
Rf=.1*Rin Rf=.1*5,050.505 which is 505.05 ohms
When the 50k pot is added, the gain becomes:
gain=(Rf+50,000)/R1 which is gain=(5,050.5+50,000)/5,050.505,
which is 10.000
So, the gain is now variable from 1/10x to 10x
Have fun trying to get resistances (and the pot!) this accurate.
If you use 5.1k and 51k 1% resistors you would get a gain range from
.1x to 9.9x, which is within 1% of what you want.
Fr. Thomas McGahee
{Original Message removed}
2002\03\28@160620
by
Rick C.
Google turned up these:
ourworld.compuserve.com/homepages/Bill_Bowden/opamp.htm
http://www.williamsonlabs.com/480_opam.htm
www.uoguelph.ca/~antoon/gadgets/741/741.html
Rick
Richard Sloan wrote:
> Is it possible to make an opamp design with one variable 50K resistor and have its gain variable from 1/10x to 10x?
>
> Where on the web is good opamp info?
>
> Richard.
>
> 
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2002\03\28@161702
by
Dal Wheeler
Yes,
check out national semiconductor for the op amp databook and the linear
applications handbook should be available on PDF.
 Original Message 
From: Richard Sloan <spam_OUTrsloanTakeThisOuTTHEMINDFACTORY.COM>
Is it possible to make an opamp design with one variable 50K resistor and
have its gain variable from 1/10x to 10x?
Where on the web is good opamp info?

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2002\03\28@170925
by
Richard Sloan

Thanks!
>> Yes.
>> If you are using an inverting configuration, you would need three
>> resistances. For example, choose a 5k resistor for the input
>> resistor. The feedback resistance would consist of a fixed 500 ohm
>> resistor in series with a 50k pot wired as a rheostat.
>> This would give a gain variable from 1/10x to 10.1x
>> If you absolutely want a gain range from 1/10x to 10x, then you
>> have to solve the set of simultaneous equations:
>> .1=Rf/Rin and
>> 10=(Rf+50k)/Rin
>> Rearranging the first equation we get:
>> Rf=.1*Rin
>> Substituting into the second equation we get:
>> 10=(.1*Rin+50,000)/Rin
>> 1=(.1*Rin+50,000)/(10*Rin)
>> 1=(.01*Rin+5,000)/Rin
>> Rin=.01*Rin+5,000
>> Rin.01*Rin=5,000
>> .99*Rin=5,000
>> Rin=5,000/.99
>> Rin=5050.5050
>> So, Rin would be 5,050.505 ohms
>> Rf=.1*Rin Rf=.1*5,050.505 which is 505.05 ohms
>> When the 50k pot is added, the gain becomes:
>> gain=(Rf+50,000)/R1 which is gain=(5,050.5+50,000)/5,050.505,
>> which is 10.000
>> So, the gain is now variable from 1/10x to 10x
>> Have fun trying to get resistances (and the pot!) this accurate.
>> If you use 5.1k and 51k 1% resistors you would get a gain range from
>> .1x to 9.9x, which is within 1% of what you want.
>> Fr. Thomas McGahee
>> {Original Message removed}
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