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'[EE]: opamp info'
2002\03\28@150128 by

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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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}
Google turned up these:
ourworld.compuserve.com/homepages/Bill_Bowden/opamp.htm
http://www.williamson-labs.com/480_opam.htm
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.
>
> --
> http://www.piclist.com hint: PICList Posts must start with ONE topic:

--
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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 <rsloanTHEMINDFACTORY.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?

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:

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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