At 09:25 AM 2/11/2004 -0800, you wrote:
>Guys,
>
>Thanks for the help. I am still a "newbie" to op-amps and analog in
>general. How would you go about figuring out the reference voltage that is
>required for any given application? I looked in my textbook and it doesn't
>cover anything about this subject. Suppose I had the following requirements:
>
>+0.6V to +1.2V (voltage is variable) as input, I need an op-amp  to give
>me an output swing of +10V (when input is +0.6V) to +0.3V (when input is
>+1.2V).

Your input voltage change is 1.2 - 0.6 = 0.6 volts. Your desired output
change is 10 - 0.3 = 9.7 volts

Thus the absolute value of the GAIN of the op amp is going to be  9.7/0.6 =
16.16666666.

Since you want to output voltage to DECREASE as the input voltage
INCREASES, you will need to use the opamp in an INVERTING configuration
(i.e. you put your input on the -ve input, and use the +ve input as the
reference.

If you use a + and - 25 volt supply (for example), to make sure that there
is enough voltage swing on the output, an input of 0.6 volts will give an
output of 0.6 * (-16.17) = -9.7 volts.
An input of 1.2 volts will give an output of 1.2 * (-16.17) = -19.4 volts.

But you want the output to be 10 v when the input is 0.6 volts, so you need
to effectively add 10 +9.7 = 19.7 volts to the output.
Thus 0.6 volts in would give -9.7+19.7 = 10.0 volts
and 1.2 volts would give -19.4 + 19.7 = 0.3 volts.

Now, given an gain of 16.166666, to add 19.7 volts to the output requires
adding 19.7/16.17 = 1.218 volts to the reference input.

I'm no good at ASCII art, but in words:

from your input to the -ve input of the opamp, use a 1k resistor. From the
output to the -ve input, use a 16.17K resistor.

 From the reference voltage of 1.21 volts to the +ve input of the opamp,
use 1K resistor. From the +ve input to ground, use a 16.17K resistor.

Now, since the output voltage swing will be from +10 to +.3, you don't need
the +- 25 volts supply, but can probably use a single +12 volt supply, and
use a rail-to-rail opamp, or you can use a normal opamp and a +12 volt, -5
volt supply. You need to make sure that the opamp you chose can deliver +
10 volts out with whatever supply voltage (e.g. +12) you choose.



>Would you then figure out the gain of the circuit then use an offset as
>needed, if so how would you figure out how you find what offset is needed?
>
>I appreciate your help and patience.
>Thanks.
>Steve
>
>
>
>
>---------------------------------
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Larry Bradley
Orleans (Ottawa), Ontario, CANADA

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