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Subject: Re: Please help me out with the Op-Amp LM348

Date: 05/09/01 at 5:46 PM
Posted by: Rich Graziano
E-mail: rgrazian@rochester.rr.com
Message Posted:

In Reply to: Re: Please help me out with the Op-Amp LM348 posted by Ray on 10/07/00 at 11:03 AM:

James:

Your question about the OP amp is straight forward.
1. You need a split power supply, i.e., +v, gnd and -V.
2. if you want the output to be inverted from the input, then use a series resister in the -in pin and a feedback resistor between the output pin associated with that input and that -in pin. Place a resistor between the +in and ground. The resistors are calculated as follows. The desired gain (amplification of signal) is determined by the ratio of feedback resistor to input resistor.
for example, a 100,000 Ohm feedback resistor and a 10,000 Ohm input resistor is a 10 to 1 ratio, so the gain is -x10. therefore a +0.1 voly input will result in -1.0 volt output. The converse is also true. The input impedance will closely approximate the input resistance for DC or low frequency AC signals. The resistor between the +in pin and gnd (common mode resistor) will be that which is equal to the value of R in and R fb in parallel.

3. If you want a non-inverting configuration (where the output polarity is the same as the input polarity) then place a resistor between the -in pin and gnd. Then place the feedback resistor between the output pin and the -in pin.
The input could go directly to the +in pin.

The gain (amplification) will be expressed by the transfer function Vo = (1+Rfb/Rin)Vin, where Vo is the output voltage amplitude, Vin is the input voltage amplitude, Rin is the input resistance and Rfb is the feedback resistance. This configuration has the advantage of a very high input impedance and is used where very small currents are measured.

I hope this is what you were looking for.

Regards,
Rich


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