# Resistor values- scratching my head

#### c131frdave

Oct 4, 2013
40
Below is a portion of a long schematic. I won't bore you with it. But my task is to change the voltage, and therefore I assume, the resistor values.

The schematic was designed for 9V, but I want to change it to 12V. The Amplifier is good for 0 to 32 V, and has an input forward current of 50mA.

As you can see from the schematic, they are currently using two 6.8Mohm resistors for power, which I don't understand at all. It seems way over kill. Using the I=V/R formula, I get like 120ohms.

Clearly I'm a noob and an idiot. Can somebody explain this to me? Thanks.

EDIT: I should mention that the input of the other side is from an antenna. This is an Rf amplifying circuit. So very low current- is that why such large valued resistors??

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#### kpatz

Feb 24, 2014
334
Those resistors act as a voltage divider to bias the input of the op-amp at 1/2 the supply voltage. Normally lower valued resistors are used for this, but perhaps to minimize current draw they used 6.8M resistors? The higher a resistor's value, the more noise there will be. What normally powers this circuit? A 9V battery? That could be why. Or whoever designed the circuit had a bunch of 6.8M resistors lying around.

Based on what your schematic shows, you should be able to run the circuit at 12V no problem. If your 12V source has a larger current capacity than a 9V battery you could replace those two 6.8M resistors with something lower, like 470K and reduce the noise.

The 6.8M feedback resistor gives the circuit a high gain, depending on the antenna's impedance. Usually an inverting amplifier has a series resistor on the input, but with an antenna that may not be necessary.

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#### c131frdave

Oct 4, 2013
40
It was designed by a Professor at Seattle University, and I think it was done on paper before it was done in actuality. If I use, say a Lipo battery of 12.2 volts, and has the current capacity of 20 amps (I have one on my desk I use to power my radio controlled Mustang), I can lower my resistance? Should I? The signal the antenna is picking up is very weak to begin with.

Thanks so much for your help!

#### kpatz

Feb 24, 2014
334
Have you tried powering it with 12V without any modifications yet? Also, is there more to the circuit? What you showed in the schematic should be fine running at 12V, but who knows about other circuits you didn't show.

Lowering the resistors in the divider should improve the S/N ratio some which may help the performance. What frequency is the signal you're trying to pick up? The LM358 isn't really good for RF use, its open-loop gain drops to zero at 1 MHz.

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