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Old 30th October 2007, 03:59 AM   #1
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Default Voltage Follower

Hi i am quite new in here. Nice meeting you all.

There is something i would like to ask you all. I am quite confuse with this Op-Amp.
From the pic 1, i understand that its a voltage follower itself since i check from the web. But i don't understand from pic 2 when you add 2 10k ohm resistors its still a voltage follower. Then whats the use of the 10k ohm resistor if its already a voltage follower.

Thanks in Advance.
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Old 30th October 2007, 04:08 AM   #2
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The first one is a 1:1 voltage follower; you feed it 1v, it puts out 1v. The second is a 1:-1 follower; you feed it 1v, and it puts out -1v.
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Old 30th October 2007, 04:30 AM   #3
fdeck is offline fdeck  United States
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In the second case, the + input of the op amp should be grounded. There is a useful rule of thumb: "The op amp does whatever it can to make its inputs equal."

In the first circuit, it is a follower because the inputs are equal if the output equals the + input.

In the second circuit, the + input is grounded, so the op amp has to hold the - input at ground potential. It does this if the voltages at the ends of the two resistors are equal but of opposite polarity. That's how the second circuit achieves an output equal to the negative of the input.
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Old 30th October 2007, 03:26 PM   #4
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From what you say about second test is becuase i put the input in the negative side that why the output provides a negative right. Then what do the resistors are for?

The circuit should look like this.
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Old 31st October 2007, 04:37 PM   #5
AndrewT is offline AndrewT  Scotland
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remove the 10k pot,
just connect the non-inverting input to signal ground.

However, the unmatched resistances seen by the two opamp inputs will generate an output offset.
To minimise the output offset, the resistor between non-inverting input and signal ground should be 10k//10k =5k0.
The two 10k resistors set the gain of the inverting stage.
gain=-Rfb/Rin=-10k/10k=-1. Zin =10k, Zout is very low.

This inverting circuit can have a gain greater than -1, or by changing the resistors, a gain of less than -1.
The non-inverting opamp can only have a gain of greater than 1.
regards Andrew T.
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Old 1st November 2007, 08:08 AM   #6
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I see. Thanks for you help.
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