I found this op amp circuit schematic online and if I am not mistaken it is
wrong because the feedback resistor is connected to the + Non-Inverting
input and positive feedback must have a voltage divider with the input gain
resistor from the Inverting input to ground with the input on the Non-inverting input
as shown in the "STANDARD NON-INVERTING AMPLIFIER" and the gain being A = (1 + (Rf/Rinputgain) ). But I also found
another schematic that had the same configuration with the feedback resistor
connected to the NON-INVERTING input , as shown in the "NON-STANDARD NON-INVERTING
AMPLFIER SCHEMATIC. I have never seen this configuration so I have to ask,
"Is the really a thing ? " or are these two schematics wrong ? and if it is a thing,
WHY is it done ?
The four attached files are two CLASSIC configurations I have been familiar with for
many years and the other two are two NON-STANDARD configurations I have never seen before.
What's up with these ?
The ones with the feedback resistor connected tobthe + NON-INVERTING input INSTEAD of the INVERTING input with the input applied to the NON-INVERTING pin as shown in the STANDARD NON-INVERTING APLIFIER schematic sttached.
The Wheatstone bridge circuit is standard for an instrumentation amplifier. I'm sure that you can find an analysis of it somewhere. Today, you buy chips with all that in one IC, like the INA115.
If you google it you'll see almost ALL the others have the input onnthe Non-inverting pin and the feedback resistor on the inverting pin with a resistor from the inverting pin to ground.
I'm not talking about the bridge. I'm talking about how the amp is wired.
The bridge works when wired as described above
Look at page 4 of the LM339 comparator datasheet, note the values of input and feedback resistors. Positive feedback is used to set hysteresis (voltage difference between flipping HIGH and flopping LOW). LM339
My post is not about positive feedback in general but rather about the way the op amp is wired inn the wheatstone bridge amplifier schematic, specifically the one with the feedback resistor connected to the non- inverting input. That circuit doesn't work. The bridge circuits works with the "STANDARD DIFFERENTIAL AMPLIFIER" configuration posted. And yes there is such a thing as 'standard when you hoogle op amp differential amplifier and all the the links show the same thing.
Just like when you google op amp non- inverting amplifier and all the the schematics are the same as they have been for more than 50 years. I'm not saying you could find another way to do it. I'm saying that when 90% of the world uses the same schematic for a non- inverting amplifier you can be pretty safe in calling it standard.
Comparator flips from one rail to the other. So "pegged" ouput is normal. Change input past threshold & it should slam to the other rail. It is NOT going to act like a fixed gain linear block.
A wheatstone bridge amplifier is exactly the OPPOSITE of a comparator.. Comparators have no relevance to a wheatstone bridge. Look at the schematic . That circuit doesn't work but the other one with a 'standard differential amolifier does work when you connect it to a a wheatstone bridge. Google "op amp differential amplifier and all the links show the circuit I posted along with the bridge cirvuif that doesn't work. If you don't believe me build it yourself and you'll see it won't balance but pegs at the negative rail. @JCA34F,
Walter Jung's cookbook is the I started with in '79,
building op amp circuits with a bag of NS LM306H
metal can oo amps I got on sale for 25 cents EACH.
Bought 166 of them and gave the last 66 to a Standford EE student 20 yrs later.
Ahh, the good ol' (?) days. Where did you get that schematic? Would be interested in the meaning of the colored "V"s looks like an instrumentation amp with 3 X 1000Ω and 1 1001Ω resistors in the bridge.
