modified 9 years ago

How to think about the instrumentation amplifier

00:45:09

The instrumentation  amplifire circuit in  the examples has unbalanced inputs one of which terminals is connected to GND. Unfortunately, if it is not connected, this simulator dosen't work. 
This circuit shows how to think about the instrumentation amplifire with balanced inputs. Simply devide the input voltage to two, and devide the resistor in the center to two. Connect both centers. Then this simulator works (Figure in the left). Because it's same as two non-inverting amplifires. But please remember, there is no current on the line that connects both centers of the input voltage and the resistor. So, it's same as when they are not  connected.
Unfortunately, if cut the line of that, this simulator doesn't work (Figure in the right).

published 9 years ago

hurz

9 years ago

Unfortunately your circuit is wrong. Swap +- input terminal of second stage OpAmp!

FitFat

9 years ago

Ooohpssss. Thanks for your comment. I corrected it. But it dosen't work still.

hurz

9 years ago

Put a ground at input, doesn't matter were you connect it. Top, down or center. And if you dont like a hard ground at any points, then use 1giga ohm and connect any point to this weak ground

FitFat

9 years ago

1 G ohm!!! :-). I was thinking what becomes matters using balanced inputs to connect audio devices. Then I found this simulator dosen't work. Actually for audio use, maybe not too small current is better for less noise. To use some kilo ohms then. In this circuit, I haven't chaned other resistor sizes. Thank you for your comments every time.

hurz

9 years ago

You are welcome. Can you publish the application were/how you plan to use this instrumentation amplifier?

FitFat

9 years ago

I've just started thinking about possibility to use instrumentation amplifire. On a headphone amplifire like, or pre-amplifire, using only first stage like this ( http://everycircuit.com/circuit/5114912924237824 ). I think it can work with only single power supply, because no current on GND. I wanted to confirm it using this simulator :-).

FitFat

9 years ago

with voltage follower http://everycircuit.com/circuit/6542807685136384

hurz

9 years ago

And why and how?

misaki_david

9 years ago

two op amp input became floated

FitFat

9 years ago

Hi, misaki_david. Yes, that's right. But operational amplifire has no GND terminals normally. It requires two input voltages are between (+) and (-). In the original case of instrumentation amplifire, the center of two inputs will be automatically created from the balance of two operational amplifires, and it will be GND level in any case.

FitFat

9 years ago

Hi, hurz. I'm a just fan of electronic handcraft. I love simple, and like to try abnormal way :-). I wonder no need (+) and (-) power supply or adding bias voltage to input signal to use balanced inputs.

FitFat

9 years ago

If no connecton to GND from the figure, circuit voltages are fixed from powe supply ( http://everycircuit.com/circuit/6137055148507136 ).

FitFat

9 years ago

Using 5 1 G ohm resistors ( http://everycircuit.com/circuit/4589531452407808 ) :-). I think it can work without these 1 G ohms.... I've gotta use 1000 1M ohm resistors in series for each (just kidding).

hurz

9 years ago

You still don't published an application circuit.

FitFat

9 years ago

I read some catalogues and data sheets to select ICs and get these specs, and to fix values of parts. Anyway I tested a circuit plan for a powe amplifire ( http://everycircuit.com/circuit/5620762226982912 ).

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