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Help for a difference amplifier circuit

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Newbie level 3
Jan 21, 2006
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I design a circuit which is shown below,and it works
,but i don't know how to analyse it is kinda different with the basic differential circuits L1 is coil which is used to induce a changing magnetic field and i'd like to ampilify the induced current signal which is very small ,about 75mV(P to P).could you tell me how it works and how to calculate the amplifier's output.thank you very much

what is it?
please describe it forther more.
and what is its output, input?

please verify that on which port you are applying the input because the cirut you have designed is i think not correct . because you have shorted both the input secondly you shorted both.
first odf all i want to know taht which type of amploifier you want to design .
than i can help you.
thank you and reply

The values of the resistors are also required.

I suggest modeling it in spice with a transformer instead of an inductor. Apply input
signals to the input side of the transmformer and see what the output of the opamp does.

At DC the inductor is zero ohms. The steady state voltage at the
non-inverting input is therefore set by the potential divider formed
by R1 and the parallel combination of R3 and R4. You can ignore the
effect of R2 since the feback from the opamp input to output should
cause the ouput to be very close to the voltage of the two inputs.

The ratio of R1 to 1/(1/r3 +1/r4) will be chosen to make the opamp
output in the middle of the supply when there is not signal, 2.5V in this case.

There will be a steady state DC current through the coil from +5V through R1,
through the coil and through R3 down to 0V.

>i'd like to ampilify the induced current signal which is very small ,about 75mV(P to P)

With the coil just connected to a meter or scope or with it connected to this circuit?
The voltage you measure will depend on the load the coil is connected to.

The induced current is more relevant.

A simplisitic analysis is that when the inverting input is at a higher voltage than
the non-inverting input the voltage from the output of the opamp decreases.
When the non-inverting input is at a higher voltage than the inverting input
the opamp output decreases.

When the bottom of the coil is more positive than the top the opamp output
increases in voltage. This means there is more voltage across R2. The
higher voltage across R2 makes a current flow through it which compensates for
the drop in the steady state current through the coil.

Making R2 larger will increase the gain of the circuit. I suspect
that adding a capacitor between the non-inverting input and ground
would also increase the gain.

Finding a gain equiation for that circuit is a tough one.

thank you for your reply,the input of this circuit is the coil L1 which induces a transient changing magnetic field , a transient current changing occurs in the coil and i want to amplify this signal, from my experiment,when there is no changing magnetic field the current across L1 is unvaried the ouput of the amplifier is zero when R3=R4 ,as long as i apply a changing magnetic field to L1 L1 will induce a current and the amplifier's output will change in terms of the current of L1 but i'd like to know their relationship

You have to use the real model of the OpAmp, an think that the current through the inductor can be considered as the OpAmp's Input Offset Current. Then analyse the circuit and see how that current appears in the output voltage.

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