# How to build a negative resistor??

1. ## negative resistor

In the analog filter design. Sometimes we need a negative resistor. Then how to build a nagative resistor? Use feedback? how?

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2. ## How to build a negative resistor??

Hi Alles,

You can use a Negative Inmitance Converter (NIC). Look for it in network theory literature. It is an active device with one operational amplifier and 2 resistors that converts an impedance Z (not necessarily resistive) into -k*Z. It uses feedback, of course.
Regards

Z
(with a NIC I become -Z, ha...)

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3. ## How to build a negative resistor??

i think if u realize the filter in sc mode then sc inverting connected circuit acting as a negtive res should be used

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there u go

5. ## Re: How to build a negative resistor??

I did not know that you can build negative resistance with MOSFETs. probably thats the best answer.

- this is what i got:

Usually, my way has been to have an OpAmp, with its inverting node connected to both the output node of the OpAmp (feedback resistance R3) and the input current (Iin) and the non-inverting node is connected to both the output node of the OpAmp through R2 resistor and to the ground via R1 resistor. the Output is given by:

Vout = -R3*R1*Iin/R2.

Vout is between the inverting node and the ground.

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6. ## How to build a negative resistor??

What srivatsan described is a NIC.
For any Z1 in the place of R1:
Zin=-(R3/R2)*Z1

-Z

7. ## Re: How to build a negative resistor??

well since the question is for a negative resistor I think the cross-coupled mos pair is simple enough.

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