Hey I was wondering if anybody can explain the concept of negative resistance. I don't get how Current in certain materials (semiconductors, gas tubes) can decrease, if the Voltage increases. Wouldn't this go against Ohm's Law ?

Thanks

 

One thing that you have to be clear. Ohm's Law applies only for Ohmic conductors. To put it more specifically, ohm's law calls any material whose voltage increases linearly with current an ohmic conductor. It is more a description of a specific behavior rather than a universal law. It's like saying any guy who is tall is called a tall guy.

I don't know much about negative resistance circuit elements. You can refer to the wikipedia explaination.
http://en.wikipedia.org/wiki/Negative_resistance
But the most common circuit elements don't have negative resistance.

 

look up tunnel-diodes they are the only devices I know of that exibit negative resistance is a small section of it V-I curve

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Now, I admit my field is not electronics, but rather communications. But a friend of mine is getting his phd in an electronics field (semi-conductor physics I believe). Which doesn't make ME and expert, but we've talked about this one time before so I'll just relate to you what he said and what a teacher of mine once said.

He said using op amp circuits you can achieve things like negative capacitance and inductance. I'm not absolutely certain if we talked about negative resistance, but I think he said the same about that.

Now relating to the diodes question, I had a teacher once say if you can figure out how to harness the energy in the junction you can actually create power. Which I believe is the same thing as saying that a tunnel diode exibits negative resistance over a region of the VI curve.

 

Negative signs are conventions used to refer to current flow or voltage drop directions. Sometimes they represent Phase shifts.

How would you go about implementing a negative resistance? How do you eat negative bananas? The point is negative values are generally not possible physical quantities unless your dealing with theory or your bank account.

As an example take an AC generator it does not generate negative voltages just the flow has changed direction hence the negative. So as far as a negative resistance or any other negative device it is highly unlikely considering your dealing with physical quatities.

Also, what would a negative resistor, capcitor, or any other device do?
Essentially it would give energy off with out taking any energy in violating the laws of energy conservation.

 

Errr no. Positive resistance is basically depicted from a Y=MX line with Volts on the X-axis and Amps on the Y. the gradient of which is positive.

IN the case of a Tunnel diode there is a portion in its V-I curve where the slope becomes negative. Since the slope depicts "resistance", during that section the tunnel diode exibits negative resistance.

Tunnel diodes were used for a very long time as amplifiers (using this effect) and extreamly (and I mean extrealy) high oscilators.

__________________
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Once a problem is realised, the rest is just details

 

Just because the slop is negative does not mean it is a negative resistance. While there are areas where the slope is decreasing that does not create a negative resistance.

While I am not saying its impossible to have a negative resistor, it is highly unlikely since where dealing with a physical quantity. Think about counting your money. You cannot have -50 dollars, because it doesn't exist. What you have is 0 dollars and owe someone 50. Hence the sign convention which represents something such as owing money, the direction of current flow, the direction of voltage drop, etc.

 

V=R*I

that is a Y = M*X + C curve with C=0

you are confusing a phyical proporty with an effect of semiconductors. True a passive resistor can never have negative resistance, but there are diodes (tunnel and gunn) that exibit a negative resistance characteristic

The slope is negative, thus R in the V=RI eqaution must be negative => nagative resistance

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Once a problem is realised, the rest is just details

 

The point I was making was to be sure it is understood that the actual resistance is not going negative. While the slope is going negative exhibiting a negative effect, the resistance is not actually negative.

In clarfying his question I was trying to point out that the devices are not actually creating negative resistances. Negative resistance and Negative differential resistance are two very different things.

 

Well yer, it seems we are talking abt the same thing, just nto mentioning cetain bit - I fialed to mention it is a dynamic negative resistance

Resistance is defined in two ways
Static: V = IR
dynamic dv/di

IN almost every situation static is the same as dynamic. it is the odd few diode cases (gunn and tunnel) that exibit this "negative resistance) or dynamic negative resistance.


For it to be "true" negative resistance the amps would have to go (magnitude ) negative with respect to increasing volts - ie sourcing.

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With an op amp and 3 resistors, you can make a linear negative resistance whose I-V curve passes through zero. See, for example, this Maxim app note.
Of course, as the op amp hits the rails, the negative resistance region ends.

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Ron

 

In regards to the original questionof this thread, I was pointing out that these device creating this Negtive Differential Resisitance are not violating ohms law. Notice all components in the url in the post above are positive values which illustrates my point. Just because the V-I curve has a negtive slope it does not mean there is physically a negative resistance in the circuit. The whole point of this was to answer the original question about these circuits which create a Negative Differential Resistance violating ohms law. Which they do not do.

 

Except what the original poster was stating was his didn't understand how a device with increased voltage could decrease the current flowing though it.

Every reply has showed the Tunnel,Gunn... that has this effect thus satifying the orignial posters question - in part

__________________
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Once a problem is realised, the rest is just details

 

I was playing around with the Maxim circuit I cited above. I thought you guys might like to see a sim of an LC oscillator made with the operational negative resistance.
One way to look at this is that the negative resistance (about -48k here) is cancelling the positive resistance in parallel with the tank (I used 50k, for a Q of 50 at the resonant frequency). I also ran the sim with a small value series resistor instead of one in parallel, with similar results.
Of course, all sine wave oscillators using amplifiers require gain with positive feedback, which this circuit has. That's another way of looking at it.
The zener prevents the oscillation from building in amplitude until the amplifier limits.

Attached Images

	neg_res_osc_sch.gif (4.4 KB, 1062 views)
	neg_res_osc_waves.gif (7.0 KB, 1062 views)
	neg_res_osc_fft.gif (8.7 KB, 1062 views)

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Ron

 

I was pointing out that it is not violating ohms law. Anywyas this is enough of this post for me.