Re: tunnel diodes retrace lines in curve tracer


 

Hi Miguel,

This is precisely the curve a Tunnel Diodes creates on a curve tracer.
From this image you can measure many of the important tunnel diode (TD) parameters. The curve tracer typically applies a rectified full-wave sine wave to the device under test. Assume we begin with the voltage at zero. As the voltage rises (we won't need more than a volt or two) the current in the TD starts to rise until it reaches a peak.
This peak on the left of the curve tracer shows the maximum current of this part of the curve known as Ip. Ip is typically 1mA to 10mA and when you replace a TD you always want to match the new one to the Ip of the old one.
As the curve reaches Ip drop down vertically to the horizontal axis and read the peak voltage. This parameter is Vp. This is typically 0.05V to 0.15V.
Once the TD reaches Ip it can't go any further without switching in less than 1nSec to a new state. That happens so fast the curve tracer can't follow it and you see a horizontal line going to the right until it stops at a new point. The current hasn't changed but the voltage is much more now. It may be as much as 0.3V to 0.5V at this point. The current can now continue to increase and the voltage will increase with it but this part of the curve is not important.
What happens next is the voltage from the curve tracer will start to drop. Remember we switched to the far right part of the curve and although the current is the same or more than the peak current, Ip, the voltage is more on this part of the curve. Now the voltage from the curve tracer will begin to drop. As it does the current can almost drop to zero and the voltage is dropping as well. Eventually we reach the next critical point where the voltage cannot drop any more without the tunnel diode jumping again. This point is called the Valley Voltage, or Vv. The current through the tunnel diode when it reaches Vv is called Iv.
At this point the TD switches back to the left part of its curve in less than 1nSec. Once again this is too fast for the curve tracer to follow so as the beam moves as fast as it can you see a horizontal line going from the Valley voltage to the left where the tunnel diode is back on its original part of the curve.
Now the process repeats. As the curve tracer voltage rises the current increases until it reaches Ip at which point the current stays the same but the voltage increases from Vp to a higher voltage. As the curve tracer voltage decreases the current drops and the voltage begins to drop until the voltage reaches Vv. At that point the TD jumps to the left part of the curve, the current stays the same at Iv but the voltage drops to almost 0V.
The negative resistance can be calculated (approximately) if you draw a line between the two points on the curve tracer where the curve seems to disappear.
Since your photo shows the vertical and horizontal sensitivity of the TD curve it is easy to calculate the approximate negative resistance of this TD.
R = E / I = (0.12V - 0.45V) / (1mA - 0.2mA) = 0.33V / 0.8mA = -412ohms. Notice that is negative resistance.
Negative resistance is an impossibility but it can exist within a very small region of the otherwise positive resistance of the complete tunnel diode curve.

For more information Jack Rogers of Tek wrote a 13 page explanation of all of this with actual circuits used in Tek instruments showing how they perform.

It is on the vintageTEK at
https://vintagetek.org/wp-content/uploads/2019/04/TunnelDiodes_JackRogers.pdf

Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Miguel Work
Sent: Wednesday, February 17, 2021 12:28 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] tunnel diodes retrace lines in curve tracer

Thanks Jeff,

I have found some information in internet, I was stubborn

https://groups.io/g/TekScopes/album?id=260881








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Dennis Tillman W7pF
TekScopes Moderator

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