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Tunnel diodes: Difference between revisions
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Before concluding that a tunnel diode is bad, it is important to be sure that it has been measured correctly. A high resistance reading on a DMM indicates that the diode is bad. A low resistance on a DMM and a low voltage on a diode tester are both normal when measuring a tunnel diode. A more thorough test of a tunnel diode is to drive it through a resistor with a ramp voltage source while observing the voltage across the tunnel diode. The resistor should be calculated so that the peak current just exceeds the peak current that the tunnel diode is rated for. Of course if a curve tracer is available, it is great for measuring | Before concluding that a tunnel diode is bad, it is important to be sure that it has been measured correctly. A high resistance reading on a DMM indicates that the diode is bad. A low resistance on a DMM and a low voltage on a diode tester are both normal when measuring a tunnel diode. A more thorough test of a tunnel diode is to drive it through a resistor with a ramp voltage source while observing the voltage across the tunnel diode. The resistor should be calculated so that the peak current just exceeds the peak current that the tunnel diode is rated for. Of course if a curve tracer is available, it is great for measuring | ||
the I-V curve of the diode. | the I-V curve of the diode. | ||
[[ | [[File:IV_Ge_TD-10mA.jpg|thumb|Tektronix 571 curve tracer run of 10mA tunnel diode.]] | ||
[[ | [[File:Ge_TD.jpg|thumb|10mA tunnel diode mounted in Tektronix 571 curve tracer fixture.]] | ||
== Modeling == | == Modeling == | ||
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and therefore the incremental resistance of the diode is very high at this point. | and therefore the incremental resistance of the diode is very high at this point. | ||
For simplicity, we can assume that the incremental resistance is infinite at this quiescent point. | For simplicity, we can assume that the incremental resistance is infinite at this quiescent point. | ||
[[ | [[File:Trig1c.png|thumb]] | ||
(The following section will be clarified in the coming edits.) | (The following section will be clarified in the coming edits.) | ||
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== Estimating Switching Speed == | == Estimating Switching Speed == | ||
Now that we have established the initial bias conditions, let's look at the event when the tunnel diode switches state. Assume that the triggering signal is coupled to the tunnel diode through a resistor. The current through the resistor adds to the current from the DC current source. Since we are assuming that the incremental resistance of the diode is infinite at the initial bias point, all of the current due to the trigger signal flows into and out of the diode's capacitance. If enough charge is added, the instantaneous voltage across the diode will be in the second region, where the slope of the VCCS function is negative. | Now that we have established the initial bias conditions, let's look at the event when the tunnel diode switches state. Assume that the triggering signal is coupled to the tunnel diode through a resistor. The current through the resistor adds to the current from the DC current source. Since we are assuming that the incremental resistance of the diode is infinite at the initial bias point, all of the current due to the trigger signal flows into and out of the diode's capacitance. If enough charge is added, the instantaneous voltage across the diode will be in the second region, where the slope of the VCCS function is negative. | ||
[[ | [[File:Trig3c.png|thumb]] | ||
Once the diode enters the second region, increases in diode voltage cause decreases in diode current. Applying Kirchhoff's current law at the node where the diode meets the DC current source, we can see that the current entering the parasitic capacitor at any instant is the difference between the DC current source and the nonlinear VCCS current at the this instantaneous voltage. We can use this fact to estimate the switching time of the tunnel diode. (The shape of the transition can also be estimated.) | Once the diode enters the second region, increases in diode voltage cause decreases in diode current. Applying Kirchhoff's current law at the node where the diode meets the DC current source, we can see that the current entering the parasitic capacitor at any instant is the difference between the DC current source and the nonlinear VCCS current at the this instantaneous voltage. We can use this fact to estimate the switching time of the tunnel diode. (The shape of the transition can also be estimated.) | ||
As an example, let's take the case of a tunnel diode with 10 mA peak current and 5 pF capacitance. | |||
A first-order estimate of the switching time can be made by assuming that to make the transition from | |||
V<sub>1</sub> to V<sub>2</sub>, a certain amount of charge needs to be added to the parasitic capacitance of the diode. | |||
:From Q = C * V, we know that ∆ Q = C * ∆ V, which is ∆ Q = C * (V<sub>2</sub> - V<sub>1</sub>) | |||
:With V<sub>1</sub> = 65 mV and V<sub>2</sub> = 465 mV, ∆ Q = 5 * 10<sup>-12</sup> F * 0.4 V = 2 picocoulombs. | |||
Now we bravely assume that the charging current during the transition is constant, and is half of the peak current. 5 mA is 5 millicoulombs per second. | |||
: t = (2 * 10<sup>-12</sup> C) / (5 * 10<sup>-3</sup> A) = 0.4 ns | |||
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<gallery> | <gallery> | ||
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File:7D20_Tunnel_Diode.jpg|Testing a tunnel diode with an audio oscillator and a 7D20 in X-Y mode | File:7D20_Tunnel_Diode.jpg|Testing a tunnel diode with an audio oscillator and a 7D20 in X-Y mode | ||
</gallery> | </gallery> | ||
[[Category:Tunnel diodes]] | [[Category:Tunnel diodes]] | ||
[[Category:Repair issues]] | [[Category:Repair issues]] | ||