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Tunnel diodes: Difference between revisions
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== Testing a Tunnel Diode == | == Testing a Tunnel Diode == | ||
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 (note the negative-resistance may not show). | ||
the I-V curve of the diode. | |||
[[File:IV_Ge_TD-10mA.jpg|thumb|Tektronix 571 curve tracer run of | A short article in [[Media:Service Scope 49 Apr 1968.pdf | Service Scope 49, April 1968]] describes a quick checking setup that approximates a curve tracer using the scope's X sawtooth output (see in photo section below). | ||
[[File:IV_Ge_TD-10mA.jpg|thumb|Tektronix 571 curve tracer run of 10 mA tunnel diode.]] | |||
[[File:Ge_TD.jpg|thumb|10mA tunnel diode mounted in Tektronix 571 curve tracer fixture.]] | [[File:Ge_TD.jpg|thumb|10mA tunnel diode mounted in Tektronix 571 curve tracer fixture.]] | ||
== Modeling == | == Modeling == | ||
The fast switching action of the tunnel diode can be understood by modeling it | The fast switching action of the tunnel diode can be understood by modeling it as a nonlinear voltage controlled current source (VCCS) in parallel with a small parasitic capacitor. | ||
as a nonlinear voltage controlled current source (VCCS) in parallel with a small parasitic capacitor. | The nonlinear VCCS is controlled by the voltage at the terminals of the diode and is responsible for the S-shaped I-V curve. | ||
The nonlinear VCCS is controlled by the voltage at the terminals of the diode | (Alternatively and equivalently, it can be modeled as nonlinear resistance. However, the nonlinear VCCS model might be preferable because it avoids | ||
and is responsible for the S-shaped I-V curve. | |||
(Alternatively and equivalently, it can be modeled as nonlinear resistance. | |||
However, the nonlinear VCCS model might be preferable because it avoids | |||
the confusing notion of negative resistance.) | the confusing notion of negative resistance.) | ||
Consider a tunnel diode biased by a DC current source that is | Consider a tunnel diode biased by a DC current source that is slowly brought up from zero to a current just a few microamperes less than the diode's peak current. | ||
slowly brought up from zero to a current just | |||
a few microamperes less than the diode's peak current. | |||
The quiescent voltage will be just less than the peak voltage. | The quiescent voltage will be just less than the peak voltage. | ||
Note that the I-V curve is nearly horizontal at this point, | Note that the I-V curve is nearly horizontal at this point, 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]] | [[File:Trig1c.png|thumb]] | ||
== Estimating Switching Speed == | == Estimating Switching Speed == | ||
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<gallery> | <gallery> | ||
Rca1963TunnelDiodeManual.p4.png | |||
Rca1963TunnelDiodeManual.p16-17.png | |||
</gallery> | </gallery> | ||