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| How to measure p-n junction diode junction capacitance? |
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| Marco:
--- Quote from: Subhadeep on May 21, 2016, 03:41:08 pm ---so far I know about diode junction capacitance is in different bias conditions(forward --- End quote --- This doesn't make sense, sure in theory there's a forward capacitive component to the impedance but unless you want to measure it at the double digit GHz range it's overwhelmed by resistive and inductive components. |
| Conrad Hoffman:
I suspect this can be done with many traditional bridges. Usually you apply a bias through a resistor using a T in the detector circuit. Easy to build a bridge circuit. |
| T3sl4co1l:
The most general method: use a VNA with a bias tee. The VNA (Vector Network Analyzer) measures the phase and magnitude of transmitted and reflected AC waves from a device under test (DUT), through as many ports as it is equipped with. In this case, we expect a capacitance at the end of a transmission line, so we merely need to use the equation for a transmission line with a capacitor load, at each frequency tested, and can solve for capacitance in that way. We can also solve for resistance, and by varying frequency we can also synthesize an equivalent circuit for the DUT. Now, a VNA is rather expensive, if you don't have access to one. But this highest-level perspective gives us insight into what methods might be applied. A VNA is fundamentally a very precise reflectance bridge. A bridge has already been mentioned above, and that is also a very nice, general method for lower frequency applications. (At high frequencies, it's desirable to do things in an RF-and-transmission-lines method, which means, using a reflectance bridge as such. But there isn't much operational difference between that, and the old fashioned e.g. Wheatstone bridge, as long as one understands the compromises of each). We can also look at the fundamentals of what we're measuring. What is a capacitance? In AC steady-state, it is a reactance inversely proportional to frequency. This is actually a consequence of a more general statement, the fundamental capacitor equation: I = C * dV/dt. (If we have I and V as sinusoids at some frequency, then the AC steady-state condition follows instantly from this. :) ) We don't need to use sinusoids here; we can just as well use sharp, piecewise waveforms, that may be easier to measure with digital circuitry, for example. Namely, if we apply a square-wave current, the time integral of that waveform is the voltage, a triangle wave. This is what most multimeters do: apply an alternating current (square wave), and measure the voltage. Typically the current will be switched when the voltage reaches a threshold, so the impedance (V / I) is held constant, and the frequency of oscillation is then inversely proportional to capacitance. Or the current is varied, in steps or continuously, to hold voltage and/or frequency in a typical range. Note that such a method loses something: we expect to measure capacitance, but if we measure resistance instead, we get nonsense results. We're also testing with a wideband signal (assuming linear components (constant C), the usual analysis methods apply (superposition, Fourier transform, Parseval's theorem, etc.), and we note the transform of a square wave has many harmonics), which means we can get very weird results for complex (RLC) networks. Some of this can be recovered (e.g., measuring the voltage waveform in quadrature with the current waveform, so we measure in-phase and out-of-phase components -- ESR and C), and some of it can be managed (modest size ESL can be avoided with a bandwidth-limited square wave test signal), but if you are expecting a network any more complex than that, a bridge or VNA method is probably best. :) Tim |
| rodpp:
--- Quote from: Conrad Hoffman on November 19, 2018, 03:56:26 pm ---I suspect this can be done with many traditional bridges. Usually you apply a bias through a resistor using a T in the detector circuit. Easy to build a bridge circuit. --- End quote --- Yes, this is the suggested method to measure diode capacitance in the Tektronix book Semiconductor Device Measurements: https://archive.org/details/tektronix_Semiconductor_Device_Measurements |
| cdev:
Many diodes act as varactors, some to a remarkable extent. This effect can be leveraged to make some useful devices. For example, its quite easy to turn some LEDs into varactors and they perform as well or almost as well as real varactors. This can be demonstrated very easily by back biasing all the segments in an old seven segment LED display and applying a variable positive voltage by means of a voltage divider, by way of an RF choke to decouple the dc supply lead from the magnetic loop part. You may then find that with some parts - using this arrangement, you can tune a good chunk of the HF shortwave bands. Under the best conditions, some diodes behave just like a decent sized variable cap, plus this loop is remotely tunable. Use a big multi turn pot or a reduction gear on the shaft if you have one around because the tuning can be very sharp, with the peak easy to miss. This uses very little current so a back biased battery will last a long time used like this if you choose a very high value for the pot. (1 meg or more, even 10 megs will work) Turn it off when its not in use. |
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