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| How to measure p-n junction diode junction capacitance? |
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| Subhadeep:
Hello, I'm Subhadeep, I'm an engineering student as well as a electronic hobbyist and very new in this forum :). I'm trying to make a p-n junction capacitance measurement device. so far I know about diode junction capacitance is in different bias conditions(forward/reverse) the depletion region width varies and the diode shows transition capacitance during reverse bias and diffusion capacitance during forward bias. when a sine wave is applied to a capacitor there is a phase shift between voltage and current, current always leads the voltage by 90degree. my question is that can i proceed to measure the junction capacitance of a p-n junction diode using sine wave and how? please help me with this.. thank you.. |
| Alex Eisenhut:
It would be a lot easier to measure a frequency. Try the usual circuits for varicaps. http://electronics.stackexchange.com/questions/167051/how-to-properly-connect-and-drive-varicap-diodes |
| uncle_bob:
--- Quote from: Subhadeep on May 21, 2016, 03:41:08 pm ---Hello, I'm Subhadeep, I'm an engineering student as well as a electronic hobbyist and very new in this forum :). I'm trying to make a p-n junction capacitance measurement device. so far I know about diode junction capacitance is in different bias conditions(forward/reverse) the depletion region width varies and the diode shows transition capacitance during reverse bias and diffusion capacitance during forward bias. when a sine wave is applied to a capacitor there is a phase shift between voltage and current, current always leads the voltage by 90degree. my question is that can i proceed to measure the junction capacitance of a p-n junction diode using sine wave and how? please help me with this.. thank you.. --- End quote --- Hi The normal approach is to apply a signal to the junction that is a small fraction of the bias voltage. If you have 2V on the junction, you apply 200 mv p-p. You can vary the test signal to evaluate the sensitivity to the voltage. Bob |
| Kleinstein:
Because of the voltage dependent capacitance the AC amplitude needs to be relatively small. In the forward direction it's more like 10 mV. The forward direction is more difficult anyway, as there is a DC current superimposed. So the phase shift will not be 90 degree. This is why using an oscillator an measuring the frequency does not work well in that range. So it's more like the basic setup of apply an DC+AC voltage and measure the current. To separate the current in the capacitive and conductive (in phase) part one needs to use some kind of pase sensitive analysis. This could be analog (e.g. using synchronous rectification) or digital by analyzing the digitized signal in software. |
| uncle_bob:
--- Quote from: Kleinstein on May 22, 2016, 08:31:09 am ---Because of the voltage dependent capacitance the AC amplitude needs to be relatively small. In the forward direction it's more like 10 mV. The forward direction is more difficult anyway, as there is a DC current superimposed. So the phase shift will not be 90 degree. This is why using an oscillator an measuring the frequency does not work well in that range. So it's more like the basic setup of apply an DC+AC voltage and measure the current. To separate the current in the capacitive and conductive (in phase) part one needs to use some kind of pase sensitive analysis. This could be analog (e.g. using synchronous rectification) or digital by analyzing the digitized signal in software. --- End quote --- Hi .... or for the forward direction, do it the normal way and get the data with a network analyzer. Then fit the S parameters to extract the capacitance. Bob |
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