Measuring diode capacitance

[sharas]

measure diode capacitance

Hi,

Does anybody know how to measure the capacitance of the diode?

I mean - both the depletion capacitance and the diffusion capacitance (in forward bias).

Thanks in advance,


Sharas

 

[IanP]

measuring diode capacitance

You can measure only an equivalent capacitanceof a component without knowing what exactly contributes to this capacitance.
I would build a LC generator based on a couple of pnp transistors, so both the inductance and capacitance are related to GND. Now, this oscillator will generate a sine wave of certain frequency. As you add a diode (through a capacitor and with the bias resisitor connecte to a diode) to this parallel LC tank, the frequency will decrease. By measuring the frequency you will be able to calculate the value of the equivalent capacitance of this diode..
Regards,
IanP

 

[neazoi]

Re: measuring diode capacitance

You can measure only an equivalent capacitanceof a component without knowing what exactly contributes to this capacitance.
I would build a LC generator based on a couple of pnp transistors, so both the inductance and capacitance are related to GND. Now, this oscillator will generate a sine wave of certain frequency. As you add a diode (through a capacitor and with the bias resisitor connecte to a diode) to this parallel LC tank, the frequency will decrease. By measuring the frequency you will be able to calculate the value of the equivalent capacitance of this diode..
Regards,
IanP

Hello, I am sorry to bring this topic back again.
What if one builds an LCR dridge? could one be able to measure diode capacitance then?

 

[jiripolivka]

You did not mention at what frequency you want to operate. To measure varicap diode capacitance vers. voltage,I would build an oscillator running at the frequency of interest, and use a variable tuning capacitor to tune it. This capacitor can be measured on a LCR meter, or, estimated from the visible mechanical parameters. The I will replace this capacitor with the varicap diode, and record the frequency as a function of voltage.

Some LCR bridges apply unknown DC or AC voltages across capacitors under test, and can damage the diode. As you plan to use the diode in an oscillator, the described procedure is the best way to measure the diode.

 

[neazoi]

You did not mention at what frequency you want to operate. To measure varicap diode capacitance vers. voltage,I would build an oscillator running at the frequency of interest, and use a variable tuning capacitor to tune it. This capacitor can be measured on a LCR meter, or, estimated from the visible mechanical parameters. The I will replace this capacitor with the varicap diode, and record the frequency as a function of voltage.

Some LCR bridges apply unknown DC or AC voltages across capacitors under test, and can damage the diode. As you plan to use the diode in an oscillator, the described procedure is the best way to measure the diode.

I am talking about ordinary diodes like schottky, not varicaps.
I am thinking of building this simple LCR https://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/L-C-R_Bridge_G3BIK.gif
Do you think I would be able to measure a diode capacitance with this? It can measure up to 1pf of ordinary capacitors.

 

[jiripolivka]

I am talking about ordinary diodes like schottky, not varicaps.
I am thinking of building this simple LCR https://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/L-C-R_Bridge_G3BIK.gif
Do you think I would be able to measure a diode capacitance with this? It can measure up to 1pf of ordinary capacitors.

I do not think the LCR meter like shown in your reference can work. A Schottky P/N junction is usually a good detector for AC and RF signals with low-level like -20 dBm. Such diode typically has the I/V curve bending at 0.3...0.7 V, so it will rectify a test signal with a comparable amplitude.

I would again recommend the substitution method, with a RF oscillator at a desired frequency. Use a power divider to get ~ -10 dBm with a 50-Ohm load. Measure the power on this load and connect the diode under test across the load. The power will drop a bit. Then connect a SMD capacitor of 1 or 3 pF across the load and measure the power again. You can obtain a line of power vers.capacitance on which you can locate the point for the diode.
DC-biasing the diode (watch the current!) will vary its capacitance. All connections must be as short as possible, mainly at RF > 1MHz. A typical small-size Schottky diode can have 1...3 pF, its junction alone , several femto-farads ( thousandth of pF).