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Old 22nd October 2004, 06:58 AM   #1
Kevinbd is offline Kevinbd  United Kingdom
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Default Measuring CCS impedance and at different frequencys

Hello,

1,

I am trying to measuring the impedance of some current sources at various frequency's.

Can someone tell me how to do this ?

2,

PS. Does anyone know a good way to cascode mosfets for a current source?

Have seen this done with jfets but have to use mosfets for higher voltage.

Thanks
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Old 22nd October 2004, 08:00 AM   #2
hjelm is offline hjelm  Sweden
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As for 1 for small currents you could usa a OPAMP to drive a voltage at the out put of the current source.

Connect currentsource resistor and opamp.

Vary the voltage on the opamp output with the desired frequency and measure the voltage swing on the output of the currentsource and the varying current you measure over the resistor. The output resistance is the Voltage swing on the current source/current through resistor.

2:
You need a second voltage reference for the cascoded mosfet. Place it so that the Vref-Vgs is higher than the output of your current generator.
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