# Unity Gain current buffer Zin

#### duffman268

Joined Feb 7, 2013
1
I have a circuit with a unity gain current buffer in the box. I want to figure out the input impedance and its equivalent circuit, but I'm not sure how I'm supposed to do that. Can anyone help me?

#### The Electrician

Joined Oct 9, 2007
2,751
Add a 1 amp current source injecting the 1 amp into the Zin node. calculate the voltage at that node due to the current source. That voltage divided by the 1 amp will be the impedance there.

#### BillO

Joined Nov 24, 2008
990
An amp! If it is a high Z buffer, the voltage might need to be in the mega volts!

No, don't try to pump an amp into it.

All you need is a known voltage (V) and a known resistance (R). Connect V to the input of the buffer through R and measure the current (I). Use Ohm's law to get the Z of the input stage of the buffer.

Z = (V/I) - R

Of course, that is just the DC part of it. Now you will need to calculate the Z of the LC circuit.

Edit: BTW, that looks like a pretty good equivalent circuit to me.

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#### Ron H

Joined Apr 14, 2005
7,014
An amp! If it is a high Z buffer, the voltage might need to be in the mega volts!

No, don't try to pump an amp into it.
Do you suppose that would cause sparks and smoke to erupt from the paper and pencil he's working with? #### The Electrician

Joined Oct 9, 2007
2,751
It would undoubtedly blow out the LCD display of his calculator!

• Ron H

#### BillO

Joined Nov 24, 2008
990
LOL, yes, literary sparks might fly!

• Ron H