Measuring input impedance of an amplifier in lab

[bababui]

Hello,

I need to measure the input impedance of an amplifier in lab. This amplifier is basicaly a black box with a known gain.
I was thinking to use a function generator with a variable resistor in series between the function generator and the input of the amplifier.
Then measure the output of the amplifier while tweaking the series resistor, and when the output/gain=input then my amplifier resistance=resistance of the adjustablel resistor.
Please let me know if I'm missing anything, or if there is another/proper way to do this.

Thank you.

 

[alexan_e]

You can try this
**broken link removed**

Alex

 

[bababui]

You can try this
**broken link removed**

Alex

Thanks for your response. Is there anything wrong with the method that I mentioned? Can I use regular ohmmeters to measure V1 and V2 in the mothod that you proposed?
Thanks you.

 

[alexan_e]

There is no reason to measure the output of the amplifier, a measurement in the input of the amplifier will give you an accurate result without having to calculate any gain.
You need a voltmeter to measure the voltages

Alex

 

[LvW]

Thanks for your response. Is there anything wrong with the method that I mentioned? Can I use regular ohmmeters to measure V1 and V2 in the mothod that you proposed?
Thanks you.

I am not sure about your method: (Quote): output/gain=input then my amplifier resistance=resistance

I think, the ratio output/gain=input always applies.
You should try to be a liitle more exact. What input? At the black box or at the additional resistor Rs ?
In any case, I think the approach could be as follows:

If you know the gain A, you can measure Vout and calculate

Rs=rin in case Vout=A*Vin/2 (with Vin=input to Rs)