# Lock In Amplifiers

Discussion in 'General Electronics Chat' started by shahdeep, Aug 13, 2015.

1. ### shahdeep Thread Starter New Member

Aug 13, 2015
3
0
Dear Users,
I am working on EQE measurements and we have a lock in amplifier which uses voltages as input source. But the EQE setup that we have has DC modulated current. So I need to know what kind of circuit will be useful for my setup to have a linear voltage response to the change in DC current.
K.R,
DS

2. ### Brainbox Member

Nov 15, 2010
25
2
For linear conversion from current to voltage You wil need a linear resistor to do the conversion and a capacitor to shift out the DC component.
Most common resistors are linear.
Btw; what is ment by EQE measurements ?

3. ### DickCappels Moderator

Aug 21, 2008
2,647
631
What is the impedance of your current source?

4. ### shahdeep Thread Starter New Member

Aug 13, 2015
3
0
Hello,
EQE: External Quantum Efficiency, used for solar cell measurements.
I have not measured the impedance yet but my working range is 100uA to 50mA.
Thank you once again.

5. ### DickCappels Moderator

Aug 21, 2008
2,647
631
If the output impedance is very high you can simply measure the voltage across a resistor as Brainbox suggested.

If you run the solar cell into a dead short instead of a resistor you will be able to obtain consistent results regardless of the output impedance of the cell. A transimpedence is the classic way of accomplishing this. The voltage at the output is equal to I x R while the cell looks into a virtual short circuit.

An op amp such as the SGS-Thompson Semiconductors L272M can supply more than the needed 50 ma.

Bob Pease's take on it:
http://electronicdesign.com/analog/whats-all-transimpedance-amplifier-stuff-anyhow-part-1

6. ### shahdeep Thread Starter New Member

Aug 13, 2015
3
0
Thank you so much. I will read about it and let you know if anymore doubts.
Thank you once again.

Last edited: Aug 13, 2015