I worked with an old russian machine in my lab, which contains an input from a photodiode with amplification circuit.

The amplification is composed into 2 stages , the first with a JFET transistor and the other with an amplifier IC (123ун1б) where the internal circuit in the figure below :


In the datasheet the only interesting Information that the gain is between 100 and 350 , and nothing either.

The amplifier ic with external components are showed in this figure :


where the correction capacitor is 120 pF, the resistor between 4 and 5 is 3.9K and the load resistor is 5.1K

My goal is to know how we can choose the gain of this circuit which given approximetly 100?? what are the value that should be changed? What are your opinion and what's the way to know the solution ?

  • 1
    \$\begingroup\$ Ask the old Russian? OR adjust R16. \$\endgroup\$ – Trevor_G Oct 16 '17 at 18:40
  • \$\begingroup\$ Or build it in a simulator and play with it. \$\endgroup\$ – Trevor_G Oct 16 '17 at 18:55
  • \$\begingroup\$ Is there an optical chopper of some kind prior to the detector diode? \$\endgroup\$ – jonk Oct 16 '17 at 19:39
  • \$\begingroup\$ @Trevor :adjusting R16 is a good way to play with gain not understand how it work, and about the simulation , I tried the LTspice but how to understand the current gain in the circuit! \$\endgroup\$ – Eva Ghanoume Oct 17 '17 at 17:29
  • \$\begingroup\$ @jonk yes, it's an encoder I think \$\endgroup\$ – Eva Ghanoume Oct 17 '17 at 17:30

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