Nanoampere sensing circuit

  
The circuit may be used as a sensitive current detector or as a voltage detector having high input impedance. Rl is set so that the voltage at point (A) is V2 to % volts below the level that fires the 2N494C. A small input current (Iin) of only 40 nanoamperes will charge C2 and raise the voltage at the emitter to the firing level
Nanoampere sensing circuit - schematic

When the 2N494C fires, both capacitors, Cl and C2, are discharged through the 27 ohm resistor, which generates a positive pulse with sufficient amplitude to trigger a controlled rectifier (SCR), or other pulse sensitive circuitry.




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