The 5 V supply provides a dc bias current to D1 via R2, and the ac signal coupled via C1 and R1 increases and decreases the diode current. Transistor Q1 is biased into an on state by the direct current through D1, and its emitter current is increased and decreased by the variation in light level produced by the alternating current in D1. An output voltage is developed across R3.

 

View attachment 213966
The 5 V supply provides a dc bias current to D1 via R2, and the ac signal coupled via C1 and R1 increases and decreases the diode current. Transistor Q1 is biased into an on state by the direct current through D1, and its emitter current is increased and decreased by the variation in light level produced by the alternating current in D1. An output voltage is developed across R3.

View attachment 213966
The 5 V supply provides a dc bias current to D1 via R2, and the ac signal coupled via C1 and R1 increases and decreases the diode current. Transistor Q1 is biased into an on state by the direct current through D1, and its emitter current is increased and decreased by the variation in light level produced by the alternating current in D1. An output voltage is developed across R3.

thank you very much i understand now that circuit , but why do we need to have a variating current at the emitor

 

thank you very much i understand now that circuit , but why do we need to have a variating current at the emitor

@Delta prime
and now that you put a capacitor

if it was like it would be the reverse ? blocking the AC and allowing the DC .

 

Hello I'm a newbie here.
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This will answer your question I hope good luck to you.
https://forum.allaboutcircuits.com/threads/capacitor-on-gate-of-mosfet.126699/post-1031362