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A current-source LED circuit with an op amp.

Hari T O0
Genius 3820 points

Dear Team,

I was going through this document()

Please see figure 7 which shows the circuit of driving LED using an opamp.We are fixing current through LED using opamp.I am clear with how they obtained the current equation.

My question is why they are not considering the forward voltage(Vf) of the LED anywhere. For LED to turn on Certain Vf is required.

If we fix the forward current will it set the Vf automatically?

Or do I need to select a Vs which is always greater than Vf.

My understanding is 

I can write KVL for "V+" =Vf+Vce+Vin.

From this equation, I can determine the minimum voltage required for V+(Voltage at the collector of Transistor/Drain of Mosfet).

Please correct me if I am wrong.

Regards

Hari