Diode Transistor Coupling

[khanrising]

I have a ckt I want to design. I want to maximize the coupling from
photodiode to transistor. How would I figure out the best transistor to give me
best response to 10 mA pulse through photodiode?

Thanks

 

[A.Anand Srinivasan]

select a transistor with very good β.... transistor with β as high as 400 are available....

 

[khanrising]

Thanks for the tip. To maximize the coupling between the diode and transistor, don't you have to look at base resistances and diode impedance and things like this? Is gain the only factor? And what about speed?

 

[Eugen_E]

You may add a resistor between the base and emitter of the transistor so that the trasistor wont saturate deeply at the current peak. "Recuperating" from hard saturation takes time, and may delay the response. The usual β value of 100-400 is enough, since you drive it with 10mA.

Also, a transimpedance amplifier based on a high speed opamp can be better.

 

[A.Anand Srinivasan]

choose a low noise transistor.... because the input from photodiode would already be noisy... you just gotto take care that there is enough current to drive the transistor.... adding a resistor between photodiode and base is a good idea....