Reducing Gain

Thread Starter


Joined Jan 10, 2007
I got 2 types of infrared Phototransistors, the QSE133, and the QSE113.
The QSE133 actually is a photodarlington and the QSE113 isn't.
When its dark the QSE133 marks a gain of about 70 and the QSE113 with a candle in front marks a gain of about 70..
I want to have a gain of about 200, but neither of the phototransistor can produce that, the QSE113 produce too little and the QSE133 produce too much.
My point is if there's any way to reduce the QSE133 gain.

Any recommendations are accepted:)
here is the circuit im using, that transistor thats there is a MJ11032 instead of the 2N2222.



Joined Apr 27, 2007
You cannot reduce the gain. However, there is a number of ways of reducing the sensitivity depending on the circuit you want to use. In your case, you can increase the value of the base resistor.

A better alternative, in my opinion, would be using a resistor from the emitter of the photodarlington to the ground. The idea here is to make a voltage divider so the transistor will have a base voltage lower than 0.7V (therefore not conducting) when the light hitting the photodarlington is less intense than the value you want. You can use a darlington in the place of T1, so you won't increase the sensitivity, but you will increase the current difference between the on state and the off state.

But that depends of the behaviour you wish your device to have.