Trying to understand how this pnp works
My answer to SE EE question Trying to understand how this pnp works I have been familiar with this BJT driving technique for a long time... but now I came up with a new original explanation through a "dynamic resistor" concept... Answer OP: How can this pnp be turned off? A more in-depth conceptual answer to this question can be given if the main BJT input property is taken into account: BJT can be more easily controlled by voltage than current since its input base-emitter junction behaves as a voltage-stabilizing element . This property can be seen in its input IV curve having an almost vertical part. This means we can control it more easily by small voltage changes instead of large current changes. So, the problem of turning off the transistor only by an "opto" is that we try to do it by decreasing the current instead of the voltage… and it opposes us. Let's see how... The output collector-emitter part of the photo-transistor can be thought of as a varying...