# External Current Sourcing Sensor parallel to PNP BJT

I have Biased the transistor Q1 in active region as shown in the following diagram. I have Sensor which outputs 1-10ma and needs to be connected across parallel as per the following diagram. the transistor is biased in such way that for 1-3mA of sensor output it remains in active region and for 3-10 mA it switches to Saturation region. Now how do i calculate the current I2, I3 and I4 theoretically ?

Current without the Connection of external sensor I4= 864uA , I2= 72.208uA
I4/I2(Ratio)= 11.96

Current With external Current sensor for 10 mA source : 10/11.96= 0.8361uA
So I2'= (72.208 + 0.863 ) uA and I4'= [864 + (10-08361) ] uA

This calculation is not precise and slightly vary from the simulated results . Kindly Suggest me theoretical Approach to solve this for mentioned Currents. will there by any current flowing inside 15 V source from the external sensor ?

• if you want a sharp transition in the operating regions, then use diffpairs. – analogsystemsrf Sep 21 '19 at 1:54
• @analogsystemsrf what should be the method to calculate the current I3 and I4 ? will there be any current going inside 15 V source ? please provide your suggestion . – Rohan Sep 22 '19 at 5:37
• @Rohan As this appears NOT to be a practical application, but rather a homework or problem question, why not just use nodal analysis? It will work fine. If you want a symbolic solution then use 'sympy' (freely available) to achieve that once you've prepared your set of equations from the nodal analysis. You can use the results to determine for yourself where active and saturated modes occur, too. (I assume your external current source sensor is also 'ideal' in the sense that it's compliance voltage is infinite.) – jonk Oct 8 '19 at 21:06