This circuit is part of the pulse train FM Detector of a project of mine. It is (part of) an existing circuit which works fine. I modeled the circuit in LTSpice and noticed that the base current is reversed during a large part of the time the transistor is in saturation. It is during the time from 700ns to 1000ns in the model.

I have not observed the reversed base current in the physical circuit yet, but that's mainly because this part of the circuit is a bit hard to desolder and adapt for the current measurement. For the rest the model seems to match the physical circuit very good.

Can anyone explain this reversed base current? I cannot find any explanation of saturated common base BJT's with reversed base current.


Cheers, Koen

 

The base current is not reverse current. It is normal positive base-emitter current.
It is an NPN transistor with its base at ground and its emitter is driven negative so the base-emitter is forward biased and the collector to emitter is normally turned on,.

 

I believe you are seeing the effects of storage-time stored charge.
If you use a switching transistor with a low storage-time (e.g. 2N2369 or 2N5769) that reverse current is much smaller.

 

I believe you are seeing the effects of storage-time stored charge.
If you use a switching transistor with a low storage-time (e.g. 2N2369 or 2N5769) that reverse current is much smaller.

Hi Crutschow,

Thanks for your answer.

This is also what I have been considering. What made me doubt about this option is that the 'reverse base current', Ib(Q1) from 0.7s to 1.0s seems to be fed from the collector current. Collector current from 0.6s to 1.0s is more or less constant at 3mA, the saturation current. The 14mA discharge current for C1, Ie(Q1) from 0.6s to 0.7s comes from 11 mA extra base current, Ib(Q1), added to 3mA collector current. This is the positive current spike Ib(Q1). Ie(Q1) from 0.7s to 1.0s is zero. So where does to 3mA collector current Ic(Q1) from 0.7s to 1.0s go to if the reverse current is from stored charge? Is this also a discharge current for stored charge, in the same direction as usual collector current?

I tried the two transistors you mention (2N2369 and 2N5769), and show the same effect as 2N3904, only with a much shorter time for the 'reverse base current'. These transistors have lower internal capacitances than the 2N3904.

 

From: https://ecee.colorado.edu/~bart/book/book/chapter5/ch5_6.htm

“This reverses the base current and the base-emitter junction capacitance is discharged. After this transient, the transistor is eventually turned off and the collector current reduces back to zero.”

 

The emitter voltage is driven down to -0.7V to turn on the transistor then the emitter voltage does not return to 0V or +0.4V, instead the emitter voltage stays at -0.7V. There is no base-emitter resistor to turn off the transistor when the emitter is floating.

 

So where does to 3mA collector current Ic(Q1) from 0.7s to 1.0s go to if the reverse current is from stored charge? Is this also a discharge current for stored charge

The stored charge is discharged by reverse current from the base.
Depending upon the relative bias conditions this current path can also be also through the collector or emitter.

Incidentally, this stored charge is independent of, and unrelated to, the value of the junction capacitances.