How to improve the Emitter−Base Voltage for NPN BJT

Thread Starter

Nick Long

Joined May 12, 2020
21
Below is the maximum ratings for 2N3904 NPN BJT. As you can see the maximal Emitter-Base voltage is 6 V. I have checked many other BJT datasheets which all show that this voltage is around this value. But in my circuit the Emitter-Base voltage of NPN BJT will be over 6 V which causes the operation is not normal. I konw by connecting a diode to the emitter of the BJT in series can improve the problem. Is there another better method? Thanks in advance!
1601989879569.png
 

MrChips

Joined Oct 2, 2009
22,886
This is the maximum reverse bias voltage allowed.

The forward voltage will be around 0.7V. You need to limit the current, not the voltage.
 

KeithWalker

Joined Jul 10, 2017
1,520
In you circuit, the emitter/base voltage should be measured from the base to the emitter, not from the base to circuit common, and will be around 0.7V,
 

Alec_t

Joined Sep 17, 2013
11,886
You can connect a diode in series with the base (or the emitter, as you'r aware) to withstand a high reverse voltage.
 

Thread Starter

Nick Long

Joined May 12, 2020
21
1601994024498.png
The reverse Emitter-Base Voltage of T3 or the Base-Emitter Voltage of T4 will be over 6 V if the input voltage is large.
 

ronsimpson

Joined Oct 7, 2019
1,131
After reading the paper on this circuit, I think it does not help much.
I think adding a "power factor correction" circuit will do a better job.
We can see that with out extra parts the power coming from the Piezo only happens at the peaks of the cycle. Pulling power 90% of the time (by theory) should allow for more power. I think, but do not know, that in the case of Piezo the extra power will be small.
Because my background is in power supplies, I would have put the parts after the full wave bridge and before Crect. (like in PFC).
Sorry I can not help. I need to get back to a paying circuit. (real job)
 
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