Negative voltage to Vbc of NPN?

[Faringdon]

Hi,
We have a typical RC filter to filter PWM to an analog voltage...sometimes, there is no PWM and its just 3v3 all the time....so then when the pwm cap is charged up, and then the PWM'ing starts, the npn Vbc can go negative........can this damage it?....its MUN2211.
As in attached LTspice and jpeg.

https://www.onsemi.com/pdf/datasheet/dtc114e-d.pdf

[magic]

Forward biasing the BC junction is called "saturation" and it happens all the time BJTs are used to switch DC loads...

Actually reverse biasing the BC junction is called, well, "cutoff" or "linear operation", I guess? Sounds normal?


edit
But, wait, what the hell is this circuit doing?
Which V is the PWM and how is it supposed to work?

[wasedadoc]

The transistor in the OP's circuit cannot function properly. There is no path for the dc component of collector current.

[wasedadoc]

Actually reverse biasing the BC junction is called, well, "cutoff" or "linear operation", I guess? Sounds normal?

"cut off" means that no collector current is flowing. Opposite of "saturated". If cut off, linear operation is not possible.

[DavidAlfa]

It's a simple RC filter... but what's the transistor for?
Maybe to discharge the cap, but you wired it wrong?
Or maybe your intention was to enable/disable the filtering on demand?

[Gyro]

As it's Faringdon, I suspect it's a troll question. He's no beginner.

[xavier60]

While off, there will be some reverse bias B-E voltage sometimes but not enough to damage the transistor.
With the transistor on, C-E will be low resistance in both directions like an analog switch.

[Faringdon]

Or maybe your intention was to enable/disable the filtering on demand?

Thanks, yes.

[floobydust]

Or maybe your intention was to enable/disable the filtering on demand?

Thanks, yes.

If you could mention that in the OP instead of generating noise pls.

While off, there will be some reverse bias B-E voltage sometimes but not enough to damage the transistor.
With the transistor on, C-E will be low resistance in both directions like an analog switch.

There's no reverse h_FE spec for the transistor, so current flow in both directions, in and out of the collector... I use a CMOS switch for controlling PWM filters.

[xavier60]

The transistor would act as a diode clamp anyway.
And  why not a small MOSFET,

[magic]

Or maybe your intention was to enable/disable the filtering on demand?

While off, there will be some reverse bias B-E voltage sometimes but not enough to damage the transistor.
With the transistor on, C-E will be low resistance in both directions like an analog switch.

I suppose it may work.

But there is a problem with off: while the collector may go positive with no issues, if it tries to go negative it will forward bias the BC junction and pull the base below the emitter.
As said above, the BE junction won't break down with merely -3V, but the base drive circuit will source current into the base and then the collector.
To make it really off, the base drive circuit needs to have high impedanece even when taken below ground. For example, a common emitter PNP.

An N-ch MOSFET would clamp drain voltage to -0.7V due to its body diode.
CMOS switches probably have some diodes too and likely wouldn't stay open circuit below ground, unless supplied by a -3.3V rail.

[DavidAlfa]

If you could mention that in the OP instead of generating noise pls.

It's the standard procedure in this forum, it's like approaching women, takes at least 50 messages

Though personally, the worst are the posts like:
 - Please help to identify a component with "D1C3" marking. The pcb is labeled "AA-1933-4CH1". Thanks
  (No pictures)

Much, much later, finally posts some pictures. After wasting everyone's time:
 - Ah, it was "OLCE", my bad.
 

[tggzzz]

Or maybe your intention was to enable/disable the filtering on demand?

Thanks, yes.

If you could mention that in the OP instead of generating noise pls.

No, Faringdon/Treez can't, since he likes XY Problems and this is his modus operandi.

He has decades of experience, and isn't a beginner.

[David Hess]

so then when the pwm cap is charged up, and then the PWM'ing starts, the npn Vbc can go negative........can this damage it?....its MUN2211.

No, if the transistor is off, then this will forward bias the base-collector junction which is fine.  If the transistor is on, then the transistor will conduct either way and the collector will be clamped.  Audio mute circuits and switched RF filters used this mode of operation.