Negative voltage to Vbc of NPN? - Page 1
Forum section: electronics / beginners

author: Faringdon
author_rank: Super Contributor
timestamp: September 05, 2023, 05:54:09 am
content: 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

author: magic
author_rank: Super Contributor
timestamp: September 05, 2023, 07:03:10 am
content: 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?editBut, wait, what the hell is this circuit doing?Which V is the PWM and how is it supposed to work?

author: wasedadoc
author_rank: Super Contributor
timestamp: September 05, 2023, 08:42:19 am
content: The transistor in the OP's circuit cannot function properly. There is no path for the dc component of collector current.

author: wasedadoc
author_rank: Super Contributor
timestamp: September 05, 2023, 08:46:39 am
content: Quote from: magic on September 05, 2023, 07:03:10 am"cut off" means that no collector current is flowing. Opposite of "saturated". If cut off, linear operation is not possible.

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 05, 2023, 09:10:30 am
content: 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?

author: Gyro
author_rank: Super Contributor
timestamp: September 05, 2023, 11:19:17 am
content: As it's Faringdon, I suspect it's a troll question. He's no beginner.

author: xavier60
author_rank: Super Contributor
timestamp: September 05, 2023, 12:33:06 pm
content: 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.

author: Faringdon
author_rank: Super Contributor
timestamp: September 05, 2023, 06:37:40 pm
content: QuoteThanks, yes.

author: floobydust
author_rank: Super Contributor
timestamp: September 05, 2023, 07:32:52 pm
content: Quote from: Faringdon on September 05, 2023, 06:37:40 pmIf you could mention that in the OP instead of generating noise pls.Quote from: xavier60 on September 05, 2023, 12:33:06 pmThere's no reverse hFEspec for the transistor, so current flow in both directions, in and out of the collector... I use a CMOS switch for controlling PWM filters.

author: xavier60
author_rank: Super Contributor
timestamp: September 05, 2023, 07:43:09 pm
content: The transistor would act as a diode clamp anyway.And why not a small MOSFET,

author: magic
author_rank: Super Contributor
timestamp: September 05, 2023, 07:56:44 pm
content: Quote from: DavidAlfa on September 05, 2023, 09:10:30 amQuote from: xavier60 on September 05, 2023, 12:33:06 pmI 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.

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 05, 2023, 08:07:48 pm
content: Quote from: 5link=topic=391546.msg5045833#msg5045833 date=1693942372It's the standard procedure in this forum, it's like approaching women, takes at least 50 messagesThough 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.

author: tggzzz
author_rank: Super Contributor
timestamp: September 05, 2023, 08:22:59 pm
content: Quote from: floobydust on September 05, 2023, 07:32:52 pmNo, 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.

author: David Hess
author_rank: Super Contributor
timestamp: September 06, 2023, 02:38:05 am
content: Quote from: Faringdon on September 05, 2023, 05:54:09 amNo, 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.