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single-, two-, and four-quadrant switches


Active Member

I was reading up on quadrant switches and need your with some points.

I was using this text: http://ecee.colorado.edu/~ecen5797/course_material/Ch4slides.pdf
(or, use Google drive PDF: https://drive.google.com/file/d/168CHus1Rzm4fCViEJdFnOA845f_R3PsN/view?usp=sharing)

Question 1:
In Slide #8 it says that a BJT is single quadrant switch which can block positive off-state voltage. Does it mean a BJT cannot block negative voltage? The shown BJT is an NPN. In Slide #19 it shows how an NPN could function as a two quadrant switch. Doesn't it suggest that a NPN cannot block negative voltage on its own? Where am I going wrong?

Slide #8:

Slide #19:
(edited after rjenkinsgb post to substitute the correct slide; previously this slide was mistakenly used)

Question 2:
Yes, the body diode won't conduct but the diode in yellow would definitely conduct reverse current. So, what's the purpose of bypassing body diode?

Slide #14:

Note to self:
Slides related to quadrant switched end around slide #23.
Related video, possibly by the author of slides: watch?)v=GLdMUd27LtM (insert www.youtube.com/ at the front)


Last edited:


Well-Known Member
Most Helpful Member
Doesn't it suggest that a NPN cannot block negative voltage on its own? Where am I going wrong?
Back to the original question:

If you reverse the voltage across a bipolar transistor, current would flow through the collector-base junction as if it's just a simple diode. An external series diode prevents that.

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