BJT as pass element

  • #1
634
1

Main Question or Discussion Point

When using BJT as a pass element in series pass regulation, why is the NPN drop more than the PNP.
Both have BE diodes. Why is the drop (VCEsat+VBE) in case of NPN and drop is equal to VCE sat in case of PNP.
Can someone also explain VCE sat using diode equivalent of BJTs.
 

Answers and Replies

  • #2
berkeman
Mentor
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When using BJT as a pass element in series pass regulation, why is the NPN drop more than the PNP.
Both have BE diodes. Why is the drop (VCEsat+VBE) in case of NPN and drop is equal to VCE sat in case of PNP.
Can someone also explain VCE sat using diode equivalent of BJTs.
It's not so much the PN drop in the BJT, it's where you have to bias the transistor in order to be able to control its current. For the PNP highside regulator topology (used in low-dropout applications), you can put the control circuitry "below" the base voltage. But for the traditional NPN highside regulator, you need your control and bias circuitry "above" the base, which adds to the C-E voltage drop.

Make sense?
 
  • #3
634
1
Kinda makes sense. But not really. Do you have an alternate explanation?
 
  • #4
berkeman
Mentor
57,275
7,265
Kinda makes sense. But not really. Do you have an alternate explanation?
:rofl:

Nope, that's the one and only. Have you looked at the schematic for typical NPN and low-dropout PNP regulator ICs? You should be able to see how the control circuitry dictates the minimum C-E bias voltage required. Maxim makes low-dropout regulators -- you could check their website for some datasheets....
 
  • #5
1,161
82
With a positive input & output voltage, an npn pass element is an emitter follower. A pnp is a common emitter. In the ef, the base drive must be one b-e forward voltage drop above the output. Not the case w/ the ce stage. A ce stage, built w/ a pnp, can have a Vce of just a few tenths, or even a few hundredths of a volt. With non, at least 0.65 volts is needed due to Vbe forward drop.

Did I help?

Claude
 

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