Why NPN BJT Pass Element Drops More Than PNP

In summary: NPN drop is more than the PNP drop because the PNP has a bias voltage "above" the base, which adds to the C-E voltage drop. The ce stage in a PNP can have a Vce of just a few tenths of a volt, while a non-PN ce stage needs at least 0.65 volts due to Vbe forward drop.
  • #1
likephysics
636
2
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.
 
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  • #2
likephysics said:
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
Kinda makes sense. But not really. Do you have an alternate explanation?
 
  • #4
likephysics said:
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
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
 

1. Why does an NPN BJT pass element drop more than a PNP BJT?

The main reason for this is due to the difference in the doping levels of the two types of transistors. NPN BJTs have a higher doping concentration in the collector and base regions, which leads to a larger depletion region and a higher voltage drop across the base-collector junction. PNP BJTs, on the other hand, have a lower doping concentration in these regions, resulting in a smaller depletion region and a lower voltage drop.

2. How does the temperature affect the pass element drop in an NPN BJT?

The pass element drop in an NPN BJT increases as the temperature increases. This is because an increase in temperature causes an increase in the minority carrier concentration in the base region, resulting in a larger base-collector current and a higher voltage drop across the base-collector junction.

3. Can the thickness of the base region affect the pass element drop in an NPN BJT?

Yes, the thickness of the base region can affect the pass element drop in an NPN BJT. A thicker base region will result in a larger depletion region and a higher voltage drop across the base-collector junction. This is why NPN BJTs with a larger base width tend to have a higher pass element drop than those with a smaller base width.

4. How does the current gain of an NPN BJT affect the pass element drop?

The current gain of an NPN BJT has an inverse relationship with the pass element drop. This means that as the current gain increases, the pass element drop decreases. This is because a higher current gain allows for a larger base current to flow, which in turn reduces the voltage drop across the base-collector junction.

5. Can the pass element drop in an NPN BJT be controlled?

Yes, the pass element drop in an NPN BJT can be controlled by adjusting the operating conditions such as the base current and collector current. By increasing the base current, the voltage drop across the base-collector junction can be reduced. Similarly, a lower collector current can also lead to a decrease in the pass element drop.

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