Continue to Site

# BJT: Are PNP transistors really indispensible when we can't do so much with NPNs?

Status
Not open for further replies.

#### matrixofdynamism

BJT: Are PNP transistors really indispensible when we can do so much with NPNs?

I understand that when we use PNP in place of an NPN transistor, we basically make the circuit upside down and reverse the current direction. I do wonder, do we ever come across a situation in which only one of the two can be used? Why?

NPN power are preferred so a PNP Darlington is preferred with PNP driver to NPN.
Only one means square root of linear current gain product and 1/10th the saturated gain at Ic/Ib=10 for each stage.

As some application cases, output of industrial equipments for triggering sensors are in the type NPN, due this way even short-circuiting such pins to the ground will not burn the driver. The PNP output on the other hand, is suitable when you plan to cut supply of an entire system without floating its ground reference.

Using PNPs can significantly simply a circuit for certain applications.
For example, suppose you want to switch a grounded load using a saturated transistor switch, to a positive supply. That's simple to do with a PNP but not with an NPN.

If we use only NPN there will be big surplus of P silicon in the world.

This is a 40Wp Audio Amp with ξ =83% using Complementary Quasi Darlingtons make better use of PNP with 0.6~0.7V less saturation voltage. Linear feedback, low output impedance.

(No EMI filters added for simplicity)

Everything that can be done with a PNP can also be done with an NPN. However in some cases, a PNP configuration may simplify the complexity arising from referencing signals to ground.

matrixofdynamism

### matrixofdynamism

Points: 2
P devices are easy to orient to a negative power supply when you wish to reference the circuitry to 0V ground.

It allows a symmetrical arrangement when you also have circuitry powered from a positive polarity.

Examples: class B amplifier, negative voltage regulation.

matrixofdynamism

### matrixofdynamism

Points: 2
The impression I get is that, mainly it is when we deal with negative supplies that using PNPs becomes very important. Otherwise, it happens only rarely that we need to use PNPs in place of NPNs. I guess this is the reason that when we are introduced to transistors, the NPN is used for this purpose, instead of PNP.

BJT's are often replaced with FETs now which give a much lower ESR or Rce or RdsOn in same size or cost but in really high powers as RdsOn get smaller, Ciss input Capacitance gets bigger since the RC product is almost constant and thus became the FET input BJT output devices called IGBT's which can switch > 1Megawatt at line freq. with liquid cooling.

High or Low side smart Switches in cars depends on the load and kickback EMI polarity and other interface requirements, which determines if P or N type are used, but due to physics N type are always more cost effective slightly.

i.e high side switches 12V P type and low side switches ground N type and high side PWM N type need a higher gate voltage generated by diode charge pump rectifier called the Bootstrap voltage generated from the low side PWM pulses AC coupled and clamped above V+.

The impression I get is that, mainly it is when we deal with negative supplies that using PNPs becomes very important. Otherwise, it happens only rarely that we need to use PNPs in place of NPNs. I guess this is the reason that when we are introduced to transistors, the NPN is used for this purpose, instead of PNP.
Not an accurate impression.
NPNs are certainly more common but PNPs are used extensively for high side switches with a grounded load and a positive supply.
And it would be difficult to build a power audio amp without some PNP (or P-MOSFET) transistors.

Status
Not open for further replies.