9.4.1. Review of NPN and PNP Transistors

Now, so far we are considering about the n-p-n transistor if you look into the p-n-p transistor on the other hand it is very similar, but of course, it is the 3 islands or 3 regions are different. Namely, we do have p-region and then n-region and then p-region, so we do have p-n-p.

NPN and PNP transistors are both types of bipolar junction transistors (BJTs). The main difference lies in the arrangement of their semiconductor materials. An NPN transistor has a layer of p-type material sandwiched between two layers of n-type material. Conversely, a PNP transistor has a layer of n-type material between two layers of p-type material. This arrangement affects the operation of the transistors as well as their biasing and current flow.

Think of a transistor as a water valve. An NPN transistor is like a valve that allows water to flow from a higher pressure (n-type) to a lower pressure (p-type) when triggered by a smaller amount of flow (base current). In contrast, a PNP transistor works in the opposite way, where it allows flow from p-type to n-type when the base is activated.

And here also to keep the device in an active region of operation base and emitter junction need to be a forward bias which means that at the emitter now we are looking for higher voltage with respect to the base. On the other hand, the other junction the base to collector junction we like to keep it is in reverse bias.

For a PNP transistor to operate in its active region, the base-emitter junction must be forward-biased, meaning the emitter must hold a higher voltage compared to the base. In contrast, the base-collector junction should be reverse-biased, where the base has a higher potential than the collector. This ensures that current can flow correctly through the transistor, enabling it to amplify signals.

Imagine a PNP transistor as a one-way street in a neighborhood. For traffic (current) to enter into the street from the freeway (emitter to base), it needs an open entrance (forward bias). However, the exit to the city (base to collector) must be blocked (reverse bias) to ensure vehicles flow smoothly onto the neighborhood street.

So, you may say that this is the actual polarity a positive direction of the current and. So, we do have I , we do have I and then we do have I like this.

In a PNP transistor, the convention for current flow is opposite to that of an NPN transistor. In the PNP configuration, the emitter current (Ie) flows into the transistor, while the base current (Ib) emerges out, and the collector current (Ic) also emerges from the collector. This flow is largely dictated by the biasing conditions set up in the device.

Imagine water flowing through a fountain. The water is supplied from a reservoir (emitter) into the fountain, misting out through a base and finally pouring into different collection pools (collector). The directions in which the water flows can illustrate how current behaves in a PNP transistor.

If you compare the notation or seem the equation we have used for BJT this n-p-n BJT with p-n-p what you can see here it is. So, these are the equations it was used for n-p-n. So, with respect to that we simply have to modify this part namely we can make it V . So, likewise here we can replace this is V and this is into V .

The mathematical representation for both NPN and PNP transistors is similar, with adjustments made for the current and voltage polarities. In a PNP transistor, for instance, the voltage and current orientations need to be adjusted so that when analyzing circuit behavior, the equations appropriately reflect the PNP setup. The fundamental relationships remain but involve sign changes due to the reversed polarity of current flow in PNP devices.

Think of the equations for transistors as instructions for a recipe where you need to swap out ingredients based on what you have at home. Instead of using certain spices for an NPN 'dish', you might adjust and change them for the PNP version, yet the flow of cooking (or electronics) remains the same.

If you say that no you like to keep the similar kind of convention of the current as well. So, then you need to change the polarity of this current similar to that of the n-p-n transistor.

When representing the characteristics of PNP transistors graphically, it is important to either adopt a consistent sign convention or understand that the resulting graphs may occupy different quadrants compared to NPN graphs. This approach allows for a coherent understanding of how PNP transistors behave in comparison to their NPN counterparts when analyzed or plotted.

Picture yourself navigating with a map. If you decide to switch your route to one that goes in the opposite direction, you may need to redraw your map. Going from NPN to PNP transistors is similar; the fundamental rules apply, but the directionality must be accounted for in your representations.

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