So it may be necessary to add another transistor to convert the 5 V to 12 V to drive the PNP. However, to turn off the PNP transistor, its base must see 12 V because its emitter is at 12 V. Current is delivered to the transistor base when the MCU I/O is 0 V, and the load turns on. When there’s no current (digital output= 0 V) the load keeps floating and is turned off.Ĭonversely, you can connect one side of the load to ground and use a PNP transistor to switch the other side to 12 V. A current through a resistor to the transistor base from the MCU I/O turns on the load. Here you can connect one side of the load to 12 V and use an NPN transistor to switch the other side to ground. The approach is to add an external 12-V supply and connect its -V output to GND of the MCU. Suppose the load works from 12 V but the MCU logic is at 5 V on, 0 V off. A PNP would go between the voltage source and the load with the other end of the load connected to ground.Ī more realistic example of the two uses is that involving a load driven by a logic device such as the digital output of an MCU. An NPN device would go between the ground and the load with the other end of the load connected to the voltage source. For example, consider the simple case of a load and and a voltage source. PNP devices are used to switch from the positive side. Thus the most common use for NPN devices is for switching the ground side of a circuit. A current flowing into the base of the NPN turns on the transistor whereas a current flowing out of the base turns on the PNP. That brings us to the NPN vs PNP decision. There are other differences between BJTs and FETs in terms of switching speed and gain, but input considerations tend to be the primary focus. Thus FETs are generally the choice when its important to minimize input current/maximize input impedance. However, its gate input current is much less than the base current of a BJT. In contrast, the voltage-controlled FET typically can require on the order of 10 V at its gate to turn on. Thus a bias of less than a volt between a BJT base and emitter is enough to turn on the transistor. The diode turns on and conducts current when it becomes forward biased, and that happens at the same approximate 0.7-V level as for an ordinary discrete diode. What this generally means in practice is that the BJT switches on when its base-emitter diode conducts current. BJTs, whether NPN or PNP, are current controlled whereas FETs are voltage controlled. A few general guidelines help in making the determination.įirst consider the FET vs BJT question. One question that sometimes arises concerns how to discern when an NPN, PNP, or FET is the best choice for a given circuit application. As for logic functions, IEEE Standard 91/91a promulgates the graphic symbols, which are referenced in ANSI Y32.2/IEEE Standard 315.

In North America, ANSI Y32.2-1975, IEEE Standard 315-1975 and CSA Z99-1975 are substantially the same. In Europe, IEC 60617, which is the same as British Standard BS 3939, publishes relevant standards. Schematic symbols and conventions for drawing schematic diagrams differ among nations and jurisdictions. If it points out, the MOSFET is N-channel. If this arrowhead points in, the MOSFET is a P-channel device. Another difference is that in the JFET the arrow head is in the gate lead, not in the drain, which corresponds to the BJT emitter. Keep in mind the mnemonics “not pointing in” and “pointing in please”.įET symbols can be distinguished by the fact that drain and source connections take off perpendicularly from the substrate, rather than at acute angles, diverging from one another. If the arrow head points out, the device is an NPN transistor. The emitter has an arrowhead embedded in the lead, so the remaining lead has to be the collector. In the BJT symbol, the base lead is perpendicular to and lands at the midpoint of a short line segment representing the semiconductor substrate. Most bipolar junction transistors and field-effect transistors (FETs) have three terminals.

Npn vs pnp mosfet