![]() The driver effectively boosts the driving voltage high enough to unbias the Vbe or Vgs junction of the transistor. Without a driver, the transistor may never turn off. Using an NPN driver with a PNP BJT or P-Channel MOSFET is very common when the load voltage is higher than the signal voltage. (I’ve updated this post, because I accidently said the wrong thing…) In a MOSFET operating as a switch, you want it to stay in the linear (or ohmic) region. It turns out, saturation for a MOSFET is different from saturation for a BJT! (Their definitions are swapped!) Note: A common mistake is saying “For a switch, drive a MOSFET into saturation.” This is NOT the case. Until the FET is saturated, its Rds-ON can be relatively high, limiting the maximum current it can handle. While 5 volts from an Arduino GPIO pin might be enough to turn on the transistor, it isn’t enough to drive it into saturation. High current MOSFETs have a substantial Vgs threshold. So if you know of a case, or suspect you need one, leave a comment with it. Below are two cases where you might need to use a transistor driver. ![]() Instead, a transistor driver is used when the driving signal’s voltage (or current) is incompatible with the load transistor. This circuit is not the same as a BJT Darlington pair, which is a high-gain BJT. Transistor driving another transistorĪ driver transistor circuit is one that controls another transistor. Next, let’s see how a driver gets used with low-side and high-side transistor switches. You need a driver when the load voltage is HIGHER than the signal voltage. For a detailed explanation, check out my post on P-Channel MOSFET Tutorial with only Positive Voltages. Well, the logic is inverted but other than that, it is fine. The circuit above works fine when using a P-type transistor on a load voltage that is the same voltage level as the signal driving the transistor. They only exist in textbooks and as data entry errors. Keep in mind that you may never find a depletion mode P-Channel. Like with the low side, you probably want to use an enhancement mode MOSFET. Like the NPN, the PNP BJT must operate in the saturation region to turn on the transistor fully.įor a P-Channel MOSFET, the source connects to the voltage source, and the drain connects to the load’s positive side. Looking at the schematic drawing for an NPN and PNP, the PNP might look like it is upside down. High-Side Transistors (Note the FET has a Pull-Up resistor.)įor a PNP BJT, the emitter connects to the voltage source, while the collector connects to the load’s positive side. Typically these use a PNP BJT or P-Channel MOSFET. Because of how transistors work, these can be a little more difficult to use in an Arduino or Raspberry Pi circuit. This transistor connects between +V and the load. The opposite of the low-side switch is the high-side switch. While you can use a JFET for this circuit, an enhancement mode MOSFET works better. Saturation means there is enough base current to turn on the transistor fully.įor an N-Channel MOSFET, the source connects to ground, and the drain connects to the negative side of the load. ![]() As a switch, the BJT operates in saturation mode. Low-Side Transistor Examples (Note the FET has a pull-down resistor.)įor an NPN BJT, the emitter connects to ground, and the collector attaches to the negative side of the load. Typically these use an NPN BJT or an N-Channel MOSFET. Since the transistor is switching the path to ground or is sitting on the low side of the load, it is called a low-side switch. When the transistor is connected to ground, that means the load is between +V and the transistor. I have a few videos I made and some from element14’s The Learning Circuit, which do a great job introducing transistors. If you are new to transistors, check out the resource links at the bottom. In this post, I draw the configuration for both transistor types, discuss which requires a driver, and explain why you would use either. Either transistor configuration can use a BJT or MOSFET. The location of the transistor determines the type of circuit and its name. There are two configurations for a transistor switch: low side and high side. A common task for a transistor is switching a device on and off.
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