Instead, a transistor driver is used when the driving signal’s voltage (or current) is not compatible 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. When the load voltage is HIGHER than the signal voltage, you need a driver. For a detailed explanation, check out this post I wrote on P-Channel MOSFET Tutorial with only Positive Voltages. Well, the logic is inverted, but other than that, is fine. When using a P-type transistor on a load voltage that is the same voltage level as the signal driving the transistor, the circuit above works fine. 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. Just like the NPN, the PNP BJT needs to be operating 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 couple of 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, talk about 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.