# how does a transistor work

#### Jervac

Joined Jan 20, 2015
2
Im having trouble understanding how the transistor component works. I think current flows out of c into the emitter and not sure how the base effects it. Any good diagrams?

#### Evil Lurker

Joined Aug 25, 2011
116
In a nutshell NPN transistors you feed current into the base pin which allows current to flow into the collector and out the emitter to ground. Generally connected "after" the load. Using a garden hose as an analogy this would function as the spray nozzle.

With PNP transistors instead of feeding current into the base pin, you let current flow out of it which allows current to flow into the emitter and out the collector. Generally connected "before" the load. Again using the garden hose analogy these would function as the spigot connected to the water supply.

NPN = "push" small amount of current into the base to allow collector to "suck" large amounts of current

PNP = "suck" small amount of current out of the base to allow collector to "push" large amounts of current

Of course "suck" and "push" are used metaphorically... electricity will always follow the path of least resistance and if not impeded via some form of resistance the transistor will fail.

Edited to add: Probably should be mentioned there are three states to a transistor "off", "active" or partially on, and "saturated" or wide open. With "active" using the garden hose analogy it would be like pressing the handle on the nozzle or turning the knob on the spigot a little bit to control the flow of water going out. "Saturated" state would be the point where pushing the handle or turning the knob further would not result in additional water flow.

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#### #12

Joined Nov 30, 2010
18,167
Love those spring animations! They almost resemble how I think.

Mr. Evils' explanation seems difficult to me so I will try to garble the message some more.

A transistor is a 3 legged animal such that when you allow base current to flow, it gets that current through its emitter. When the emitter provides current for the base, it also provides a LOT more current for the collector. This is called current gain, a little for the base and a lot more for the collector. That is the basic function, but the transistor is mostly unable to limit the current by itself. That is what resistors are for. In most cases, you limit the base current with a resistor and you limit the collector current with a resistor. Having 2 of the legs controlled will force the third leg to comply with your wishes.

You are allowed to ask more questions because this can be a conversation if you want that to happen.

#### wayneh

Joined Sep 9, 2010
16,128
... electricity will always follow the path of least resistance ...
Most of it will, but electricity will follow EVERY path available to it, just like water running down a hill. This matters, especially if you happen to be part of one of those paths. The fact that most of the current is going one way won't save you if some of it is going your way.

#### Evil Lurker

Joined Aug 25, 2011
116
For what it is worth my brains dynamic link library uses visual object association. The terms "base", "emitter", and "collector" mean absolutely nothing to me by themselves as they are concepts, not things that can be directly observed. Would be much simpler for me to understand if they just called the pins "in", "out", and "control".

#### wayneh

Joined Sep 9, 2010
16,128
Like source, drain and gate?

#12

#### Evil Lurker

Joined Aug 25, 2011
116
Exactly like source, drain, and gate. When I first started learning about MOSFETS gate is pretty much self explanatory, but "drain" sounds like an orifice where something leaves and "source" is where something goes in. Basically FETs are transistors too but unlike bipolar junction transistors the "base" or "gate" is voltage controlled. FETs are also typically used as low resistance voltage controlled switches that are either all the way off or all the way on ("saturated") with great pains being taken to avoid the "active" region. The problem with FETs is that the "on state resistance" is much greater in the "active" region than when a FET is "saturated" and that the "gate" functionally acts as a very tiny little capacitor. This little capacitor must be charged up to a certain voltage before the FET saturates and likewise it must also be discharged before a FET can turn off. In the meantime while waiting for the gate voltage to get to where it is supposed to be thanks to the active region resistance a bunch of power is wasted and converted into heat. So basically the deal with MOSFETs is to turn them on and off as quickly as possible. One quirk is that the gate capacitance is typically proportional to the blocking voltage... higher blocking voltage = greater gate capacitance. And thanks to gate capacitance it is actually possible for the device driving the gate of a MOSFET (especially high blocking voltage ones used in mains power supplies) to source or sink so much current while trying to overcome the FETs gate charge that without some additional circuitry in place the driver will let out the magic smoke.

Also some other quirks with MOSFETs are that some operate in "enhancement" or "depletion" mode, and they may or may not contain a diode in parallel to the drain and source. This diode is used for "freewheeling" when the load is an inductor or transformer. Much like one of those little toy cars where you push it on the ground and you hear a flywheel spin up inside then let go of it and the energy stored in the flywheel will continue to propel the car forward, the "spinning" magnetic field of the inductor will want to pull the drain pin below ground and into negative voltage territory when the FET transitions into the off state. MOSFETS are good at blocking voltages on the drain, lousy at blocking voltages on the source. Without a diode in place it is possible for the MOSFET to let out the magic smoke as the potential voltages would exceed the source/gate and/or source/drain limits. In addition the diode itself generates heat when current flows though it when it is "freewheeling".