How Transistors Work - The Learning Circuit

the following program is brought to you by element 14 the electronic design community where you can connect and collaborate with top engineers from around the world join now at element14.com hi and welcome back to the learning circuit where we learn about basic electronics I'm Karin and today we're going to start learning about transistors the term transistor comes from Korea transferring resistor the basic type of transistors are bipolar transistors previously we've talked about how diodes work diodes have a PN Junction bipolar Junction transistors or PJ T's are bipolar because they have two PN junctions eg T's are essentially two diodes in a single package the two main types are NPN and PNP transistors NPN transistors have two n-type regions on either side of one p-type region while PNP transistors have two p-type regions on either side of one n-type region bipolar transistors have three leads one going to each region typically the middle layer is the base p-type in the NPN and n-type in the PNP one of the other layers forms the emitter and the third the collector these are labeled B E and C this is the symbol for a diode the arrow points towards the n-type side when we combine the P sections of two diodes there are two arrows pointing out compare this to an NPN transistor symbol notice there is one arrow pointing out towards one of the n-type layers we know that the middle layer is always the base but the emitter and collector can appear on either side of the symbol on the circuit symbol the arrow is always on the emitter so we can tell which lead is the emitter and which is the collector by seeing which one has the arrow of NPN transistor is similar except it has two n-type layers on either side of one p-type layer on the PNP transistor symbol the arrow on the emitter still points towards the n-type layer which in this case is in the middle so the NPN transistor symbol has an on the emitter pointing out while the PNP transistor symbol has an arrow on the emitter pointing in so how do transistors work rather than using a physical mechanical switch a transistor can act as an electronic switch using signals to turn it on and off the main current in your circuit would flow through the collector and emitter while a signal current flows from the base to the collector the signal current at the base is what controls our switch a current flowing from the base to the emitter can open the flow of current from the collector to the emitter from our diodes lesson we know that diodes require a forward voltage of 0.7 volts before they turn on allowing current to flow in a standard NPN transistor when 0.7 volts is applied between the base and the emitter the transistor turns on allowing current to flow from the collector to the emitter let's look at an example we'll start with an NPN transistor with an NPN transistor we normally bias the device so that the collector voltage is positive with respect to the emitter the voltage across these two points is referred to as the collector emitter voltage or VCE we would then connect the base to be positive with respect to the emitter this voltage is referred to as the base emitter voltage or vbe here's a circuit where a 9-volt battery connects with an LED and it's resistor but it connects through a transistor this means that no current will flow in that part of the circuit until the transistor turns on to turn the transistor on you need a vbe base emitter voltage of at least 0.7 volts imagine you have a small 0.7 volt battery when you apply the 0.7 volt battery from base to emitter the transistor turns on this allows current to flow from the collector to the emitter and thereby turning on the LED so once the vbe reaches 0.7 volts the nine volts from the main power source is available but don't forget your voltage drop 0.7 volts is lost to heat in the transistor leaving 8.3 volts as your VCE [Music] now let's look at PNP transistors for a PNP transistor rather than needing a minimum of 0.7 volts on the base of the transistor there needs to be a minimum difference of 0.7 volts between the VCE collector emitter voltage and the vbe base emitter voltage let's say our circuit power supply is a 9-volt battery again the base emitter voltage would need to be no more than 8.3 volts for the transistor to turn on and allow current to flow between the collector and emitter so if the base emitter voltage is 8.6 volts that's only a difference of 0.4 volts the transistor would be off and no current would flow if the base emitter voltage is 7 volts that is a difference of 2 volts which is more than 0.7 volts so the transistor would be on and current would flow between the emitter and collector a difference of less than 0.7 volts off a difference of more than 0.7 volts on another use for transistors is amplification one method for current amplification is by combining two transistors into a Darlington pair this can be done with two NPN or two PNP transistors the two transistors are combined by connecting the emitter of the first transistor to the base of the second transistor in a Darlington pair the current is amplified by the first transistor and then further amplified by the second when a Darlington pair is contained in a single package it makes a Darlington transistor like other transistors it has three leads one base one collector and one emitter Darlington transistors are useful because only a small amount of current at the base is required in order to get a current gain of up to a thousand times or more however there is also a significant voltage increase to be considered there's a good amount of math involved when calculating currents and voltages when using Darlington transistors and really transistors in general here are some of the abbreviations used in those equations you should recognize some that I've already mentioned many of these can be seen on a transistor datasheet the last two types of transistors I'll mention our field effect transistors the two most common types are J fetes and MOSFETs but we'll have to save those for another time transistors can be a little complicated to understand so I hope I gave you some good first steps to get you on your way have any questions about what we talked about in today's video post your questions on the element14 community on element14.com forward slash the learning circuit happy learning [Music]

Channel: element14 presents

Views: 941,182

Keywords: electronics, hardware, gaming, hacking, mods, weekly, element14, maker, engineering, circuit, battery, switches, diodes, vce, mosfets, jfet, pnp transistors, thelearningcircuit, tlc, stem_projects, darlington transistors, npn transistors, darlington pair, vbe, field-effect transistors, collector-emitter voltage, base-emitter voltage, The Learning Circuit, Karen Corbeill, Bipololar junction transistors, NPN transistors, PNP transistors

Id: R0Uy4EL4xWs

Length: 7min 12sec (432 seconds)

Published: Wed Oct 24 2018