# trying to figure out the differences between the six transistors provided by the schematic tool

by KevinSim   Last Updated May 15, 2019 22:25 PM

My background consists of a BS in Computer Science and an MS in Computer Science and Engineering, both from the University of Washington. My MS consisted mostly in the very high level design of a hardware sorting machine. Now I'm trying to figure out whether it would make sense to implement that machine, and I'm trying to take a look at it all the way down at the transistor level. My schematic diagram consists of all six transistors the schematic tool lets me draw.

Can anyone tell me what the essential differences are between the six transistors are?

simulate this circuit – Schematic created using CircuitLab

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Q1 is an NPN BJT transistor
Q2 is a PNP BJT transistor
M1 is a N channel or NMOS mosfet transistor
M2 is a P channel or PMOS mosfet transistor
J1 and J2 are JFET transistors

BJT's have a voltage drop (normally 0.7V) across them, then need current to turn on
An NPN has current flowing into A that controls current flowing from B to C
A PNP has current flowing out of D that controls current flowing from E to F

What you need to know: It's easy to fry a BJT if you don't limit current with resistors, there is a voltage drop similar to a diode, but this creates loss and heat. You control a BJT through the base.

Mosfets need voltage on the gate (G and J) to switch on and off N channel mosfets need the gate voltage (G) to be a higher voltage than the Drain (I) to switch on. P channel mosfets need the gate voltage (J) to be a lower voltage than the Drain (L) to switch on.

What you need to know: Mosfets should not be installed backwards (because they have a reverse protection diode) N channels need a higher voltage on the Drain than the source. P channel mosfets need a lower voltage on the drain than the source.

Use N-channels, they are easier to wrap your head around. They are low resistance. Mosfets work best if they are fully off or fully on, you can run into thermal issues if they are in between. They are also sensitive to static (well all transistors are) but the gates are nm thick and can be blown away easily.

Jfets are simmilar to mosfets, but have lower noise and higher input impedance (two things you probably wouldn't care about unless designing an amplifier)

laptop2d
May 15, 2019 21:45 PM

• Q1 & Q2 -- bipolar junction transistors (BJT), one NPN, one PNP.
• M1 & M2 -- metal oxide field effect transistor (MOSFET), one N-channel, one P-channel
• J1 & J2 -- junction field effect transisor (JFET), one N-channel, one P-channel

Each with strengths and weaknesses. The REALLY SHORT answer is that:

• BJTs are the oldest commercial technology, they're good for linear circuits, linear power output stages, and general "knock around" design.
• MOSFETs have really low losses at high currents when they're turned on all the way, so you see them a lot in switching power supplies and switching amplifiers.
• JFETs tend to be specialized, and rarely used -- which makes them hard to get, and even more rarely used.

Beyond that -- please look them up on the web. The full description of each one takes a book, or more.

TimWescott
May 15, 2019 21:46 PM

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