How do transistors work?

Rondog · #1 (**permalink**) 6th June, 2006, 07:06 PM

I have read through wikipedia's article on transistors and I do not understand it. Could someone please tell me how they work, as in which way do voltages go etc etc based on this diagram.

image copyright: http://hsc.csu.edu.au/physics/core/i...4_3/943net.htm

http://www.aoaforums.com/forum/attac...1&d=1149638779

Gizmo · #2 (**permalink**) 6th June, 2006, 08:56 PM

Hmm.........not sure how good a job I can do with this, but I'll take a stab at it.

The transistor you have pictured is called a PNP transistor. It is a variety of bipolar transistor. It is called a bipolar transistor because it operates using both majority and minority charge carriers, although that fact is not terribly important to the basics of this discussion.

A bipolar transistor has three terminals called the Collector, Emitter, and Base. Transistors work by regulating current flow. In bipolar transistors, the current flow you are trying to control exists between the collector and emitter, and is controlled by current supplied to the base. For PNP bipolars, the current flows from emitter to collector and the base will have a current flowing OUT, with a bias voltage that is less than the emitter voltage when the transistor is conducting. For NPN transistors, the current flows from collector to emitter and the base will have a current flowing IN, with a bias voltage that is greater than the emitter voltage when the transistor is conducting.

A bipolar transistor consists of two separate circuits; one is the collector-emitter circuit, which represents the load current. The other is the base-emitter circuit, which represents the control current. The amount of current flow in the collector-emitter circuit is in direct proportion to the amount of current flowing in the base-emitter circuit. This is generally called the 'gain' of the transistor, or more appropriately the beta-gain, or sometimes just β. Power transistors generally have a gain of 10 or so, but small signal transistors can have gains in excess of 200.

This is a very brief and greatly simplified explanation. I hope it answered your questions. If not, try asking your question a different way and I'll try to answer.

QSDT · #3 (**permalink**) 6th June, 2006, 09:19 PM

Thanks gizmo that's a good read helped me a bit aswell.

Rondog · #4 (**permalink**) 6th June, 2006, 09:20 PM

Ok...that confused me a little bit. But anyway: is a transistor something that inhibits electricity flow until a certain voltage is reached then it allows electricity flow?

EDIT: Is this a good analogy of how it works? http://www.satcure-focus.com/tutor/page4.htm

If so, I understand it now, but what does a transistor do? I've heard them refered to as 'electronic gates'. What does this actually mean?

noob · #5 (**permalink**) 6th June, 2006, 10:31 PM

Rondog, that is a good analogy of how they work.

Why that whole series of electronic components are called gates I'm not exactly sure, but they can and are used as switches or amplifiers.

What you have is a voltage (or amperage) that goes into the collector (call it Vcc). It then hits the 'gate' and doesn't go anywhere.

You then need a certain voltage (depends on the transistor) going into the base (PNP transistor) to open the gate (call it Vb). This will then allow the Vcc to flow through and most of the Vb will also join it creating a small amount of amplifcation.

So you can use a transistor as either a switch (will post a schematic of a blinking LED using this when I find it), or as a basic amplifier (transistor radios).

If I can find my old notes regarding this I will post them too. They are pretty basic (stage 1 electronics), but give a good explanation.

Áedán · #6 (**permalink**) 7th June, 2006, 04:20 AM

Quote:

Originally Posted by noob

What you have is a voltage (or amperage) that goes into the collector (call it Vcc). It then hits the 'gate' and doesn't go anywhere.

There are different types of transistors. The bipolar transistor that Rondog used as an illustration works as a current amplifier. That is, the flow of current into (or out of, depending on the transistor construction) the base terminal sets the flow of current between the emitter and collector. The bipolar transistor does not have a gate.

However another type of transistor, the Field Effect Transistor (FET) doesn't work quite the same way. The FET transistor doesn't have a base connection, but a gate connection instead. As such, the gate is insulated from the rest of the structure. This leads to the idea of the field effect, as it's the electrical field from the gate that affects the working of the transistor, rather than the current. As it's an electrical field, there is no (or marginal) current flowing into/out of the gate connection, instead the transistor is sensitive to the voltage present on the gate terminal.

Yes, the MOSFET is a form of FET, in case you were wondering!

Rondog · #7 (**permalink**) 7th June, 2006, 05:17 AM

Arghhh... Im so confused!

Can anyone draw arrows on my diagram so I can see where the current flows?

noob · #8 (**permalink**) 7th June, 2006, 03:56 PM

The arrow labelled 'I' is the direciton of the current.

Rondog · #9 (**permalink**) 7th June, 2006, 05:44 PM

Ok...

So current flows from the battery to e then b then c. A small current flows through e to b and opens the gates, and then flows back to the battery. With the gate open, it allows a large current to flow through to the rest of the circuit. Where does this large current come from?

Is the small current that opens the gate, positive charge from the postive holes? And the large current is a negative charge from the electrons in the conduction band?

Gizmo · #10 (**permalink**) 7th June, 2006, 10:27 PM

Hole flow and electron flow are just two different ways of looking at the same problem. Classical Electronics works with hole flow (current flows from positive to negative). It is only when you get into some very esoteric areas of electronics that the difference between electron flow and hole flow becomes important.

ALL current flow in ANY circuit STARTS and ENDS with the power source. In order for ANY current to flow, there has be a complete circuit from the positive terminal to the negative terminal.

With the PNP transistor shown above, the emitter will be connected to the positive terminal, the collector will be connected to the negative terminal, and the base will be connected to the negative terminal (that is an oversimplification, but it will suffice for now). There will be a SMALL current flowing from the positive terminal through the emitter and base to the negative terminal. There will be a LARGE current flowing from the positive terminal through the emitter and collector to the negative terminal.

The emitter-base current is referred to as the minority carrier current, and the emitter-collector current is referred to as the majority carrier current. Emitter-collector is the load and does the work. Emitter-base is the control, or on-off switch.

Does that help?

Rondog · #11 (**permalink**) 8th June, 2006, 01:39 AM

Quote:

Originally Posted by gizmo

Hole flow and electron flow are just two different ways of looking at the same problem. Classical Electronics works with hole flow (current flows from positive to negative). It is only when you get into some very esoteric areas of electronics that the difference between electron flow and hole flow becomes important.

ALL current flow in ANY circuit STARTS and ENDS with the power source. In order for ANY current to flow, there has be a complete circuit from the positive terminal to the negative terminal.

With the PNP transistor shown above, the emitter will be connected to the positive terminal, the collector will be connected to the negative terminal, and the base will be connected to the negative terminal (that is an oversimplification, but it will suffice for now). There will be a SMALL current flowing from the positive terminal through the emitter and base to the negative terminal. There will be a LARGE current flowing from the positive terminal through the emitter and collector to the negative terminal.

The emitter-base current is referred to as the minority carrier current, and the emitter-collector current is referred to as the majority carrier current. Emitter-collector is the load and does the work. Emitter-base is the control, or on-off switch.

Does that help?

Very much so. Thank you kindly Gizmo.