Can Someone Explain This Transistor Diagram to Me?

[turkey3_scratch]

I found this on Wikipedia:

I am trying to learn about transistors and CPU architecture so I have a few questions about this image.

1) what does the squiggle represent?
2) What is a base, collector, and emittor?
3) Why does it cut to the left and cut back near the base?
4) How is this transistor useful to arithmetic and logical units for calculating data?

 

[Someone Somewhere]

The following is from memory, by someone who is not an electronics engineer, and may not be perfectly correct:

1) The squiggle is a pull-up resistor. If there is no current passing through the transistor, then the pull-up resistor ensures the output stays at input voltage.
2) The base is where the signal enters the transistor. Very little current flows through it. When there's voltage between it and the side of the transistor with the arrow on it, current can flow from the collector to the emitter.
3) That's the symbol for a transistor. It doesn't have much to do with the actual shape on the silicon.
4) Used with its counterpart, you can construct logic gates. The most simple is a NOT gate, which simply inverts the signal. Complex arrangements can be used to AND or OR together two signals, or go even further.

 

[hunter315]

Thats a BJT, that isn't useful for much in digital logic, you end up losing a lot of power since you can never truly shut it off.. In a BJT there is a gain factor called Beta(B because greek letters are hard), when current(Ibe) flows into the base towards the emitter, current is pulled in from the collector(Ice), Ice=B*Ibe.

Your most common gate is going to be your inverter(NOT gate)

The squiggle at the top is the pullup resistor, when the input a=0, the transistor is off so no current is flowing through the resistor so the voltage at F is Vcc(1) since there is no voltage drop due to current, when a=1 the transistor is on, there is a large current draw but little drop in the transitor so F is at ground(0) this results in F!=a.

As for how its useful for arithmetic, the one you posted isn't, but AND, OR, and XOR gates will let you build a complete added, you just take these and stack them so you have the appropriate bit with and you can do binary addition using just 3 simple gate types

 

[turkey3_scratch]

Thanks to both of you!

So looking at the NOT transistor, let me get something I'm confused about straight. So the source of power is a it seems. When the electricity reaches the base, what makes the electricity travel up instead of down? Also, if a=1 and the electrical current is ON, would F receive any power or no? Or would power split and go to both F and the pullup resistor?

 

[Pinhedd]

I found this on Wikipedia:

I am trying to learn about transistors and CPU architecture so I have a few questions about this image.

1) what does the squiggle represent?
2) What is a base, collector, and emittor?
3) Why does it cut to the left and cut back near the base?
4) How is this transistor useful to arithmetic and logical units for calculating data?

Ah, you've discovered TTL logic. TTL is an old and depreciated form of constructing integrated circuits. Almost all modern digital electronics are fabricated using a variation of CMOS logic.

1. The "squiggle" is the electrical circuit symbol for a resistor. A resistor impedes the flow of current and dissipates energy as heat.

2. Base, Collector, and Emitter are three of the four terminals of a Bipolar Junction Transistor. BJT transistors are nominally current amplifiers.

The symbol that you see there, with the arrow pointing from the base towards the emitter, is an NPN transistor. In an NPN transistor a current across the base-emitter junction is used to control the current across the collector-emitter junction. If the base-emitter junction current is positive and high, then the collector-emitter junction will be really high. The current flowing out of the emitter terminal is the sum of the base-emitter current and the collector-emitter current.

A PNP transistor (identified by the arrow pointing from the emitter to the base) works the other way around. The current flowing into the emitter is the sum of the current flowing out of the base and out of the collector.

3. That's just the symbol. It is often enclosed within a circle to avoid confusion with normal wires.

4. Great question!

If my answer to #1 and #2 made any sense to you, you may have guessed that the transistor lets the designer control how much current flows from the node Vout to the reference point (ground). If Vin is at a suitably high potential, then current can flow [almost] freely across the transistor. If Vin is at a suitably low potential, then current can barely flow across the transistor at all.

Understanding what this allows the designer to do requires knowledge of Kirchoff's circuit laws, as well as Ohm's law. However, I will simplify this a little bit. When Vin is suitably high, an ideal BJT transistor appears to be a short circuit, it may as well not even exist. When Vin is suitably low, an ideal BJT appears to be an open circuit.

In the former case, all of the energy carried by the electrons would be dropped across the resistor and Vout would be equipotential to reference, or nominally 0 volts. In the latter case, the electrons would not be able to flow at all so no energy would be lost and Vout would be equipotential to supply, which may be 5v.

So, to describe the logical operation of that circuit,

If Vin is the equivalent of a logic high, Vout is the equivalent of a logic low.

If Vin is the equivalent of a logic low, Vout is the equivalent of a logic high.

The circuit above is an inverter, or a NOT gate.

The complexity of the logic can be increased by adding more transistors.

If two transistors are placed in series, both must be supplied with a logic high for current to flow, this forms a NAND gate.

If two transistors are placed in parallel, supplying either one with a logic high is sufficient to drag Vout to a logic low. This forms a NOR gate.

The arrangement of transistors that is responsible for connecting the output to the reference (ground) is called the pull-down network. In the example above, if nothing is pulling the logic down, it will be pulled up by the resistor.

If the diagram is switched around with a resistor on the bottom and a PNP transistor on the top it becomes a pull-up network with a pull-down resistor.

However, resistors are not flexible devices. They impede the flow of current so using them to pull logic up or down is very slow as they do not readily allow charge the flow through to the output; they also dissipate a lot of heat. This is why most modern circuits are constructed using both pull-up and pull-down networks. The networks are constructed in a complementary fashion so that at any point in time either the pull-up network is driving the output, or the pull-down network is driving the output. These kinds of circuits are called complementary logic, and when constructed with MOSFETs (a different type of transistor that is more suited for digital applications) the design is called CMOS or Complementary Metal Oxide Semiconductor. All modern microprocessors are designed primarily using CMOS logic.

 

[hunter315]

Transistor are like light switches, they are on or off, but very little current and power moves about. The current that flows into a is on the order of microamps, the current that flows into F is on the order of microamps as well so its not really letting power flow, its more of adjusting where the voltage is.

Electricity always travels towards ground from higher potential so it never flows up(well it does, but i blame Ben Franklin....).

When a=1 there is current flowing from Vcc to ground, almost all of the voltage drops across the pullup resistor so point F is slightly above ground so current flows out of F and towards ground. When a=0 there is no connection to ground so current flows into F until it is charged to the same level as Vcc, F is generally connected to another gate like a is, a gate has some capacitance so it takes a brief moment to charge up but once it is charged it pulls zero current so the entire link from Vcc to F ends up sitting at Vcc.

Wikipedia has pretty good articles on the gates with lots of good pictures, they helped me through digital electronics a couple years back.

 

[turkey3_scratch]

Thx Pinhedd! A lot of both confusing but great information. The only thing that still confuses me in the diagram is going from Vin to Base. Does the electrical current then move up or down in the diagram? And how exactly does this effect the Vout if it is on or off the current?

 

[Pinhedd]

Thx Pinhedd! A lot of both confusing but great information. The only thing that still confuses me in the diagram is going from Vin to Base. Does the electrical current then move up or down in the diagram? And how exactly does this effect the Vout if it is on or off the current?

One of the aspects of electricity that constantly confuses newcomers is the concept of current. Current (measured in amperes) is defined as the time rate of change of charge (measured in coulombs). Current always flows from high potential to low potential, this is a fundamental law of physics. It is analogous to gravity; charges cannot flow from low potential to high potential for the same reason that we cannot fall upwards into the sky.

However, excepting anti-matter, mass is a positive quantity. We don't know much about anti-matter, so we don't have anything to call negative-mass and accordingly we have no real concept of negative gravity. Why am I going on this strange analogy? The greatest troll of all time.

While the coulomb may be defined as a measure of positive charge carriers, the principle charge carrier of electricity is the electron, and the electron is defined as being negatively charged. The rather immobile proton is the positive charge carrier and those rarely move (in fact, their moving in an electric circuit at all is bad). What this means is that while current flows from high potential to low potential, the actual charge carriers that comprise that current are negatively charged and moving from low potential to high potential.

Neat huh?

Anyway, moving on.

BJTs have four main operating modes that correspond to the four inequal permutations of base, emitter, and collector potentials.

Case1: Emitter < Base < Collector. This mode is known as forward-active and is the mode that allows the collector-emitter junction to conduct current. Current will flow into the base and out of the emitter, and current will flow into the collector and out of the emitter.

Case2: Emitter < Collector < Base. This mode is known as saturation and occurs when base is at a higher potential than both the emitter and collector. Current flows into the base and out of both the emitter and collector. This is usually bad.

Case3: Base < Emitter < Collector: This mode is known as cutoff. Since the base is at an equal or lower potential than the emitter, current flows into the emitter and out of both the base and out of the collector (or from the collector to the base depending on the polarity). Due to the design of the BJT, this amount is negligible.

Case4: Collector < Base < Emitter. This is the opposite of Case1, and is known as reverse-active. It works just like you might think that it would, a small amount of current flows into the base and out through the collector. A small amount of current also flows into the emitter and out of the collector. However, due to the design of the BJT this amount of current is far less than Case1.

The ratio of collector current to base current is known as the Beta. In the forward-active mode it is known as Beta Forward, or Bf, and is very large. In the reverse-active mode it is known as Beta Reverse, or Br, and is several times smaller than Bf.

For small variations in the base-emitter potential, the following equation can be used

IE (emitter current) = IB (Base Current) + IC (Collector Current)

IC = Beta * IB

 

[turkey3_scratch]

Thanks Pinhedd! I really appreciate you informing me of all this stuff. One last quick question. What do you exactly mean by high and low potential?

 

[Pinhedd]

Thanks Pinhedd! I really appreciate you informing me of all this stuff. One last quick question. What do you exactly mean by high and low potential?

Voltage is a measure of electrical potential. Going back to the gravity example, it's somewhat similar to height. Like height, all voltage is relative. And like the ground floor, it usually helps to pick a common reference point to work with, we call this reference, or ground. Vref, GND, Earth (especially if it is physically connected to earth via a ground wire), and Ground are all more or less synonymous.

One coulomb of charge that passes through a potential of one volt (eg, from 1.0v to Vref) loses (or in the case of an electric field going from 0.0 to 1.0, gains) one joule of energy. If this is through a purely resistive load such as a resistor the energy will be dissipated in a higher entropy form such as heat or light. However, if it passes through a reactive load such as a capacitor or an inductor the energy will be stored in an electric or magnetic field.