Electronics Lessons: The Transistor Simple Circuits Long Tailed Pairs

This could really be an advanced lesson.
I'm not suggesting that you try to build this, BUT this is the building block of the next major component that I'll be looking at.

Long Tailed Pairs
This is a circuit that forms an amplifier, but, that amplifier amplifies the difference between the set of two inputs rather than just amplifying the signal applied to one input, this gives the amplifier good noise rejection since you can (not unreasonably) assume that the same noise would be present in both conductors carrying a signal.

Long tailed pairs of transistors work very well in integrated circuits where they are built on the same piece of silicon substrate, the reason for this is so that both transistors such that they are electrically the same and subject to the same environmental changes in temperature.

As a side note, in Valve amplifiers long tailed pairs will usually use duel triode valves in the same envelope with the same grid and heaters etc.

Long tailed pair amplifiers are also called differential amplifiers, as they can be used to amplify the difference in a signal.

the amplifiers are basically two ordinary transistor amplifiers that share a common junction at Re, and this Re junction is isolated from ground by a second Re resistor.

The amplifier is a difference amplifier because it amplifies the difference between the two inputs.

(V1 - V2) x gain = Voutput


The long tailed pair can make a very good amplifier.
when the input V1 is connected the amplifier will be a non-inverting amplifier, when the input V2 is connected then the amplifier.

In addition to this, if you apply a signal to both inputs the amplifier will cancel common components.
Consider
v1 and v2 tracking perfectly

v1 - v2 = 0
So the output of the amplifier is

(0) x gain = Voutput so Voutput = 0
In a pair of cables the noise induced by external sources (stray EMF perhaps by mains cabling) will be the same, the signal source will not be the same.

Electronics Lessons: The Transistor Simple Circuits (Lab 2)

So in the last lesson I talked about the transistor, I talked about how you could turn the transistor on to a fully conducting state.

I also mentioned that transistors could be used as current amplifiers.

You'll use a current amplifier when you need a little more power to switch on an output than that output can supply.

We discussed before how the transistor was off, and how it could be turned on.

So, lets make a device that puts a light on with the presence of a voltage, even if that voltage source doesn't have the power (current sourcing capability) to light up the light itself.

In this tutorial we're interested in two areas of the transistors output, because we're going to use the transistor as a switch we'll either have the transistor in the cut-off region (off), or the saturation region (on) of it's output characteristics. (There are the areas marked in red in the chart below).

The Circuit
What we do is connect the input to the base of the transistor, we use a resistor to ensure that not too much current is pulled from the voltage source that we're detecting.

We'll also put a resistor between the circuit voltage source, and the transistor to limit the current being drawn from the source.

Our light will be an LED, we're expecting that the voltage going to the LED will be 5V at most, but as little as 3v so we look at some data sheets for LEDs and we find that the there are a couple that we can't use, (some have Vmax as 4v), and others will tolerate a higher voltage, but won't light with only 3v.

Eventually we come across the L-53GD-5V made by Kingbright

It has the following characteristics.

So we can see that it'll be on, and bright at 5v, and it will turn on, (though only be half as bright) at 3volts, (we're interested in sensing 5v Logic levels and 3.3v Logic levels right?)

Elsewhere in the data sheet it tells us that the maximum current is 17mA
We know that our greatest voltage going through the LED will be 5v
So we use ohms law to determine the resistor needed.

5/0.017 = 294

So we really want to make the total resistance between the supply voltage and the indicator LED around 300Ohms
In case you're interested

3.3/300 = 0.011 or 11mA available for the 3.3v logic level to light the LED.

I say the 3.3v, remember this is a current amplifier, not a voltage amplifier. The voltage is going to remain the same as what's going into the transistor base.
Anyway, at 3.3v the LED only draws some 6mA, so there is plenty of current available. (and that 11mA is below the devices max draw of 17mA

Schematic
Here is the schematic of the circuit.

And here's what happens when a logic level of 1 (+5v) is applied to the base resistor

When the circuit is on it pulls around 6ma from the logic source, and the current going through the LED is about 14mA.

Stay tuned to see this idea scaled up!

Electronics Lessons: The Transistor

This lesson deals exclusively with Bi-Polar Junction transistors (BJT)
So I covered the diode, a couple of lessons ago.

I'm trying to make these lessons as simple as possible,

I'm trying to keep these beginner introductions to components as simple as possible, trying to keep them at a type GSCE level, so I'm not going to go into exactly how silicon is created and doped to P or N types, I'm not going to go through all the in depth technical stuff that a university course would cover.

These are beginner lessons, assuming that I do stick with this, those more in depth lessons should come a bit later.

The Diode
While I did just say that I wasn't going to go in depth as to how diodes are made it's important to understand a little bit about diodes.

Basically there are three types of silicon in this world.
There is silicon as you find it, that's pretty useless for electronics, then there is doped silicon, doping silicon enables to to be able to transport electrons and therefore allow electricity to flow.
There are two types of silicon, P type, (that contains what we call holes as the transport mechanism) and N type (that contains negative charge carriers [electrons]).

When you put these together you form a PN junction diode, which lets electricity flow one way.

The Transistor

Once upon a time transistors came in metal cases, the little tab on the case was next to the emitter leg, the collector was usually attached to the case and the base was the remaining leg, now they tend to come in plastic cases, and you need the data sheet to tell which leg is what.

The transistor is like two diodes placed back to back, (or front to front depending on the transistor type), arranging the blocks of doped silicon like this allows you to make the transistor act like a switch, but let's not get ahead of ourselves too much just yet.

I'm going to look at the NPN transistor for now.

NPN Transistors
The NPN transistor is called such because the sandwich of silicon is a P-type piece of silicon sandwiched between two N-type pieces. this forms two PN junctions where the P type silicon is shared. (no you can't actually wire two diodes back to back to make a transistor, the semiconducting silicon actually has to be shared).

Each piece of silicon is attached to a leg, and on BJTs these legs are given the name collector, base and emitter.

The collector collects current, and the emitter emits it, (that's overly simplistic, but vaguely true) the base leg controls the amount of charge in the bit of silicon in the middle and therefore controls the amount of current, or electrons that can flow through the device.

If you apply voltage to the base, then you essentially turn the tap on, (assuming you apply enough voltage!).
If you apply 5v then the tap is on. (hard on) the component will what is called hard on.

When you look at the data sheet for the component, you're going to see that there is (of should be!) two values of particular interest, one of these is VBE(sat)

That's the VBE saturation voltage, the amount of voltage needed to turn the transistor hard on.

For BC108 transistors this value is 0.7 volts, (so this or any voltage over this is going to turn the transistor into saturation mode, where it's conducting as much as it can, and can't conduct any more).

You'll also notice that in the VBE values in general there are three values.
Min, Typ (typical) and Max.
Max is the same as the saturation voltage, which we've already covered.
Min is the minimum voltage needed to turn make the transistor start conducting at all.

There are a few more values of interest (depending on the project that you're doing) I'll tell you what these are, and how to use them in a later tutorial.

We've covered that if you apply less voltage that VBE(min), then the transistor won't turn on at all, and if you apply more than VBE(max) that transistor goes into saturation mode.

So what if you apply VBE(typ)?
Again this will be an over simplification, what we'll say is that the transistor is half on.
it's not conducting in it's full on saturation mode, nor is it in it's non conducting off mode. it's half on, go a little over VBE(typ) and it'll conduct a little more, go a little under VBE(typ) and it'll conduct a little less.

Note: That VBE is the voltage difference between the base and the emitter, not necessarily the voltage applied to the base, increasing this voltage above VBE(max) will force more current through the junction and destroy the component.

The transistor is a variable switch, a current amplifier, a voltage amplifier.

Schematic symbols
The symbol for a BJT transistor is a little more complicated than for other components, that's because there are three legs for a start, and they come in two varieties, NPN and PNP.

We'll start with the emitter leg, if you think back to the diode, the symbol for that was an arrow with a bar, the arrow pointed to the N type silicon,

The part of the symbol that shows the emitter is an arrow, the arrow points to the N type silicon.
so on an NPN transistor the arrow points outwards from the base (because the base is P and the emitter is N), on a PNP type the arrow points inwards (because the emitter is P and the base is N)

The Base is a thick straight line.
the collector is an angled straight line.

so here is the symbol for a NPN transistor (remember the arrow point out)

And here is the symbol for the PNP type transistor (remember the arrow points in)