Welcome back.
I'd like to cover a concept called feedback.
It's not feedback about me telling you your performance in class, but feedback in electronic
circuits.
Why do we need feedback?
We need feedback for the purposes of control.
You don't know where you're at, you can't manage the circuitry situation.
You need to feedback a signal.
Nature is incredible, and there is an interesting phenomenon that we tend to see with transistors
when heat occurs.
Heat occurs from a number of, not just the outside environment but also from just he
flow of electrons.
If you think about it, or the flow of current, through a device, it's bumping into molecules.
That's energy.
That energy gets expanded or changed into another form and that form can be heat.
That's why when you see devices, when you open up your computer, for example, you see
a very large piece of metal on the central processing unit.
Many thousands, actually millions of transistors in that CPU, but you've got to get the heat
out.
If you don't get the heat out, the device destroys itself.
Why?
In silicone, just looking at this device here, as you apply heat.
Let's go this way.
Temperature will go up.
The voltage across that particular junction will actually go down.
In fact, it is a number of 2 millivolts per degree Celsius.
For every increase in T, you're voltage will go down by 2 millivolts.
The other phenomenon is the beta will go up.
What's going on here?
We have this device in operation, it's heating up.
This goes down, and the game goes up.
Remembering that beta is equal to IC over IB.
If you've got less of a voltage drop here, you're going to have more base current.
You get more base current, you're going to have more collected current.
That's one problem.
Also, as the beta goes up, then the multiplication of the collected times the base will go up.
Collected current is going to increase and base current is going to increase.
We're going to have a problem.
The device is going to effectively burn up.
That's not controlling the flow.
Let's try and do something about this.
There is a number of ways that you can control, or at least feedback to offset this temperature
problem.
This one here is called collector feedback.
Makes sense, we're taking it off the collector.
You think about the tap, or even this device.
I can control it on this input here, and manage it, so when temperature goes up, maybe what
I should do is reduce the push here.
Think about it, if I have more current flowing down through here, this voltage will drop.
If this voltage drops then I'm going to have less base current.
I'll use that as a feedback mechanism.
As you saw from the other diagram we had, we can also put a resister in the emitter,
so as more current comes down in here, that will push back on the drop across here.
We can control that way.
Let's look at this one.
Now that we're using collector feedback, which is one way of controlling this NPN transitor.
We still have to understand, for example, someone will say, "Can you calculate for me,
what is that collected current then anyway?
Given that you're using collector feedback?"
Let's look at this device.
Let's call this our CRB and we know we're looking for VC, VB, VE, those types of numbers.
Let's see what's going on.
This is VCC.
Let's try and write an equation first, before we plug in the numbers.
We will know this, we will know this, we will know what our beta is as well.
It will be given.
We need to calculate our voltages and currents as we've done before.
If I look at this equation here, I have the pressure at the top.
I have a drop of current there.
I have this voltage here.
All of that entirety should fall and equal zero.
Or this part here, this drop, this drop, and this drop will equal VCC.
Either way.
I'm just writing the equation this way.
Now here's the problem with math.
I don't know what this is.
This, of course, I know, know, know and know.
You cannot have 2 unknowns in 1 equation.
You can't solve it.
Do we really have 2 unknowns?
We know that IB times beta equals IC, so we subsitute.
We're going to substitute into this equation.
What does that give us?
What he have here then is VCC minus ICRC minus, in this case here we're substituting for IB.
IB will be equal to IC divided by beta.
IC divided by beta RB minus .7 equals zero.
Now we only have one unknown.
Let's rearrange the terms.
What we will have here, so if we take IC out, then we would have RB over beta.
RC, okay.
Here we have VCC minus .7.
Let's see what we can do.
Let's rearrange the terms, and you can do this.
You'll have VCC minus .7 as you numerator, and here you will have RC plus RB over beta.
Plugging some numbers in, if I give you these numbers here, you can plug them in and you
can actually have your collected current.
Sum up to a mirror current, divide by beta, you can figure out IB etc.
The concept is here is feed back.
You have feedback with your eyes, and your hands, and your ears, and your nose when you're
dealing with the faucet.
In electronics we have to do something a little bit more than.
Unfortunately we have a physical situation where temperature increases, and the same
thing, if the temperature goes down then the beta goes down and the VBE goes up.
Same thing, lets accommodate for that, and this is one of the simpler ways to do that,
to minimize the effect of moving that bias point.