Peter Wheeler: All right, so we need to calculate the gains now because it's all about being
gain amplifiers.
I picked two transconductance numbers for these devices.
They can be much higher than that, but that's fine for our purposes.
I just used those numbers.
That means that I need a gm times an Rd. in this case here, Rd is 1K.
There is no load because it's such a high impedance here of the second amplifier, there's
really no load to it.
We're going to use simply this 40, times 10 to the minus 3 Siemens, times 1K, which is
10 to the 3.
The gain is going to be 40, and if you, dimensionless, if you take the log of that and multiply that
by 20, it's going to equal 32dB.
Now let me back up just a touch.
We calculated, from our previous equation, voltage at the gate source of 3.365, which
was greater than V threshold, so things were working, and things were on.
What I did was, when I stuck that in this particular formula here, there's a slight
... With round-off, which was, with all the solving of that quadratic equation, I changed
my drain number slightly, but you can still use the 4.67 number, which was good.
If you use either one of them, and you multiply that by this 500-ohm resistor, you'll end
up with the voltage at the source, in this case here we have 2.195.
That all ties together, that's fine, but this is the gain.
The gain is going to be 32dB on the first stage.
If we want to look at the voltage on the drain, so we can figure out the voltage on the gate
here, we know what our current is, we know it drops through the 1K resistor, whatever
that voltage drop is, subtract it from 18, results in that.
It'll be something like, the voltage at the drain will be equal to 18, minus, because
we're using 4.39 as a number, I'm going to use that, minus 4.39 volts, and that's going
to be equal to 13.61 volts.
Now that's that circuit, so we've gone back full loop, we've got V gate, gate 2, right?
Put that into Kirchhoff's Voltage Laws, there's V gate source 2, and then there's I drain
2 through RS2, and that is your loop to solve for.
You substitute the expression for ... Move your variables around so that you've got VGS
equals ID and something else, stick that back into this particular formula.
Solve using a biquadratic equation, come up with two currents, and you have what's here.
That's fine.
You can set up your gate source there, if that's what is asked.
But the final part is, and this is what I want to get across, is: what is the voltage
gain for the second stage?
That is going to be this 560-ohm resistor in parallel with the 10K-ohm resistor, times
a gm of 29.5.
Then you take it in dBs, and your total gain, your overall gain then, so A total, let me
just put it here, will be A1, plus, in dB, A2.
Okay, that's in dB, and that's you'll have the total gain for this particular multistage
amplifier.
So, a bit of a beast, probably not too easy to find on the web if you go looking around,
simply because that's a fairly involved concept.
But it is Kirchhoff, and we started out this course with Ohm, and Kirchhoff, and voltage
dividers, and we're no further along, except with a couple of active devices to basically
amplify the particular voltages that we're using.
I think you're in very good shape in the understanding of the types of transistors we have out there,
basically two types.
My favorites are bipolar junction and EMOSFET.
There are a few other MOSFET styles, metal-oxide-semiconductor field-effect transistors, but really, they're
too confusing in order to turn them on, and EMOSFET is the most widely used in industry
today.
I leave that with you.
All the best with your exams, and we'll see what happens for the next series of videos.
Thank you.