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Very high frequency operation, sometimes called the radio frequency or RF

operation, necessitates considerations in addition to those we have given in the

previous videos. These considerations are summarized in

this video. In order to proper model the transistor

for very high-frequency operation, we need the non quasi-static small signal

equivalent circuit for the intrinsic part, which we have already derived.

And a model for the extrinsic part of the transistor, which has already been

discussed back when we considered the dynamic operation of the, the large

signal dynamic operation of the transistor.

So I will not repeat the discussion of these elements.

for hand analysis, this model is too complicated and sometimes the two source

resistances are lumped into one. The same is done for the drain

resistances and the four body resistances are lumped into one.

And you end up with this model. In addition, if you have interconnects

connecting the device to other elements, you need to model the inductance of those

interconnects. But this is supposed to be outside the

main transistor, so we don't show such inductances here.

I would like now to discuss the transition or cut-off frequency of the

device, including extrinsic parasitics. I remind you we have already discussed

the transition frequency for the intrinsic part of a transistor.

Now we will do so, for the complete device.

So here we have a device assumed to have source and body connected together.

A simplified model for it, is shown here. gate to source capacitance, gate body

capacitance. And gate drain capacitance are assumed to

be in parallel because we assume that the drain is grounded for small signals.

This simply means that there is a DC source, VDS, which does not have small

signal variations in it. Therefore it is represented here by a

short. So this allows us to use a single gate

capacitance which is the sum of Cgs, Cgb, and Cgd, and this is the source, excuse

me, the gate resistance. I'm driving the device with a current,

and I would like to look at the output current and consider the current gate.

The ratio of this current Io to this current, ii.

Now, the current ii goes through rge, the same current appears here.

And then it will create the voltage here which would be the current times the

impedance you see at this node. And the impedance you see at this node is

1 over j omega cg, where cg is the total gate capacitance.

So this is then the voltage at the internal gate note, the external note is

g, the internal node after the gate resistance, is g prime.

Notice that because the resistor is in series with ii, this voltage does not

depend on the resistor. So this current simply goes through the

resistor, comes out of here, and creates this voltage strong.

Now, to find this current here, I'm going to ignore the small current through Cgd

because we will not need to go to very high frequencies where this current would

be very large. And then the current you see is just gm

Vg prime s for Io. If for, instead of Vgs prime.

Vg prime s, we use this quantity here. We end up with this result.

So, Io is equal something times II. This something is the current gain.

Now I would like to find the frequency at which the magnitude of the current gain

becomes one. Notice by the way, that there is no Rge

here, no gate resistance, for the reason I have already explained.

So, at what frequency does this have a magnitude of one.

If you set this equal to one and solve for a magnitude you find this frequency.

So, omega T, the transition or cut-off frequency, which is the frequency at

which the current gain is reduced to the value of 1, is simply Gm over Cg.

So the detailed expression will, will depend the particular region of operation

we're in and what assumptions we make in terms of channel length and so on.

Let me give you two examples. If we assume no velocity saturation then

gm is given by this expression. We have already derived this way back.

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I would now like to talk about an important quality called the maximum

frequency of oscillation. In the development we did, we presented

above for the counter frequency, we saw that this quantity ignores the gate

resistance. Remember we driving the device the

current and the gate resistance was in series with that current.

So it did not figure in the calculating. Nevertheless, we know that the gate

resistant dissipates signal power, so it should have important effect on how the

device behaves. So, omega T, because it ignores Rge is a

poor figure of merits for some applications.

And after all, omega T has something to do with the current gain, and current

gain is not something that we're interested or in all of the time.

So we will talk about power gain now. The particular gain I will consider here

is called unilateral power gain and a simplified picture is as follows.

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You use a matching circuit at the input to make sure that the impedance seen by

the driving source, which is represented here by a voltage source in series with a

source resistance, is equal to Rs. It is known that then you get the maximum

power transfer into the device. Similarly we have another matching

circuit at the output so that the impedance seen by the transistor at the

input of the matching circuit, is ideal in terms of deriving as much power as

possible into the matching circuit. Because the matching circuits are assumed

to be made of lossless elements, all of the power that goes in comes out, and it

becomes the load power. Finally, we have a feedback cancelling

circuit, to cancel any feedback from the output to the input.

A more general description of this embeds the device into a four port.

We will not go into such details here. So now, when you divide the output power

by the input power, Po over Pi, you have the so-called unilateral power gain.

Which can be calculated to be given by this expression.

There are references in the book if you're interested in more detail.

This result assumes that the source series resistance is very small.

So notice that instead of the total gate resistance, we may have to use an

effective value for it, because of the distributed effects.

This was discussed back when we considered dynamic large signal operation

for the device. Now the maximum frequency of oscillation

is defined as the frequency at which the power gain, the unilateral power gain,

becomes equal to one. And that is the highest frequency at

which you can make the device oscillate, in a sense taking the output and feeding

it back into the input. If you have a gain of at least 1, power

gain of at least 1, you can maintain oscillations in such a circuit.

Now, to find the maximum frequency of oscillation we have to take the

unilateral power gain, set it equal to 1, and solve for omega.

And that is called omega max. It is the frequency at which the

unilateral power gain becomes 1. And it turns out to be given by this

expression, just by solving as I suggested.

Notice that omega max is omega T divided by this quantity and this quantity can be

smaller or larger than what. So omega max can be larger than omega T

or smaller than omega T. If we want a high omega max, we have to

shoot for a small series resistance. So here the resistance, the gate

resistance, not only clearly appears but it has a dominant effect.

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Now, sometimes in the literature, Gsd is ignored, which at high frequencies, is

okay, because this term will be much larger.

To minimize the gate resistance, devices can be laid out in this way.

This is a device with a large W, which has been split into several devices in

parallel. This is part of the source, this is part

of the drain, source, drain, source drain, and source.

Between this part of the drain and this part of the source, this is the channel.

So you're seeing here the gate polysilicon from the top, and between

them, between the source and the drain is the channel.

Now, although it is not shown here, all of the source strips are connected

together, and all of the drain strips are connected together.

And as you can see, all of the gates parts are connected together, so all of

these devices are in parallel. The length of the channel is this

distance here, and the W of the device is the sum of all of the devices that are

connected in parallel. Notice that the drain strip here has

channel both to its left and to its right.

So this is one way, and variations of this are used to make a device with a

small gate resistance, because all of the gate resistances are in parallel here.

So in this brief video, we talked about considerations for radio frequencies,

very high frequencies. And we showed that the non quasi-static

model for the intrinsic part of the device must be augmented by extrinsic

parasitic. We calculated the counter frequency of

the transistor including such parasitic. And then we defined and calculated the

maximum oscillation frequency, an important parameter in high frequency

work.