Re: mosfet RF amplifier with resistance

jure <jure@xxxxxxxx> wrote:

[...]

Mike,

If the input common source (CS) transistor sees an almost 0 ohm load
( the input Z of the transimpedance pair , if the loop gain is high
enough ) the time constant of that node will be very minimized.
Also, because this first stage has no voltage gain, Cgd1 is not
multiplied by Miller feedback when reflected into the input.
The CS input device has been unilateralized ... and its output behaves
close to an ideal current source.

Then, if ( BIG IF ) the second and third transistors could be modeled
as a TIA, with gain Rm, the total gain at midband frequencies is
(very) approximately equal to gm1.Rm .

The problem has now been transfered to the proper design of the
combination of M2 and M3.
Yet another twist would be to have M2 and M3 as cascode pair, instead
of a cascade CS - CS.

Here is my second attempt at ASCII art of the AC circuit.


|-----------|
|----VWV---|-----|| M3 |
| RL2 | |-->--| ///
/// | |
| |
|----| |
|----|| M2 |
| |-->-| |
| | |
| /// |
| |
| |
|----|-----------VWV---------|
IN ------|| M1 Rm
|-->---|
|
///

Thanks , Jure Z.

Thanks, Jure. I see that.

The only transistor supplying voltage gain is M2. M1 and M3 basically kill
Miller Effect and provide a low impedance drive to the next stage.

I wonder if that is the most effective use of the gain capability of the
three devices. For example, my understanding is the Tek 7400-series scopes
made a dramatic change in the way gain was obtained. Instead of going for
voltage gain in each stage, the designer went for current gain instead.

This obviously killed Miller effect, and reduced power consumption since
the only place voltage swing was needed was the last stage driving the crt.
Correct me if I'm wrong - the schematics really didn't tell much about what
was going on inside the chips. But that seemed a very effective way of
getting the high gain-bandwidth needed with a flat response.

Another configuration is travelling-wave, where low-gain transistors are
placed along a transmission line and operate in parallel. There, each one
contributes to the output signal. Horribly wasteful of power, but it
apparently satisfies a need.

So how do you decide which is the most overall effective use of the gain
potential of each device? Assume a 50 ohm environment and a reasonably low
noise figure.

BTW, I like your ascii art. Other people use a lot of funny characters,
and I have to reprogram my editor so it doesn't think they are erroneous
punctuation marks and try to correct them:)

Regards,

Mike Monett

.

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