Hi,

I am looking for some help in understanding amplifier concepts (solid
state only) and product specifications. I have searched for
definitions and essays on the topic, but , having no electronics
exposure, I find it hard to understand the concepts clearly. So if
someone can please help:

1. What actually is amplification of a RF signal: is it the scaling up
of the amplitude?


I want to amplify a signal where the output has very low distortion
and noise. It should be as close to the input signal as possible:

2. Does push-pull setup mean that it's a class AB amplifier in which
two amplifiers amplify the same signal, but the opposite two halves of
the input signal?

If this is true, in an amplifier like this:
http://www.communication-concepts.com/ar347.htm

If two solid state amplifiers are rated at 600W each, and it's a push-
pull arrangement, how come the output of this amplifier is 1000 watts
(or 1.2 kw theoretically)
If one Motorola amplifier is amplifying half the signal, the other
amplifying the other half, wouldn't the total output be 600W for the
whole signal?


3. In class AB amplifiers, is the output signal an exact replica (but
amplified) version of the input signal?

4. What is an octave when dealing with amplifiers?


5. Is this communication concepts HF 1000 watt amplifier a class A,
class AB, or class C amplifier?
http://www.communication-concepts.com/ar347.htm

6. Does anyone know the detailed specs for this same communication-
concepts 1000 watt HF amplifier?
Specs like:
a. Duty cycle
b. Phase stability
c. Phase change with power
d. Gain flatness
e. Pulse droop
f. Harmonics
g. Spurious
h. Rise/fall times
i. VSWR or load SWR
j. Gate delay



7 Can this amplifier be "controlled" to give graded amplification less
than 1000 watts specified, or will it work only at this maximum level?

Any help is appreciated.

Thanks

Vijay

wrote:
Hi,

I am looking for some help in understanding amplifier concepts (solid
state only) and product specifications. I have searched for
definitions and essays on the topic, but , having no electronics
exposure, I find it hard to understand the concepts clearly. So if
someone can please help:

1. What actually is amplification of a RF signal: is it the scaling up
of the amplitude?

In electronics, amplification of _any_ signal is it's scaling up in
amplitude or power.

I want to amplify a signal where the output has very low distortion
and noise. It should be as close to the input signal as possible:

2. Does push-pull setup mean that it's a class AB amplifier in which
two amplifiers amplify the same signal, but the opposite two halves of
the input signal?

Take out the "class AB" from your above statement, and the answer is
"yes". You can have push-pull class A or C amplifiers as well.

If this is true, in an amplifier like this:
http://www.communication-concepts.com/ar347.htm

If two solid state amplifiers are rated at 600W each, and it's a push-
pull arrangement, how come the output of this amplifier is 1000 watts
(or 1.2 kw theoretically)
If one Motorola amplifier is amplifying half the signal, the other
amplifying the other half, wouldn't the total output be 600W for the
whole signal?

There are a number of reasons for not pushing the output devices to
their limits, and there is always a certain amount of loss in the
coupling network. Just because the transistor _could_ generate 600W in
the right circuit doesn't mean that every designer _would_ make it so.

3. In class AB amplifiers, is the output signal an exact replica (but
amplified) version of the input signal?

We wish. Ideally, yes that is the case. Practically the output signal
will be filtered, delayed, and distorted but not too much.

4. What is an octave when dealing with amplifiers?

A two-times change in frequency, just like it is on a piano.

5. Is this communication concepts HF 1000 watt amplifier a class A,
class AB, or class C amplifier?
http://www.communication-concepts.com/ar347.htm

The page mentions that it's a linear amp, so it's probably class AB.
Class A would be terribly inefficient, and class C wouldn't be linear.
Communications Concepts builds amplifiers from Motorola applications
notes -- your best bet would be to find them and read what the app note
has to say. I _think_ that the high-end RF transistor line was retained
by Motorola when it spun off On semiconductor, so you probably need to
check the Freescale website (Motorola then spun off Freescale). It may
now be an On part, however. If you don't find it, ask here.

6. Does anyone know the detailed specs for this same communication-
concepts 1000 watt HF amplifier?
Specs like:
a. Duty cycle
b. Phase stability
c. Phase change with power
d. Gain flatness
e. Pulse droop
f. Harmonics
g. Spurious
h. Rise/fall times
i. VSWR or load SWR
j. Gate delay

Check the app note.

7 Can this amplifier be "controlled" to give graded amplification less
than 1000 watts specified, or will it work only at this maximum level?

The amplifier is linear, so it will have a more or less fixed
amplification. To get less power out in it's linear region, just put a
smaller signal in.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

I like Tim;s explanation, but since it isn't real clear where you are
heading, there are a couple other points I'd like to make.

"Tim Wescott" wrote in message
...
wrote:

[ BIG CLIP ]

In electronics, amplification of _any_ signal is it's scaling up in
amplitude or power.

Understand that theoretically, increasing the amplitude increases the power.
The reality, especially with RF, and especially with solid state amplifiers,
is not quite so simple. In order to transfer power from one RF stage to
another, or from an output stage to an antenna, we need to match the input
impedance of the following stage with the output impedance of the preceding
stage. If the impedances don't match, not all the power will be
transferred, and the portion that is transferred is likely to be distorted,
perhaps badly.

I turns out that the output impedance of a solid state amplifier is
dependent on the output power, with higher power implying a lower
impedance -- often quite low. Generally, our antennas, and most larger
scale modules, have, by convention, a 50 ohm impedance. (Internal stages
often do as well, but there is no reason a designer might not pick something
else 'inside the box').

Normal practice is to follow the amplifier with a matching network
(typically still inside the amplifier box, however), so that the amp you buy
has a 50 ohm impedance output. This matching network actually distorts the
signal, really, deliberately. The result is what we want for a typical RF
amplifier application.

Your question sounds a bit as if you might not be interested in the
"typical" RF application, so the typical answers might not be quite what you
are looking for.

[ANOTHER BIG CLIP]

We wish. Ideally, yes that is the case. Practically the output signal
will be filtered, delayed, and distorted but not too much.


And again, this linearity is right for typical applications, but again, for
TYPICAL applications. Obviously, if an amplifier produces 1000 watts out
with 100 watts drive, and it is linear, it will produce 500 watts out with
50 watts drive. But if you give it one watt drive, it might not produce 10
watts, and what it does produce might be quite distorted and masked with
noise or other artifacts. Again, within reasonable limits, in typical
applications, these things behave as you would expect. But in some odd
application, or at the liimits of their useable range, the results might not
be close enough to theoretical to satisfy you.

The same is true if your input waveform is not what one would expect. For
communications service, the expected waveform is really quite predictable,
and the design of the impedance matching circuits makes assumptions about
these waveforms. If your input waveform is far from what would be expected,
then you might experience a lot of distortion of the input waveform, even
though a non-critical look appears to be within the operating range. For
example, if you were to drive that amp with a 50 MHz square wave, the output
waveform would probably look more like a sine wave than a square wave.

...

Thank you guys for the detailed explanations. The underlying concepts
are much clearer now

vij