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Old February 26th 05, 01:09 AM
gwhite
 
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Cecil Moore wrote:


What is the steady-state impedance of the
source at the fundamental frequency?



Exactly Cecil. That's exactly why I said the concept of impedance is dubious
for large signal devices and thus provides a second argument, albeit weaker,
against the "impedance matching" idea for power amps.

"The concept of 'output impedance' breaks down
for large signal devices. For example, what is
the output impedance of a class C or D amp taken
when the transistor is on or off?"

The strongest argument for dropping the impedance matching concept is PA
efficiency, and therefore maximum signal swing. Obtaining maximum swing is a
load line issue.
  #22   Report Post  
Old February 26th 05, 01:20 AM
Ken Smith
 
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In article , Cecil Moore wrote:
Ken Smith wrote:
RF transmitter power amps are certainly "impedance matched" to the
intended load. Take a look in the ARRL "The radio amateur's handbook".
If you have the 1944 addition, you will need to start reading at page 96
in the lower right column. If you don't have that, try Motorola's AN-721.


A CMOS Class-E amp is in full saturation (0.5v at 2a)
for 10% of a cycle and off (12v at 0a) for the other
90% of a cycle. The tank circuit changes the digital
energy to analog energy by filtering out everything
except the fundamental frequency component. How
in the world does one determine the steady-state
impedance of the CMOS source? Isn't the best one
can do with a digital switch is to keep it within
specified parameters? The CMOS device dissipates 2
watts for 10% of the time - therefore 0.2 watts
steady-state.


For what you say here really to be true the transistors must switch very
fast. About 25pS switching speed is needed at about 400KHz. If we take
that to be the case however, I think you will see why matching still
applies.

Lets take the reactive component first. If there is a reactive component
to the loading, the current in the switch will have a higher RMS value
without that increase in RMS increasing the radiated power of the system.
So the reactive component of the matching is fairly obvious.

Imagine that you have a well designed Class-E circuit loaded with the load
the designer optimized it for.

Now imagine that you slightly increase the resistance slightly. When you
do so, the current into the load will decrease but the voltage will not
increase enough to compensate for this.

Now lets assume that you slightly decrease the resistance. Since we are
assuming that this is a well designed case, we can assume that the
designer took steps to ensure that the output devices would be protected
from excess currents. This could be done by reducing the operating
voltage of the output section, for example. In any case, the voltage on
the load will decrease by a larger factor than the current will increase.

So it is obvious that the reactive part is matched and the resistive part
is matched just as it would be in a non-class-E output section.


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Old February 26th 05, 01:21 AM
Ken Smith
 
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In article , Cecil Moore wrote:
Ken Smith wrote:
RF transmitter power amps are certainly "impedance matched" to the
intended load. Take a look in the ARRL "The radio amateur's handbook".
If you have the 1944 addition, you will need to start reading at page 96
in the lower right column. If you don't have that, try Motorola's AN-721.


It appears that I may have canceled an earlier posting
by accident so will repeat it.


I answered your other copy of the posting.


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Old February 26th 05, 01:23 AM
Ken Smith
 
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In article ,
Jim Thompson wrote:
On Fri, 25 Feb 2005 09:47:16 -0600, Cecil Moore
wrote:

[...]
A certain Class-E CMOS amp is in full saturation for
10% of a cycle, 0.5v at 2a. For the rest of the time
it is off. The supply voltage is 12v. What is the
steady-state impedance of the source at the fundamental
frequency?



Now, now, Cecil! Don't sully the thread with facts !-)


This needs a 12V transistor that switches in about 25pS. Would that be
likely on a current chip?

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Old February 26th 05, 01:34 AM
Ken Smith
 
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In article , gwhite wrote:
Cecil Moore wrote:

[...]
"The concept of 'output impedance' breaks down
for large signal devices.


Motorola seems to disagree with you. Perhaps more importantly, I disagree
with you. The large signal impedance of a transistor working into a tuned
load still applies quite well.

For example, what is
the output impedance of a class C or D amp taken
when the transistor is on or off?"


When dealing with an RF output section you don't deal with just one part
of the cycle. You can tell if the real part of the impedance is matched
if:

(1) If you increase the resistance of the load does the power
decrease?

AND

(2) If you decrease the resistance of the load does the power decrease?


The strongest argument for dropping the impedance matching concept is PA
efficiency, and therefore maximum signal swing. Obtaining maximum swing is a
load line issue.


What do you mean by "maximum signal swing" in this context. I can get a
bigger swing by leaving the output completely unloaded and hence causing
the actual efficiency to be zero.

The reactive component issue is still there too. Reactive loads cause
increased currents in the output stage without delivering any power to the
load so they still need to be reduced as much as practical.




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Old February 26th 05, 01:40 AM
Ken Smith
 
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In article , gwhite wrote:
Ken Smith wrote:


RF transmitter power amps are certainly "impedance matched" to the
intended load.


I'm sorry, but they are not. Nor are any power amps that I know of. Efficiency
(and thus necessarily output swing) is what matters for power amps. To maximize
swing requires load line matching, not impedance matching.


I still say they are. Motorola AN-721 takes on the theory. AN-758 does a
practical example matching 12.5 Ohms into 50 Ohms


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Old February 26th 05, 02:05 AM
Jerry Martes
 
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"Tam/WB2TT" wrote in message
...

"gwhite" wrote in message
...
Richard Clark wrote:

On Wed, 23 Feb 2005 19:08:20 GMT, gwhite wrote:

RF transmitters are not ....

Sorry OM,

This was all nonsense.


Nice articulation. I don't know who OM is, but RF transmitter power amps
are
not "impedance matched." Neither are audio power amps for that matter.


My stereo amp has a spec on output impedance. As I recall, it was around
0.16 Ohms. Intended load is 4 - 16 Ohms.

Tam


Tam

An audio amplifier can have an improved capability of minimizing a
speaker's deviation from its desired displacement if it has a low output
impedance. That is, the speaker's cone tends to swing past its desired
displacement when its terminals are loaded to a high impedance.

Jerry




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Old February 26th 05, 02:26 AM
Tom Ring
 
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Ken Smith wrote:

In article , gwhite wrote:

Ken Smith wrote:

RF transmitter power amps are certainly "impedance matched" to the
intended load.


I'm sorry, but they are not. Nor are any power amps that I know of. Efficiency
(and thus necessarily output swing) is what matters for power amps. To maximize
swing requires load line matching, not impedance matching.



I still say they are. Motorola AN-721 takes on the theory. AN-758 does a
practical example matching 12.5 Ohms into 50 Ohms



Sure are. A 30 year ago 500 level course I took called Non-Linear
Transistor Design said so. The way you handle class C etc. is by
handling each harmonic separately in your analysis of the transistor
plus tank circuit. You match to the harmonic you want. It may be the
fundamental, or the third for a tripler, etc.

tom
K0TAR



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Old February 26th 05, 02:33 AM
Tom Ring
 
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Jerry Martes wrote:

"Tam/WB2TT" wrote in message
...

"gwhite" wrote in message
...

Richard Clark wrote:

On Wed, 23 Feb 2005 19:08:20 GMT, gwhite wrote:


RF transmitters are not ....

Sorry OM,

This was all nonsense.

Nice articulation. I don't know who OM is, but RF transmitter power amps
are
not "impedance matched." Neither are audio power amps for that matter.


My stereo amp has a spec on output impedance. As I recall, it was around
0.16 Ohms. Intended load is 4 - 16 Ohms.

Tam



Tam

An audio amplifier can have an improved capability of minimizing a
speaker's deviation from its desired displacement if it has a low output
impedance. That is, the speaker's cone tends to swing past its desired
displacement when its terminals are loaded to a high impedance.

Jerry


Correct - in the case of an audio amplifier damping factor is critical
to accurate reproduction of transients. In an RF amp, it is normally
not required or desired.

tom
K0TAR

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Old February 26th 05, 03:09 AM
Rich Grise
 
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On Sat, 26 Feb 2005 00:40:09 +0000, Ken Smith wrote:

In article , gwhite wrote:
Ken Smith wrote:


RF transmitter power amps are certainly "impedance matched" to the
intended load.


I'm sorry, but they are not. Nor are any power amps that I know of. Efficiency
(and thus necessarily output swing) is what matters for power amps. To maximize
swing requires load line matching, not impedance matching.


I still say they are. Motorola AN-721 takes on the theory. AN-758 does a
practical example matching 12.5 Ohms into 50 Ohms



Evidently, the guy's never tuned up a 40 meter pi-net output transmitter. ;-)

If that's not impedance matching, I don't know what it is! (Oh, "Load line"
matching? What are the two parameters of the load line? Voltage and Current,
right? What's the slope of the load line? Impedance!)

Cheers!
Rich


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