1/4 vs 1/2 wavelength antenna

[Ken Smith]

[...]

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?

 

[Ken Smith]

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.

 

[Ken Smith]

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

 

[Jerry Martes]

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

 

[Tom Ring]

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

 

[Tom Ring]

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

 

[Rich Grise]

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

 

[Rich Grise]

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

That works because the transmission line is less than 0.01 wavelength.
So impedance matching becomes moot. If the speaker line were 1/4 wavelength
long, there would be almost no signal transferred at all.

Cheers!
Rich

 

[Reg Edwards]

Arguments about whether the power ammplifier is matched or not matched to 50
ohms arise due to misunderstandings about the meaning of "matched".

Meaning 1.
-------------
The PA has been designed for maximum, linear, undistorted power output when
loaded with Ro ohms and the load reistance has actually been adjusted to
equal Ro. Ro is usually 50 ohms. (There may be additional criteria to
define what constitutes an optimum match.)

Meaning 2.
-------------
The load impedance Z = R+jX has been adjusted to equal the conjugate of the
internal impedance resistance of the PA. (The internal impedance of the PA
is usually unknown but the circuit is assumed to behave as if a conjugate
match exists.)

The two meanings are entirely different from each other. If there is danger
of confusion then the meaning should be stated. Some people already use the
descriptions "Zo match" and "Conjugate match".

 

[Ken Smith]

That works because the transmission line is less than 0.01 wavelength.
So impedance matching becomes moot. If the speaker line were 1/4 wavelength
long, there would be almost no signal transferred at all.

( Unless you use the specially tapered euphonic cable! )


Actually, if you used a thick enough wire, you could go 1/4 wavelength at
audio frequencies. If you connect a speaker straight onto the mains,
chances are you are going further than that from the generator.

 

[Ken Smith]

Arguments about whether the power ammplifier is matched or not matched to 50
ohms arise due to misunderstandings about the meaning of "matched".

Meaning 1.
-------------
The PA has been designed for maximum, linear, undistorted power output when
loaded with Ro ohms and the load reistance has actually been adjusted to
equal Ro. Ro is usually 50 ohms. (There may be additional criteria to
define what constitutes an optimum match.)

Meaning 2.
-------------
The load impedance Z = R+jX has been adjusted to equal the conjugate of the
internal impedance resistance of the PA. (The internal impedance of the PA
is usually unknown but the circuit is assumed to behave as if a conjugate
match exists.)

Meaning 3:
The PA has been designed to deliver the maximum power at that load
impedance and the distortion is not an issue.

The two meanings are entirely different from each other. If there is danger
of confusion then the meaning should be stated. Some people already use the
descriptions "Zo match" and "Conjugate match".

Actually meanings 2 and 3 are effectively equal in the case of the tuned
system, if you define the Zo based on the change in output power vs
connected impedance for small changes. Since a lot of such systems aren't
linear, this is the way you end up having to define the impedance. You
can't use open circuit voltage and short circuit current.

Remember that this all started with the OP having a "transmitter". This
would include any needed filtering. He was just connecting a 1/4 and 1/2
wave lengths of bent up wire. His output filter, I assume is just a bunch
of LC sections.

 

[Tam/WB2TT]

That works because the transmission line is less than 0.01 wavelength.
So impedance matching becomes moot. If the speaker line were 1/4
wavelength
long, there would be almost no signal transferred at all.

Cheers!
Rich

There is nothing wrong with driving a transmission line/antenna from a zero
impedance source. It does NOT change the SWR. The point is that an audio
amplifier with a damping factor of 50 is NOT conjugate matched.

Somebody mentioned Motorola Application note 721. This is what it says:

****************************************************************************************
" ..the load, in first approximation, is not related to the device, except
for VCE(sat). The load value is primarily dictated by the required output
power and the peak voltage; it is not matched to the output impedance of the
device. "
*******************************************************************************************

When device people talk about "matching", they mean matching the load to
what the transistor wants to see, which is not the conjugate of the output
impedance. The way this is done is to build an amplifier, and vary the load
until maximum output power is reached. The transistor is then removed, and
the impedance looking into the coupling network is measured. The conjugate
of this is sometimes listed as "output impedance" on data sheets. Newer data
sheets will have an asterisk * next to that, and a note explaining what it
means. If you look at Philips literature, you will see exactly the same
explanation.

This whole thing has been hashed out here about 5 times during the past
year.

Tam

 

[Tam/WB2TT]

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
[/QUOTE]

Ken,
Motorola 721 specifically says the device IS NOT MATCHED.

Tam

 

[Ken Smith]

Ken,
Motorola 721 specifically says the device IS NOT MATCHED.

It say " Strictly speaking..." is that the section you are refering to?

Yes they do say that but then if you follow the design through, I think
you will find that they try to cancel the reactive component. If you then
put in the output device protection they didn't include, you end up with
the matching as I explained elsewhere. As the impedance moves away from
what the designer intended, the radiated power decreases.

The point is that the OP was taking about hooking a "transmitter" to a
length of wire. I was talking about the matching from a complete
transmitter which I assume contains such protection to this length of
wire.

 

[Cecil Moore]

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.

So what impedance does the reflected wave encounter?

 

[Ken Smith]

When device people talk about "matching", they mean matching the load to
what the transistor wants to see, which is not the conjugate of the output
impedance.

Actually, in this case, I was speaking of matching the transmitter's
output to the load. The transmitter already contains gawd knows what L
and C components etc. The OP has a completed transmitter and a hunk of
wire. If he matches the wire to what the transmitter wants to see, the
transmitter will be happy. If he causes a reactive current to flow that
the designer did not design for he will cause added heating in the output
device. If the designer did a good job, the transmitter will protect its
output devices and thus end up producing less power.

Also if he makes the real component of the impedance vary from what the
designer intended, the output power will decrease. Which direction gets
limited by the Vcc and which by the protection circuit depends on the
collection of Ls and Cs inside the transmitter.

 

[Cecil Moore]

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.

Let's assume the designer is an amateur who didn't provide
any protection for his tube's output. The lower the resistive
load, the more current the output device draws until it fails.
What is the output impedance of the device?

 

[Cecil Moore]

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!)

And there's the catch. If the load line is the source
impedance, the load (not the designer) effects the source
impedance.

 

[Reg Edwards]

The phrase "output impedance" in connection with amplifiers is ambiguous and
likely to result in arguments.

The correct description is "internal impedance" or "internal resistance" and
should always be used.

 

[Cecil Moore]

If you then
put in the output device protection they didn't include, you end up with
the matching as I explained elsewhere.

SWR foldback is part of impedance matching?