Efficiency of Power Amplifiers
 

The latest QEX has an article by k9la: "The Impact of Load SWR
on the Efficiency of Power Amplifiers". The amplifier is designed
to drive a 50 ohm load and simulated measurements were made for
8 reflection coefficients of 0.333 with phase angles in 45 degree
increments.

One thing I don't understand. When driving a 100 ohm load, the
drain current is 33 amps and the efficiency is 62%. When driving
a 25 ohm load, the drain current is 12 amps and the efficiency
is 83%.

The amp is a Class E/F design from Jan/Feb 2004 QEX. Why would
the drain current fall when the load is decreased from 100 ohms
to 25 ohms?
--
73, Cecil, http://www.qsl.net/w5dxp


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Cecil Moore wrote:
The latest QEX has an article by k9la: "The Impact of Load SWR
on the Efficiency of Power Amplifiers". The amplifier is designed
to drive a 50 ohm load and simulated measurements were made for
8 reflection coefficients of 0.333 with phase angles in 45 degree
increments.

One thing I don't understand. When driving a 100 ohm load, the
drain current is 33 amps and the efficiency is 62%. When driving
a 25 ohm load, the drain current is 12 amps and the efficiency
is 83%.

The amp is a Class E/F design from Jan/Feb 2004 QEX. Why would
the drain current fall when the load is decreased from 100 ohms
to 25 ohms?

The output filter of an amplifier can act like a 1/4 wave (or 3/4 wave)
transmission line, so a high impedance at the antenna terminal would
look like a low impedance at the final, and visa-versa. This extends to
the time-delay properties: older oscilloscopes with time delay used long
L/C filters operated way below cutoff to simulate even longer stretches
of transmission lines.

In fact, a filter that looks like this

___
--------o---UUU---o--------
| |
--- ---
--- ---
| |
| |
=== ===
GND GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

with all reactances equal to the design impedance (say 50 ohms) at the
design frequency is _called_ a 1/4-wave filter. Put two of them
together and you get a 1/2 wave filter:

___ ___
--------o---UUU---o---UUU---o------
| | |
--- --- ---
C --- 2C --- C ---
| | |
| | |
=== === ===
GND GND GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Stick on another section and it'll act like a 3/4 wave line, etc.

--

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

Tim Wescott wrote:

Cecil Moore wrote:
The amp is a Class E/F design from Jan/Feb 2004 QEX. Why would
the drain current fall when the load is decreased from 100 ohms
to 25 ohms?

The output filter of an amplifier can act like a 1/4 wave (or 3/4 wave)
transmission line, ...

I should have described the output circuitry. It's a push-pull
LC tank circuit with DC supplied by the coil center-tap. The
output is just a link coupler. The low-pass filter is a single
series L and single C to ground.
--
73, Cecil, http://www.qsl.net/w5dxp


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Cecil Moore wrote:

The latest QEX has an article by k9la: "The Impact of Load SWR
on the Efficiency of Power Amplifiers". The amplifier is designed
to drive a 50 ohm load and simulated measurements were made for
8 reflection coefficients of 0.333 with phase angles in 45 degree
increments.

One thing I don't understand. When driving a 100 ohm load, the
drain current is 33 amps and the efficiency is 62%. When driving
a 25 ohm load, the drain current is 12 amps and the efficiency
is 83%.

The amp is a Class E/F design from Jan/Feb 2004 QEX. Why would
the drain current fall when the load is decreased from 100 ohms
to 25 ohms?

How has the QL changed as a function of load? How has the power output
changed from 110 to 25 ohms load? Have you calculated the effective load
lines for the amplifiers?

We need a lot more information.

I don't subscribe to QEX so I don't have the circuit available.

I shouldn't, but will. You don't say, but I think you are puzzled by a
decreasing load impedance causing a decrease in collector current. I'd have
to think about it, but I'd first Try modeling that LC filter at resonance to
see if it gives an inverse function like the 1/4 wave line.
A "gut" reaction tells me that a decreasing collector current for *either*
an increased or decreased load Z would not be a surprise...

Steve K9DCI


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

Cecil Moore wrote:
The amp is a Class E/F design from Jan/Feb 2004 QEX. Why would
the drain current fall when the load is decreased from 100 ohms
to 25 ohms?

The output filter of an amplifier can act like a 1/4 wave (or 3/4 wave)
transmission line, ...

I should have described the output circuitry. It's a push-pull
LC tank circuit with DC supplied by the coil center-tap. The
output is just a link coupler. The low-pass filter is a single
series L and single C to ground.
--
73, Cecil, http://www.qsl.net/w5dxp


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Steve Nosko wrote:
I shouldn't, but will. You don't say, but I think you are puzzled by a
decreasing load impedance causing a decrease in collector current. I'd have
to think about it, but I'd first Try modeling that LC filter at resonance to
see if it gives an inverse function like the 1/4 wave line.
A "gut" reaction tells me that a decreasing collector current for *either*
an increased or decreased load Z would not be a surprise...

The output is link coupled through a series 216 nH coil and a 2.1 nF cap
to ground. The load is connected from the coil/cap junction to ground.
It's a Class E/F 40m amp.

Load Drain Current Drain Efficiency Load Power
25 ohms 12.2 amps 82.6% 129 watts
50 ohms 20.1 amps 77.7% 200 watts
100 ohms 33.1 amps 62.3% 264 watts

Can these be thought of as load-pulling data points? Maximum load
power appears to occur when the load is greater than 100 ohms.
--
73, Cecil http://www.qsl.net/w5dxp

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Can I drive a class E amp with an SSB signal?

Thanks,

The Eternal Squire

wrote:
Can I drive a class E amp with an SSB signal?

You can but you won't like the results. :-)
--
73, Cecil http://www.qsl.net/w5dxp

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"Cecil Moore" wrote in message
...
wrote:
Can I drive a class E amp with an SSB signal?

You can but you won't like the results. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Funny, Nate Sokal, who owned the patent thinks you can.

W4ZCB

Harold E. Johnson wrote:
"Cecil Moore" wrote:

wrote:
Can I drive a class E amp with an SSB signal?

You can but you won't like the results. :-)

Funny, Nate Sokal, who owned the patent thinks you can.

I'm assuming that "drive a class E amp with an SSB signal"
means an SSB signal on the input, i.e. linear amplification
is required. How does Mr. Sokal linearize the non-linear
characteristics of the Class-E amp? I'm not saying it can't
be done - but I don't know how to do it and there's no claim
of linear operation on any of the Class-E Amp web pages.
--
73, Cecil http://www.qsl.net/w5dxp


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"Cecil Moore" wrote in message
...
Harold E. Johnson wrote:
"Cecil Moore" wrote:

wrote:
Can I drive a class E amp with an SSB signal?

You can but you won't like the results. :-)

Funny, Nate Sokal, who owned the patent thinks you can.

I'm assuming that "drive a class E amp with an SSB signal"
means an SSB signal on the input, i.e. linear amplification
is required.

its only required if you want the output to sound like the input. there are
lots of amplifiers on 27mhz and other bands where this is not a requirement.

Dave wrote:

"Cecil Moore" wrote:
I'm assuming that "drive a class E amp with an SSB signal"
means an SSB signal on the input, i.e. linear amplification
is required.

its only required if you want the output to sound like the input. there are
lots of amplifiers on 27mhz and other bands where this is not a requirement.

Well, maybe I presumed too much. I certainly wouldn't like it but
of course, I wouldn't force my likes and dislikes on everyone. :-)
--
73, Cecil http://www.qsl.net/w5dxp


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"Cecil Moore" wrote in message
...

Load Drain Current Drain Efficiency Load Power
25 ohms 12.2 amps 82.6% 129 watts
50 ohms 20.1 amps 77.7% 200 watts
100 ohms 33.1 amps 62.3% 264 watts

Can these be thought of as load-pulling data points? Maximum load
power appears to occur when the load is greater than 100 ohms.
--
73, Cecil http://www.qsl.net/w5dxp

Have you considered the possibility that he might have the data for 25 Ohms
and 100 Ohms interchanged? The amplifier is voltage limited at, it appears,
12.8 V. I would believe 25 Ohms, 33.1A, and 264W.

Tam/WB2TT

Tam/WB2TT wrote:
Have you considered the possibility that he might have the data for 25 Ohms
and 100 Ohms interchanged?

Nope, I accepted his data at face value and am looking
for an explanation.
--
73, Cecil http://www.qsl.net/w5dxp

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