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Author Topic: Do solid state amps need be push-pull?  (Read 9850 times)
NO9E
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« on: November 28, 2012, 06:14:55 AM »

Virtually all HF solid state amps are push-pull. The transistors operate in AB class and the output is filtered. Nice broadband operation but low efficiency (40-50%) and associated large PS + large heatsinks. Then noisy fans if high power.

For CW, class B or even C would be good enough but this does not work with broadband transformers.

For single frequency one can have an amp with tuned circuits and 70% efficiency, even on 2 m (http://www.w6pql.com/1_kw_2m_ldmos_amplifier.htm). The key here is tuned and not broadband circuit on output. Forget about linearity here and agree that we consider CW initially.

Is it possible to have switched tuned circuits on HF? Are currents too big to be switched, stray inductances too big, or is there something else?

This thread is motivated by by KX3 and 2K-FA amps. KX3 takes about 13V and 2.3A (30W) to produce 10-11W. Svereal operators pointed to overheating and then power reduction when running in warm environments.

A 2 KW Expert 2K-FA has 4 fans that are really loud when transmitting.   With higher efficiency, weight could drop from 56 to 35 or even 30 lb.

Ignacy, NO9E

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WX7G
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« Reply #1 on: November 28, 2012, 07:16:32 AM »

For CW class-B would be acceptable but class-C would not. CW is amplitude modulated and class-C will decrease the waveform rise/fall times and cause key clicks.

What makes HF push-pull amps 45% efficient? It is that to produce rated power into a 1.5:1 VSWR the amp is designed to produce full power into 75 ohms. Into 50 ohms the collector/drain does not swing as far and the current is higher. Run a SS amp into a 75 ohm load at full power and the efficiency will be up there with tuned output vacuum tube amps.
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G3RZP
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« Reply #2 on: November 28, 2012, 08:18:09 AM »

The matching circuits aren't that easily switched at impedances of  2 or 3 ohms. Push-pull gives an advantage in even order harmonic reduction, too.

There have been one or two tuned single ended SS linear PAs done, but at relatively low power and long ago - the Labgear LPR30 packset of the 1960s comes to mind. About 6 to 10 watts, depending, and that was pretty well Class A.
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KE5JPP
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« Reply #3 on: November 28, 2012, 08:45:15 AM »

Virtually all HF solid state amps are push-pull.

This is not necessarily true for power levels < 5W.  For example a single RD16HHF1 will make a clean ~5 W (-40 dB or better IMD3) in class A operation (600 mA bias) at 13.8 VDC.

Gene
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M0HCN
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« Reply #4 on: November 28, 2012, 11:29:00 AM »

One could fairly easily place current and voltage transformers on the line after the amp, measure the required drain voltage swing and then control the power supply regulation loop so as to run lower drain voltage at 1:1 then at high Z 1.5:1 (And lower still at low Z 1.5:1).

Designing a buck converter to do this is not hard, and while it is not anyhting like as good as a full on envelope tracker it is much easier to implement in an external amp.

Regards, Dan.
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WX7G
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« Reply #5 on: November 28, 2012, 11:41:27 AM »

Dan, that is an excellent idea and as you say easy enough to implement.
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KB1GMX
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« Reply #6 on: November 28, 2012, 02:24:07 PM »

I'm running a MRF140 single ended on 6M and its as good as any and
limited in IMD by the radio.  However that's a narrow band match (6m only).

I've also built wide band single ended (class AB) and they work fine but the
efficiency is more like 35% maybe 40 and the power is half.  Push pull has that
advantage of two devices making near twice the power.

Allison
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M0HCN
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« Reply #7 on: November 28, 2012, 03:31:01 PM »

Also PP helps massively with second harmonic, which makes sharing output filters between some bands rather easier, the savings in toroids and high voltage caps might well pretty much cover the extra transistor cost.

Single ended class A is fine for QRP and low power driver stages, but usually the finals will want to be push pull AB if you are trying to design a multi band amp in the usual sort of power ratings, narrow band amps can easily be single ended if a suitable device is available.

73 Dan.
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TANAKASAN
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« Reply #8 on: November 28, 2012, 11:06:23 PM »

From WX7G

"What makes HF push-pull amps 45% efficient? It is that to produce rated power into a 1.5:1 VSWR the amp is designed to produce full power into 75 ohms. Into 50 ohms the collector/drain does not swing as far and the current is higher. Run a SS amp into a 75 ohm load at full power and the efficiency will be up there with tuned output vacuum tube amps."

I've just had an idea (and feel free to call me crazy but explain why). The low pass filters we must use on an amp are normally designed for fifty ohms in and out, so why not reconfigure these filters for a seventy five ohm input impedance then use them to match to fifty ohms?

Tanakasan
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WX7G
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« Reply #9 on: November 29, 2012, 01:57:06 AM »

I think the best solution to provide high DC-RF efficiency is to use an antenna tuner to provide the load impedance at which amplifier efficiency is maximized. This is what tube amps do with their built-in tunable pi-networks.
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VE1IDX
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« Reply #10 on: November 29, 2012, 01:36:12 PM »

For CW class-B would be acceptable but class-C would not. CW is amplitude modulated and class-C will decrease the waveform rise/fall times and cause key clicks.

What makes HF push-pull amps 45% efficient? It is that to produce rated power into a 1.5:1 VSWR the amp is designed to produce full power into 75 ohms. Into 50 ohms the collector/drain does not swing as far and the current is higher. Run a SS amp into a 75 ohm load at full power and the efficiency will be up there with tuned output vacuum tube amps.


Since when was class C not acceptable for CW and when did CW become amplitude modulated? I must have missed those memos.
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WX7G
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« Reply #11 on: November 29, 2012, 01:41:51 PM »

OOK CW is keyed on and off. The RF waveform from the exciter has rise/fall times that are intentionally about 5 ms. It is amplitude modulated. If you view this in the frequency domain there is a carrier, an USB, and a LSB.

A non-linear amplifier will decrease this intentional rise/fall time and in the frequency domain will widen the signal. How far one runs an amplifier into Class-C will depend on how wide a CW signal can be tolerated. 
« Last Edit: November 29, 2012, 01:46:10 PM by WX7G » Logged
VE1IDX
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« Reply #12 on: November 29, 2012, 02:09:30 PM »

I figured you were considering the rise/fall times as a form of amplitude modulation however it is not really. A moderate rise/fall time is preferred over hard and instantaneous ON/OFF keying which can cause key clicks and a wider than necessary bandwidth. Any mode of transmission has a finite rise/fall time including an FM transmitter. This is not the same as modulating the carrier. Class C is and always has been a perfectly fine class of amplifier for CW transmission as well as FM and phase modulation.
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M0HCN
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« Reply #13 on: November 29, 2012, 02:30:10 PM »

A 100W CW transmitter sure looks like 100% amplitude modulation of a 25W carrier with the keying envelope biased appropriately (The waveforms are identical).

Now for a CW transmitter you can of course use class C or even class E by modulating the supply to the final amplifier with the keying waveform, but straight class C as one would use for FM results in horrific key clicks.

FM only has a rise or fall at the start and end of the transmission, and even these are usually slowed in rigs using AB power amps by the bandwidth limitation imposed by the transmit IF filtering, a click once per over is much less objectionable then the clicking from 30wpm morse on something excessively wide.

73 Dan.
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KE5JPP
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« Reply #14 on: November 29, 2012, 03:17:50 PM »

I figured you were considering the rise/fall times as a form of amplitude modulation however it is not really.

How then do you account for the sidebands that CW keying creates if it is not a form of amplitude modulation?

Gene
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