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Author Topic: T-1000 filter question  (Read 7326 times)
W8JI
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« Reply #15 on: July 05, 2011, 04:41:25 AM »

I am thinking that it may yield a unregulated DC higher than 42 Volt from the two 31 VAC windings. I may need to regulate it to make sure it stays at 42 VDC. There is room in the amp to add a DC regulator. But the regulator will need to deal with 40 amps reliably so it is not a trivial one to add. I would rather avoid it.

You may need to regulate it so it does not cause splatter, plus besides reducing splatter regulation will improve transistor reliability.

You might look at an ALS600 power transformer and filter choke system, or the regulated switching supply that could be reset to 42 volts. Except that supply only handles about 30 amperes.
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TWORLD1000
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« Reply #16 on: July 05, 2011, 09:40:36 AM »

Thanks again, Tom, for your input to this thread. Let's take a look at the amplifier filtering for a moment.

I am not sure a unregulated DC power supply will be a significant source of an amplifier to splatter. T1000's DC power supply is not regulated. It uses two 68,000 MF electrolytic capacitors in the power supply. My understanding is the amp may add some 120 Hz hum to the audio to be received by receivers. I believe most receivers exclude the low frequency end of the audio. It is not likely heard on the speaker.

If I understand it correctly harmonics distortion, resulting in multiples of the fundamental frequency, is the cause of splatter. Harmonics distortion is largely created by non linearity of an amplifier. So filtering is a solution to deal with amplifier splatter because of the linearity issue of all amplifiers. The design of the filter and the cut off frequency of the filter will determine if it is effective in preventing splatter.

T1000 is an amp with a push pull design. It naturally has low even order harmonics. It uses matched pair of MRF428s. It is the odd number multiples of the fundamental frequency that needs to be seriously dealt with by the filter. The manual says the filter achieves -50 db (meeting FCC requirement) for harmonics suppression. I assume that it does take care of 2x and 3x harmonics of the transmitting signal. I can probably safely assume the cut off frequency is between the transmitting frequency and 2x of it.

My T1000's filter (military version) does not specifically show the frequency information. It only says channel A, B, C and D. This is why I asked the filter question in the other thread in the first place. I will need to sweep the filters to find out their exact cut off frequencies in order to determine if I can transmit all ham frequencies without causing splatter.

I agree with most of what you posted about amplifier's gain, bandwidth, efficiency, distortion and splatter. Thanks for your effort to clarify. I hope I do understand all those correctly.
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W8JI
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« Reply #17 on: July 05, 2011, 10:17:44 AM »

I am not sure a unregulated DC power supply will be a significant source of an amplifier to splatter. T1000's DC power supply is not regulated. It uses two 68,000 MF electrolytic capacitors in the power supply. My understanding is the amp may add some 120 Hz hum to the audio to be received by receivers. I believe most receivers exclude the low frequency end of the audio. It is not likely heard on the speaker.

No, that is not an accurate understanding.

Voltage changes can add IMD because it causes power gain to vary with load current (power input). This is gain or transfer function distortion. This is why power supply regulation should be reasonable. The large electrolytics probably help make it not so bad, but it would be better if it did not change.

Quote
If I understand it correctly harmonics distortion, resulting in multiples of the fundamental frequency, is the cause of splatter. Harmonics distortion is largely created by non linearity of an amplifier. So filtering is a solution to deal with amplifier splatter because of the linearity issue of all amplifiers. The design of the filter and the cut off frequency of the filter will determine if it is effective in preventing splatter.


No. That is absolutely wrong. It is a common misunderstanding, but wrong.

The intermodulation is created inside the output device. It is because the output device, if not perfectly linear, becomes a mixer. Let's consider a two-tone signal at 7.000 and 7.003. If the gain transfer is not linear or does not have the correct slope, we have 2*7.000-7.003 or 6.997 as a new frequency created by mixing. We also have 2*7.003-7.000 = 7.006 as the other third order product. This is generated INSIDE the amplfying devices. Obviously any filter outside the devices will just pass the 6.997 and 7.006 out to the antenna. We can't build a high power filter sharp enough to clean that up.


Quote
T1000 is an amp with a push pull design. It naturally has low even order harmonics. It uses matched pair of MRF428s. It is the odd number multiples of the fundamental frequency that needs to be seriously dealt with by the filter. The manual says the filter achieves -50 db (meeting FCC requirement) for harmonics suppression. I assume that it does take care of 2x and 3x harmonics of the transmitting signal. I can probably safely assume the cut off frequency is between the transmitting frequency and 2x of it.


Assume what you want.

I'm just telling you how things really work. :-) You will play hell to get one push pull class AB stage balanced enough to make   -30 dBc or so on the second harmonic. It's worse yet to get several combined stages to do that, and worse yet to do that on multiple bands. -30 db is 1000 times down, or 0.1% harmonic distortion. Good luck on that with a semiconductor biased not too far from cutoff over a wide range of input levels and operating frequencies, and especially with multiple parallel stages on multiple bands.  :-)

The FCC wants up around -45 dBc, so if you use a filter without any attenuation for second harmonic you need a semiconductor stage that has less than .01% harmonic distortion.

Quote
My T1000's filter (military version) does not specifically show the frequency information. It only says channel A, B, C and D. This is why I asked the filter question in the other thread in the first place. I will need to sweep the filters to find out their exact cut off frequencies in order to determine if I can transmit all ham frequencies without causing splatter.


Low pass filters have nothing to do with splatter.

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I agree with most of what you posted about amplifier's gain, bandwidth, efficiency, distortion and splatter. Thanks for your effort to clarify.


You might want to agree with even more of it, because that is how it actually works. :-)

73 Tom




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TWORLD1000
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« Reply #18 on: July 05, 2011, 10:52:19 AM »

Tom:

I like your words about push pull amp that does -30 db on 2nd harmonic. That is so true and the T1000 manual says that too. T1000 is a single stage amp. It has just two RF power transistors in push pull configuration. I hope Trans World Electronics used a good matched pair so that it does -30db as claimed.

Let's look at IMD. If you transmit only the fundamental RF frequency, the carrier frequency, through the amp there is no harmonics generated by IMD. Keep in mind it is a single frequency only. All the harmonics seen at the output are caused by non-linearity of the amp called harmonics distortion.

When you transmit with audio then IMD kicks in. However, keep in mind audio frequencies are a few kHz only. Harmonics caused by IMD will be close to the base band frequency. Think of 20 MHz +/- 5Khz and all multiples. They will be well within 19.95 and 20.05 MHz. They only cause impact to audio quality at the receiver end.

A splatter filter is a low pass filter. Trust me Tom. A web search will reveal the fact. CBers are known to cause VHF splatter. If they use properly designed low pass filters in their amps the splatter can be reduced or eliminated. All ham amps have LPF to eliminate splatter.
« Last Edit: July 05, 2011, 10:54:57 AM by TWORLD1000 » Logged
G3RZP
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« Reply #19 on: July 05, 2011, 11:20:29 AM »

Intermodulation distortion results in products close to the transmitted frequency - officially in what is called Out of Band Domain. The OoB domain extends from the edge of the 'necessary bandwidth' of the signal - usually considered in practice to be the point where the energy has dropped to 20 dB the below the signal power - to 2.5 times that bandwidth. Frequencies further away are in the 'spurious emission domain'. See the international Radio Regulations and ITU-R Rec. SM329-11.

'Splatter' is unwanted emissions close to the signal, i.e. in the OoB domain, not harmonics, which are in the spurious domain. So a low pass filter cannot cure splatter or, to give it its correct name, close in IMD. Close in IMD is usually measured in terms of the relative to the wanted output PEP of the powers of the 3rd and 5th order products, although especially with the logarithmic transfer characteristics of bipolar transistors, products up to the 7th and 9th orders can be appreciable.

Thus the ONLY output filter that will attenuate 'splatter' from an SSB signal would be 3kHz wide at the signal frequency - which isn't possible at these power levels.
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TWORLD1000
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« Reply #20 on: July 05, 2011, 11:24:23 AM »

Did I just misunderstand the definition of splatter? Hum... I will need to go back and study. I will be back.

Thanks again Tom. I have had a good discussion with you. Thanks for the explanation, G3RZP.
« Last Edit: July 05, 2011, 12:19:50 PM by TWORLD1000 » Logged
M0HCN
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« Reply #21 on: July 05, 2011, 11:30:19 AM »

Nope, sorry, Tom is correct in this.

Splatter is an effect that occurs close in to the transmitted frequency and is due to intermod between components of the signal being amplified.  A good solid state amp running class AB might have low order 2 tone IMD products at around the -36dBc level, but these fall off fairly quickly as you move outside the nominal bandwidth so are usually more of an audio quality problem then a source of interference as long as the amp is not over driven.
The really nasty splatter that results from overdrive is really RF clipping as the stage runs out of supply voltage to allow it to track the envelope, this will also cause an increase in generated harmonics (as the stage becomes very non linear), but the mess in band will be a bigger problem.

The harmonic generation tends to be due to imbalance between the two phases of the circuit (even harmonics) and the same non linearities that cause splatter (odd harmonics), but this time the mixing products are typically products of signals mixing with themselves rather then 2 tone products.

A push pull stage helps with lowering the even harmonics, but will typically not be good enough on its own to meet spec, and the output low pass will need to go over somewhere below twice the operating frequency to add additional suppression to the second harmonic output, for all that the third harmonic needs more suppression then the second.  

These filters are only peripherally concerned with splatter in that unless you design the filters as diplexers the harmonic energy gets reflected back into the output of the PA and can itself cause in band IMD2 components! This is however a second order effect and about the only amps that really deal with this tend to be homebrew and sometimes end up in QEX, DUBUS or such.  

Look at it this way, splatter is in the band you are working, harmonics are at multiples of the working frequency, filters help with harmonics but not with splatter. Of course the harmonic levels may track a function of the modulation envelope, but that does not make them splatter.  

73's Dan. M6ATV.

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TWORLD1000
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« Reply #22 on: July 05, 2011, 12:11:41 PM »

Thanks, Dan. It looks like I did misunderstood the definition of splatter. You see, passing two VEC exams 10 years ago did nothing to me in terms of knowing what a ham should know. I hope I am not bothering anyone. I wanted answers and that's a reason why I started the thread.

Am I correct to say splatter causes audio problems at the receiver end? I thought I said that. If this is correct then I agree that LPF of the amp does not address splatter issues, if splatter is in the band being transmitted and harmonics are outside. Splatter does not cause interference to outside bands.

Tom is correct in explaining what splatter is. I had water spilling out of a pan in my mind and kept thinking how it would interfere other bands. Now I need to think of ripples and splashes inside the pan. Sorry, Tom. You must feel frustrated trying to correct me. I do understand what IMD and non linear harmonic distortion are. I just did not know the term splatter correctly.

I should add splatter is splashes inside the pan, maybe a little bit outside but at the edge of the pan still. Harmonics on the other hand are spurious spill 2x, 3x and more away from the pan. 

I believe T1000 has its own IMD and splatter issues. I can only assume the manufacture has addressed it by the design of the amp module. It should not be over driven to minimize splatter. The LPF output filters are for out of band harmonics suppression. This is the question I had regarding its channel ABCD LPF filters. Whatever I will do to the filters will do nothing caused by IMD of the amp modules.

« Last Edit: July 05, 2011, 12:25:01 PM by TWORLD1000 » Logged
W8JI
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« Reply #23 on: July 05, 2011, 01:23:42 PM »

Tom:

I like your words about push pull amp that does -30 db on 2nd harmonic. That is so true and the T1000 manual says that too. T1000 is a single stage amp. It has just two RF power transistors in push pull configuration. I hope Trans World Electronics used a good matched pair so that it does -30db as claimed.

That's right, if everything is really perfect. Now the filter has to add another 13dB or so to pass minimum legal requirements, hopefully more to have headroom. Clearly we cannot run the filter at the range you suggested.

Quote
Let's look at IMD. If you transmit only the fundamental RF frequency, the carrier frequency, through the amp there is no harmonics generated by IMD. Keep in mind it is a single frequency only. All the harmonics seen at the output are caused by non-linearity of the amp called harmonics distortion.


IMD does not generate harmonics. You may have a fundemental misconception or confusion about harmonics and intermodulation, because it seems you mix the two.

Harmonics are caused by fractional cycle distortion.

Intermodulation is caused by a non-linear transfer function, which is an envelope distortion problem. The envelope is comprised of many RF cycles. RF cycles are the harmonic issue, envelope shape of many cycles is the intermodulation issue.

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When you transmit with audio then IMD kicks in.



Not quite. More than one frequency, or amplitude changes to the envelope, cause the IMD problem.

Quote
However, keep in mind audio frequencies are a few kHz only. Harmonics caused by IMD will be close to the base band frequency. Think of 20 MHz +/- 5Khz and all multiples. They will be well within 19.95 and 20.05 MHz. They only cause impact to audio quality at the receiver end.

Harmonic distortion by definition falls outside the band in this application. Odd-order IM falls inside the band.

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A splatter filter is a low pass filter. Trust me Tom.



Not if you think that I sure won't trust you!!!


Quote
A web search will reveal the fact. CBers are known to cause VHF splatter. If they use properly designed low pass filters in their amps the splatter can be reduced or eliminated. All ham amps have LPF to eliminate splatter.


Utter total nonsense.

Splatter is generally near the transmitting frequency, although I've seen it extend out a great bandwidth at times.

Harmonics are multiples of the input frequency.

Low pass filters fix harmonics, if properly working. Only better linearity can fix IMD in the cases being discussed.

73 Tom
« Last Edit: July 05, 2011, 01:46:39 PM by W8JI » Logged
M0HCN
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« Reply #24 on: July 05, 2011, 01:31:18 PM »

Am I correct to say splatter causes audio problems at the receiver end? I thought I said that. If this is correct then I agree that LPF of the amp does not address splatter issues, if splatter is in the band being transmitted and harmonics are outside. Splatter does not cause interference to outside bands.
Sort of.... Splatter is usually seen as IMD causing totally excessive transmitted bandwidth (Usually caused by clipping something), by definition the only things that can cause received audio problems are things that fall within the receivers IF passband (So some IMD counts), where splatter is normally relatively close but outside the receivers passband (so the receiver may be unaware of it), it poses a problem for other users of the band trying to work close to the frequency of the problematic station.

For example if the receiver has a 2.4Khz SSB filter then only IMD falling within that 2.4K bandwidth will be a problem for the receiver but if the transmitter is generating sidebands full of splatter out to 10s of Khz or so either side of its nominal frequency then someone trying to work DX 5K up the band will be cursing you.

Also be a little careful of thinking too strictly in terms of bands, the RF really does not care, it is perfectly possible to splatter over a band edge.
Quote
I should add splatter is splashes inside the pan, maybe a little bit outside but at the edge of the pan still. Harmonics on the other hand are spurious spill 2x, 3x and more away from the pan. 
Be a little careful here, spurs and harmonics cover slightly different cases.
A harmonic is an integer multiple of the input frequency, a spur (while it may be a harmonic) does not have to be an integer multiple. For example some RF amps can suffer from low frequency instability in the audio or ultrasonic region at some drive levels,  which can cause masses of spurs that are not harmonically related to the output (They are in effect modulation sidebands) sometimes extending Mhz from the transmitted frequency.

The T1000 looks to be basically a datasheet implementation of a push pull PA of a fairly crude sort, and like all amps will have IMD and harmonic generation issues, the key thing is to understand what the filters can remove and what they cannot.

If you want to dig in more depth then the following are good references:
"Radio Frequency Transistors", Dye and Granburg - They wrote most of the Motorola app notes for RF transistors.
"Experimental Methods in RF Design", ARRL, excellent.
"The ARRL Handbook", ARRL, much good discussion on this and much else that any Ham inclined to technical fiddling should know.

Regards, Dan. M6ATV.

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TWORLD1000
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« Reply #25 on: July 05, 2011, 09:53:54 PM »

Dan, Thanks for your effort in clarifying up my misconception of harmonic distortion, IMD, splatter and all that. It is very educational. I believe I learned all these long time ago but over time it proves that some have disappeared from me. Much of the remaining are based on my knowledge from audio amplifiers as my other current hobby. In audio amplifiers there is another distortion called TIM caused by bandwidth and rise time of the amp.

Now, I think I have got the correct concept of splatter. I would like to get a little clearer about IMD and harmonic distortion. I think I am safe to say harmonic distortion is a result of signal clipping by the amplifier and IMD is caused by the non-linearity of the amplifier. Harmonic distortion produces multiples of the base frequency as harmonics. IMD on the other hand produces sum and difference between two or more different base frequencies and multiples of these sum and differences.

Well, somehow deep in my memory I remember been taught that all these distortions (HD and IMD) are both caused by the non-linear property of an amplifier. I would like to clarify if this is a correct concept.

I have no question that IMD is caused by non-linearity of an amp. I understand that signal clipping or crossover error of class B and C amplifiers cause harmonic distortion. But aren't clipping and crossover error are special cases of non-linearity of amplifiers? There is always some amount of harmonic distortion even in class A amplifiers. There is no amplifier that is perfectly linear. When an amp is over driven it causes the amp to shift to a more non-linear operating point (causes clipping as an example).  It not only produces much worse IMD it produces more harmonic distortion too. 

I still think harmonic distortion is caused by the same non-linearity property of the amplifier that causes IMD as well. Harmonic distortion creates byproducts formed from each individual base frequency of the signal but IMD occurs between every frequency that enters the amp. Despite the difference of the by products they are all caused by the same non linear property of the amp. I still have a faint memory of my Fourier Analysis course back in my college time which proved how harmonic distortion and IMD are produced as a result of amplifier non-linearity. I could have messed up my memory though.

Am I thinking right in this round? Thanks a lot for the educational reply. 

 
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G3RZP
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« Reply #26 on: July 06, 2011, 12:12:11 AM »

It should be noted that the requirements deriving from the Radio Regulations for harmonics for stations in the Amateur and Amateur Satellite Services operating below 30MHz are that the spurious emissions should be 43 + 10 log P down, without exceeding 50dB.

When the FCC updated Part 97 to 'meet new international requirements', they got it wrong at 43dB - despite the fact that the FCC guy at the ITU meetings was an amateur!
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W8JI
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« Reply #27 on: July 06, 2011, 04:26:27 AM »


Now, I think I have got the correct concept of splatter. I would like to get a little clearer about IMD and harmonic distortion. I think I am safe to say harmonic distortion is a result of signal clipping by the amplifier and IMD is caused by the non-linearity of the amplifier.


No. In an RF amplifier harmonic distortion is caused by a change is shape of each RF cycle from a perfect sine wave. As I said, it is fractional cycle distortion and can be anywhere on the RF cycle.
IMD is caused by evelope distortion and mixing in the amplifying device.

Quote
Harmonic distortion produces multiples of the base frequency as harmonics. IMD on the other hand produces sum and difference between two or more different base frequencies and multiples of these sum and differences.


OK.

Quote
Well, somehow deep in my memory I remember been taught that all these distortions (HD and IMD) are both caused by the non-linear property of an amplifier. I would like to clarify if this is a correct concept.

Right.

Quote
I have no question that IMD is caused by non-linearity of an amp. I understand that signal clipping or crossover error of class B and C amplifiers cause harmonic distortion. But aren't clipping and crossover error are special cases of non-linearity of amplifiers? There is always some amount of harmonic distortion even in class A amplifiers. There is no amplifier that is perfectly linear. When an amp is over driven it causes the amp to shift to a more non-linear operating point (causes clipping as an example).  It not only produces much worse IMD it produces more harmonic distortion too.
 
 
Driving harder can make some amps cleaner as a function of signal to distortion ratio. While one problem can have multiple undesired results, it does not mean the mechanism is universal.

Quote
I still think harmonic distortion is caused by the same non-linearity property of the amplifier that causes IMD as well.


If I have a perfectly linear transfer function RF amplifier that is class B, with half of the RF cycle undistorted and half missing, it has significant harmonic distortion without IMD. Harmonic distortion is caused by individual RF cycle distortion. Undesired IM products are caused by non-linear transfer function that distorts envelope shape.

73 Tom
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TWORLD1000
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« Reply #28 on: July 06, 2011, 08:24:31 PM »

I have dug out some 2N5302 power transistors. I will proceed to build a DC regulator for restoring the power supply my T1000. The unregulated DC will be about 46 - 47 volt. The regulated output will be 42 volt. This means 4 - 5 volt of DC voltage drop on the 2N5302. This will be a few watts taken up by the 2N5302s when the amp is idle and 100 - 200 watts full driven. This is not too much for 2 or 3 2N5302s. 

I just looked at the basic amp module circuit again. It looks so simple. It is very different from audio power amps which have all the complexity necessary to make them linear and distortion free. Most RF amps I have seen including those on Motorola databook all look very simple. The secret (or known fact) is in the RF power transistors themselves. Most Motorola RF power transistors have built-in emitter resistors. The emitter resistor provides negative feedback in common emitter amp circuits. Amp designers can't change the resistor value externally. This basically forces the RF amp to be designed by the book. All can be done by amp designers are setting the bias, impedance matching and choose a supply voltage. I have always been amazed by how simple the amp circuit is. I am sure I only see the surface by saying this. All the complexity are in the books mentioned by one of the posts earlier.

T1000 is a textbook amp for me. I hope to complete the project soon. Many thanks again to all who responded to this thread to help.
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W8JI
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« Reply #29 on: July 07, 2011, 03:48:39 AM »

I have dug out some 2N5302 power transistors. I will proceed to build a DC regulator for restoring the power supply my T1000. The unregulated DC will be about 46 - 47 volt. The regulated output will be 42 volt. This means 4 - 5 volt of DC voltage drop on the 2N5302. This will be a few watts taken up by the 2N5302s when the amp is idle and 100 - 200 watts full driven. This is not too much for 2 or 3 2N5302s. 

Better look at the full load to no load volatge regulation of the supply itself and see if it says in range of what the transistors will need in order to stay  at the desired output. Peak dissipation will be at midway point in load.

Quote
I just looked at the basic amp module circuit again. It looks so simple. It is very different from audio power amps which have all the complexity necessary to make them linear and distortion free. Most RF amps I have seen including those on Motorola databook all look very simple.


Because many are test circuits and don'ty have protection, negative feedback, filtering, stable biasing, and frequency-gain compensation systems.

Quote
The secret (or known fact) is in the RF power transistors themselves. Most Motorola RF power transistors have built-in emitter resistors. The emitter resistor provides negative feedback in common emitter amp circuits. Amp designers can't change the resistor value externally. This basically forces the RF amp to be designed by the book.


That's not what the ballasting reistors do.

The emitter ballasting inside the transistor is to prevent hot spotting of the multiple emitters, so each individual emitter in each transistor has similar current. They divide current. Otherwise some emitters would have far too much current because of uneven heating of the emitter.

They have NOTHING to do with RF feedback.

Quote
All can be done by amp designers are setting the bias, impedance matching and choose a supply voltage. I have always been amazed by how simple the amp circuit is. I am sure I only see the surface by saying this. All the complexity are in the books mentioned by one of the posts earlier.


You misunderstand what the emitter ballasting resistors do. Ballasting resistors are required to be sure the emitter system, which is interdigitated with the base for power handling improvements and other reasons (speed, linearity),  has even current distribution as the multitude of emitters heat at different rates.  Without ballasting one emitter or a few emitters would heat, the heat would further increase current in that area, and the resulting hot spot destroy the transistor.

Negative feedback, gain determination, and biasing are all done externally.

73 Tom
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