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Author Topic: T-1000 filter question  (Read 7676 times)
TWORLD1000
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Posts: 31




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« Reply #30 on: July 07, 2011, 10:44:44 AM »

Tom, I do know the ballast resistor design of RF transistors although it is like a picture painted in my brain 20 years ago. I don't remember all the details about it.

However, an emitter resistor in a common emitter amplifier circuit serves a basic purpose in a form of negative feedback, for stabilizing the bias of the transistor and sets the gain of the stage. If you look at most (likely all) audio power amp circuits you can easily find those external emitter resistors on every transistor in common emitter configuration.

The ballast resistors of a RF power transistor, although are designed for many other important purposes, have the same effect of an external one for transistors without them in common emitter configuration. Most audio power amps use emitter resistor values ranging from o.1 - 1 ohm in the final stage of power transistors. I think in the T1000's amp circuit, which has no external emitter resistor, does not use one because the ballast resistors of the emitter already does it for the amp designer.

In the T1000 amp diagram the TR428's emitters are grounded. This is not a common practice in audio amplifier circuits. My guess is that the ballast resistance is probably chosen to help eliminating the need of an external one.  You sure can add an external if desired. It will further reduce the gain of the amp in a common emitter configuration.
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G3RZP
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« Reply #31 on: July 07, 2011, 11:27:20 AM »

I have to agree with Tom here.

Where you have interdigitated emitters, you have the problem that the emitters in the middle of the chip get  hotter than the ones at the edges. That means that the Vbe for that particular base/emitter junction drops as the temperature rises, while the hfe increases. This leads to increased current, and thus more heating and the 'hot spot' can rapidly become a 'melted spot'.

Providing a ballast resistor on each emitter gets around this: some designs actually used somewhat higher values in the middle of the chip than at the edges to try to minimise the problem. It is not limited to RF power transistors either: similar problems on a smaller scale can occur in ICs, where you can lose temperature tracking across a die.

Another problem with bipolars is the biasing. You need a very low impedance voltage supply, but you would like to have a constant current into the base with a negative temperature coefficient, and it is impossible to have both. What has been done in at least one IC for CDMA is to monitor the   collector current and reduce the bias derived from a low impedance voltage source to keep collector current constant as temperature goes up. This would introduce distortion if it followed the AM on the signal, so an opposing loop is provided that takes into account the amplitude of the input signal and and modulates the bias the other way, so keeping collector current constant as far as the DC bias conditions are concerned, but allowing full output power with low distortion.

Complicated, but you can do it on an IC where you wouldn't have a hope discretely. Before you ask, I've had 32 years in semiconductors.
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W8JI
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« Reply #32 on: July 07, 2011, 11:59:58 AM »

Tom, I do know the ballast resistor design of RF transistors although it is like a picture painted in my brain 20 years ago. I don't remember all the details about it.

However, an emitter resistor in a common emitter amplifier circuit serves a basic purpose in a form of negative feedback, for stabilizing the bias of the transistor and sets the gain of the stage. If you look at most (likely all) audio power amp circuits you can easily find those external emitter resistors on every transistor in common emitter configuration.

Just like the long thing about filters and IMD and splatter, you have the wrong picture. :-)

The MULTIPLE internal ballast resistors are intentionally selected to have the lowest possible resistance that still equalizes the multiple emitter currents enough to prevent hot spotting the die and ruining the transistor. They have next to nothing to do with negative feedback or bias, and especially have nothing to do with eliminating external components.

RZP brings up a good point often overlooked about bias. Drive power tries to force the base negative, and so the bias supply must be a very stiff voltage at the base. With transistors over a few watts, this can almost never be done without active components in the bias system.

You of course are not actually designing something, but since this is a learning experience for you it is really important to point out areas where you misinterpret what the system does and how it works.

(If I don't know something for sure, I won't tell you I do. That would be counterproductive and pointless.)

73 Tom
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TWORLD1000
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« Reply #33 on: July 07, 2011, 04:40:22 PM »

Well, I just did a search on Firefox browser with theese key words: emitter ballast resistor negative feedback

On the very first page of the search result I see these:

"with intrinsic emitter resistance, re, and extemal emitter ballast resistance, RE. The emitter ballast resistors provide negative feedback to the negative ..."

"emitter ballast resistors have a value ranging between 1 to 10 ohms; ... (Figure 2) The ballast resistor achieves a negative feedback to limit ..."

"Dec 5, 2001 – When a ballast resistor is connected to each base or emitter electrode of a multi-finger HBT, it provides a negative feedback to the base ..."

"the voltage between emitter and base is decreased by the ballast resistor (deffusion resistor) at emitter, which causes internal negative feedback and ..."

"If a ballast resistor is connected to either the base or emitter, it gives a negative feedback to the base or collector current. The negative feedback ..."

These are on the first page of the sear result. I have not looked into the details of each. But I have found enough references of the negative feedback I was talking about.
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M0HCN
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« Reply #34 on: July 07, 2011, 05:08:20 PM »

The emitter resistance does (as does the intrisic emitter resistance of any transistor) of course reduce the gain via negative feedback, but that is NOT a desirable thing and great care is taken to make the emitter connection have an impedance as low as is possible while providing adequate control of any hotspotting.

The key thing to realize is that unlike in the audio case, these transistors are usually operating close to Ft and usually have very limited power gain (10 - 20dB typically), which inherently limits the ability to apply negative feedback as it robs gain.

When a stage only has a power gain of 13dB open loop, feedback to improve linearity gets tricky, and feedback around multiple stages tends to lead to instability as the phase shift increases.

RF is not audio, and the design approaches are very different. For example one trick (more useful in a rig then an external PA) is to apply envelope feedback around a few stages, this does nothing for the shape of the individual rf cycles, but can reduce the IMD by reducing the envelope distortion. The low pass filters are of course still needed to remove the harmonics due to the distortion of the individual cycles. You don't see this much in ham gear, but it is popular in commercial equipment.

Regards, Dan.
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G3RZP
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« Reply #35 on: July 08, 2011, 01:54:24 AM »

I suspect that a number of the reasons for ballast on interdigitated RF transistors haven't been widely published. I seem to remember that there was a presentation at one of the RFExpo events in the 1980 on this. Being in pre-internet days, I don't believe the presentations are available on the web, or indeed, anywhere in electronic format.

Even the companies presenting don't exist anymore - at least, not under those names.
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W8JI
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« Reply #36 on: July 08, 2011, 04:56:38 AM »

In low level or medium level amplifier stages, like preamplifiers, external emitter resistors can be used to provide negative feedback. I often use this technique myself to flatten gain over wide frequany rages, but to do that the net resistance is significant.

That does NOT mean every time we see a resistor in an emitter system we can leap to this universal conclusion:

Quote from: TWORLD1000 on Yesterday at 10:44:44 AM
Quote
Tom, I do know the ballast resistor design of RF transistors although it is like a picture painted in my brain 20 years ago. I don't remember all the details about it.

However, an emitter resistor in a common emitter amplifier circuit serves a basic purpose in a form of negative feedback, for stabilizing the bias of the transistor and sets the gain of the stage. If you look at most (likely all) audio power amp circuits you can easily find those external emitter resistors on every transistor in common emitter configuration.

The ballast resistors of a RF power transistor, although are designed for many other important purposes, have the same effect of an external one for transistors without them in common emitter configuration. Most audio power amps use emitter resistor values ranging from o.1 - 1 ohm in the final stage of power transistors. I think in the T1000's amp circuit, which has no external emitter resistor, does not use one because the ballast resistors of the emitter already does it for the amp designer.

In the T1000 amp diagram the TR428's emitters are grounded. This is not a common practice in audio amplifier circuits. My guess is that the ballast resistance is probably chosen to help eliminating the need of an external one.  You sure can add an external if desired. It will further reduce the gain of the amp in a common emitter configuration.


An external resistance could never fix the problem which the internal emitter balasting resistors are designed to correct, uneven current in the multiple internal emitters.

As I said:
Quote
The MULTIPLE internal ballast resistors are intentionally selected to have the lowest possible resistance that still equalizes the multiple emitter currents enough to prevent hot spotting the die and ruining the transistor. They have next to nothing to do with negative feedback or bias, and especially have nothing to do with eliminating external components.


73 Tom
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G3RZP
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« Reply #37 on: July 08, 2011, 08:03:08 AM »

Unbypassed emitter resistors also increase both the output and input resistances.
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TWORLD1000
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Posts: 31




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« Reply #38 on: July 08, 2011, 09:28:52 AM »

Emitter ballast resistors may be designed with thermo stability as the primary purpose. It must have its implications. One of them I can imagine is that it produces a negative feedback to the base unless the ballast resistors have negligible value. Other implication includes thermo noise and reduced gain (broaden or flatten bandwidth). Also while this negative feedback reduces the gain it increases linearity and helps to reduce IMD and THD.

I believe the resistance value for the emitter ballast resistor is determined by the designer to achieve the goal of thermo stability as the primary goal but it also needs to deal with negative impacts and implications.

The T1000 amp circuit is so simple. There is the input transformer, the output transformer and that's pretty much it. The emitters are grounded. Bias is provided through the center tap of the input transformer and the VCC supply through the center tap of the output transformer. In comparison with a final stage of an audio power amplifier it appears like a no brainer circuit.

I wonder if this is because the RF transistor is designed to take care of some of the complexity already. I image that the ballast resistors plays a roll in that. A specific resistance is chosen to accomplish that.

Just think about IMD and TMD. The negative feedback produced by the ballast resistors certainly helps to reduce them. Well, R6, R7, C11 and C12 produce the main negative feedback for the amp.  Perhaps the negative feedback produced by the emitter ballast resistors is too small and is negligible. Typical emitter resistor value for the last stage of audio power amplifier is typically 0.5 ohm. The RF power transistors typically have a ballast resistance of 1 - 10 ohms. This is much higher and produces more negative feedback.

I am modeling RF power transistors with AF. This may be a totally bad idea. I believe most of the complexity of RF power amp is in the input and output transformers. This is covered in the Motorola data book I think.

 

   
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M0HCN
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Posts: 473




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« Reply #39 on: July 08, 2011, 09:55:22 AM »

Ze in a RF power device is usually MUCH lower then 1 - 10 ohms! Consider what that would do for a device running say 100W on a 24V supply rail, so a peak current of around 8A.

A 1 ohm Ze would drop 8V, a full third of the supply voltage!

Consider also that that the base impedance is typically low (~10 ohms on a good day), and thus that the base voltage swing is small for a given drive power.... Re wants to be made as small as possible of you will loose all the power gain to negative feedback.

There is a reason that RF power devices come in funny packages with two or more wide low impedance emitter/source connections, it is that Ze must be kept very low.

Seriously you want a copy of Dye and Granburg for the gory detail.

Regards, Dan.
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TWORLD1000
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Posts: 31




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« Reply #40 on: July 08, 2011, 10:21:15 AM »

Dan This is where I got the numbers from. It is an IEEE article:

ESD Performance Optimization of Ballast Resistor On Power AlGaAs ...
ieeexplore.ieee.org/iel5/6641/17711/00819066.pdf
by CY Chu - 1999 - Cited by 5 - Related articles
emitter ballast resistors have a value ranging between 1 to 10 ohms; ... (Figure 2) The ballast resistor achieves a negative feedback to limit ...

I know what you mean 10 is too high. It seems reasonable between 0.1 and 1.
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M0HCN
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Posts: 473




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« Reply #41 on: July 08, 2011, 10:33:19 AM »

You are looking at a GaAs device for microwave operation there (Probably for the mobile phone or CATV market), power probably means a hundred mW to a watt or so, rather then the sort of silicon based thing operating at a 20dB higher power level used in your HF amp.

Different technology in a different operating region.

You really do need a copy of either EMRFD or Dye and Granburg......

Regards, Dan.
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W8JI
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« Reply #42 on: July 08, 2011, 11:13:31 AM »

Dan This is where I got the numbers from. It is an IEEE article:

ESD Performance Optimization of Ballast Resistor On Power AlGaAs ...
ieeexplore.ieee.org/iel5/6641/17711/00819066.pdf
by CY Chu - 1999 - Cited by 5 - Related articles
emitter ballast resistors have a value ranging between 1 to 10 ohms; ... (Figure 2) The ballast resistor achieves a negative feedback to limit ...

I know what you mean 10 is too high. It seems reasonable between 0.1 and 1.

I think this is unnecessarily argumentative because people with actual experience are dismissed based on Internet searches for totally unrelated applications. Unrelated applications are needlessly being drawn into this application where they obviously do not apply. That's a needlessly slow way to learn or teach.

I agree with this advice:

Quote
You are looking at a GaAs device for microwave operation there (Probably for the mobile phone or CATV market), power probably means a hundred mW to a watt or so, rather then the sort of silicon based thing operating at a 20dB higher power level used in your HF amp.

Different technology in a different operating region.

You really do need a copy of either EMRFD or Dye and Granburg......

73 Tom


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TWORLD1000
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Posts: 31




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« Reply #43 on: July 09, 2011, 10:07:22 AM »

Tom, I am sorry if I sounded argumentative. No, I am not dismissing Dan's or your replies to my quest of learning. While I have no doubt about the overall correctness of Dan's and your replies they are not of no contradictions that need further clarification in my position.

For example, regarding unregulated DC supply. You said in your post #15:
You may need to regulate it so it does not cause splatter...

You probably did not feel that I was somewhat frustrated too by this statement of yours. Splatter is the result of IMD of the AMP. Thanks for clarifying the definition of splatter for me. But

IMD is a serious matter for all audio power amplifiers. I own a SAE 2400L,  along with a couple of Hafler, Yamaha and Denon, audio power amplifiers. The SAE 2400L and all my Hafler power amps all use unregulated DC power supply. I worked on them and that's what I found. My T1000 uses unregulated DC power supply too. I am afraid that this statement of yours is not one of no questions. Why all these amps are designed with unregulated DC supply if it has the effect of reducing splatter (IMD)?

Another example regarding amp linearity in my other thread of Motorola TRF428 question you said:
Linearity in a solid state power amp is a gain or amplification transfer function between input and output. It is not a harmonic issue...

It is a harmonics issue. THD is a harmonic distortion term associated with non linearity of an amp.

There are a few more... No need to bring them up to cause further frustration to you.

Tom, if you want to be an elmer you need to prepare to be frustrated. I guess I should get the suggested books instead of trying to learn from here. Thanks for all your posts on this thread. I got answers for my initial question already. 
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M0HCN
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« Reply #44 on: July 09, 2011, 10:57:13 AM »

Forget audio amplifiers, the ONLY similarity between audio and RF power stages is that both tend to use impure sand as an amplifying medium....

Look an audio power stage typically has open loop gain for days, and applies huge negative feedback around the whole mess to linearize itself, an RF stage does not have that option as the phase shifts through multiple stages will make the loop impossible to stabilize.

About the best that can be done is to feed the envelope back (and even this is not trivial), and in practice most RF amps run almost open loop (Apart from the intrinsic feedback from emitter impedances and miller capacitance and such), at least the budget stuff for amateur and most commercial service does!

Now consider that the Vas and input stage in a standard audio amp will be running from a regulated supply in all probability, it is only the current amplifier stage that is actually unregulated and that these things use voltage feedback.....

In the case of your T1000, there is sufficient capacitance in the power supply to effectively bypass the audio bandwidth envelope current (at least mostly), so while it is not ideal, it does save on a high power DC regulator and its associated heat making it a reasonable tradeoff as long as the amp still met its design specifications. 

Regards, Dan.
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