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Solid State PA Bias pot




 
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KA3EKH
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« on: September 10, 2018, 09:19:23 AM »

The common assumption is that I should know this but looks like I got no clue. I am working on a little back pack transceiver that develops around tem watts of AM or 20 watts of SSB for operation on both 80 and 60 meters. I use this radio a lot for field operations. The radio has two outputs, one 50 Ohm output that works well with inverted V and stuff like that and provisions for a whip antenna attached to the radio. The 50 Ohm output connects to an input for the radios internal antenna tuner that has ten presets (one for each channel) and that connects to a seven foot vertical antenna attached to the side of the radio. I have been trying to get the antenna tuner set for the channels I have installed in the radio but in the process have done damage to the PA deck. Ok, so I repaired the PA deck and its working again but this is the question that I have. The PA deck has two stages with the first being a driver in push pull and the second being a power amplifier in push pull. Both stages have a bias pot that allows a small level of DC, maybe under 2 volts to ground depending on the setting of the pot to forward bias the amplifiers. I thought this may affect gain of the stages but apparently has almost no effect on the output, maybe a watt at the most. What are these bias pots for? How do I set them? Were they intended to set idle current for the amplifiers? Are they used for adjust linearity? Or distortion?
This radio was built by Sunair back in the seventies for the USCG to operate on HF AM/SSB before VHF took over everything, at least in portable operation. All runs from one twelve volt gel cell and have been using it for a while now on the 60 Meter USB M&S Net and want to use it at a lot of the Military vehicle shows on 3.885 AM so that why I am doing all this damage to it to get it working with a whip.





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« Reply #1 on: September 10, 2018, 11:42:19 AM »

If you can please point to a link for a schematic some here might be able to help but crystal balls are few and far between.  Smiley

For example, Sunair model what?


Phil - AC0OB
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« Reply #2 on: September 10, 2018, 04:04:25 PM »

It's either a bias pot or a balance pot.

In the case of the bias pot, it will set the quiescent current through the output transistors, setting
them for lowest distortion, probably. Depends entirely on what class of operation it is in. IF it is AB
(which seems likely) then it would be setting a minimum current to prevent crossover distortion. I'd
look at what transistors are in there, and look at the data sheet, there's almost certainly a current
level suggested for "typical" operation. Set it there.

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KA3EKH
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« Reply #3 on: September 10, 2018, 05:01:25 PM »

Ok, this is a part of the schematic. The Amplifier has two pots R1107 for the driver and R1108 on the PA Looking at them I am assuming they are bias and not balance? but you tell me. was going to play around with them and see how they affect splatter with modulation and if its them that drive the amplifiers into Class AB. But that's why I am  asking here to see what the "Greater Minds" think. working with solid state amplifiers in AM/SSB is a new experience for me.




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« Reply #4 on: September 10, 2018, 05:14:37 PM »

Both pots adjust BIAS and are not balance pots.

I would think the manual has a procedure and or a voltage chart for adjusting those to a specific bias voltage at the bases of the transistors Q1101 through Q1104.

It appears this circuit is a linear amplifier so proper adjustment is critical for least distortion.

Upon closer inspection it appears Q1101 and Q1102 require 0.7 Volts on those bases and Q1103 and Q1104 require 0.6 volts on those bases.

I would only adjust those pots if those voltages are off spec as measured with a good DMM.


Phil- AC0OB
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« Reply #5 on: September 10, 2018, 09:53:13 PM »

It’s on the schematic, Duh! Imagine that they bias the amplifiers just a bit on into a more linear state. Spent most of my time working with class C solid state and just starting to get further into solid state amplifiers for AM and SSB and now think the big thing may be to get a two tone test generator and start looking at SSB and AM response that way. Use to just use a distortion analyzer and a good demod back in the old days when you had to do “proofs” on AM broadcast stations, still a requirement but no longer required to do it so no one dose.
Thanks for the help; maybe someday I will grow up to be a knowledgeable Ham!


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« Reply #6 on: September 10, 2018, 10:04:56 PM »

It’s on the schematic, Duh! Imagine that they bias the amplifiers just a bit on into a more linear state. Spent most of my time working with class C solid state and just starting to get further into solid state amplifiers for AM and SSB and now think the big thing may be to get a two tone test generator and start looking at SSB and AM response that way. Use to just use a distortion analyzer and a good demod back in the old days when you had to do “proofs” on AM broadcast stations, still a requirement but no longer required to do it so no one dose.
Thanks for the help; maybe someday I will grow up to be a knowledgeable Ham!

No problem, we learn by studying, experience, and a, "keep-at-it" attitude.


Phil - AC0OB
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« Reply #7 on: September 11, 2018, 06:04:27 AM »

If you can independently set bias per active device then even a single tone generator can help.  At least set it for lowest crossover distortion.

Most bipolar devices 'act right' between 65 hundredths and 75 hundredths of a volt bias.  FETs can be in the range of volts of bias. 

The generally better method of setting it, without a tone generator and a scope is for quescient current.  Turn both pots to 0 mils, turn one up to the set point where the amp is  drawing half of specified current, increase the second pot to full quiescent.

The dataset will probably tell you a good ballpark value.  However, the actual circuit may cause these values to need to be adjusted.

Also, almost all solid state bias designs suck.  It's pretty hard to build a good regulator for both static and dynamic regulation for operation between a half and three quarters of a volt at multiple amps of current draw varying at a syllabic rate.

--Shane
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« Reply #8 on: September 11, 2018, 08:50:41 PM »


...Also, almost all solid state bias designs suck.  It's pretty hard to build a good regulator for both static and dynamic regulation for operation between a half and three quarters of a volt at multiple amps of current draw varying at a syllabic rate.

--Shane
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It is actually quite easy and relatively simple for Bipolar circuitry:


Phil - AC0OB



* General SS Bipolar Bias Supply.pdf (40.69 KB - downloaded 24 times.)
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« Reply #9 on: September 11, 2018, 11:06:46 PM »

Phil,

That circuit is one of the worst their is.  Unless the diode is bonded to the active device or the heat sink, it will run into thermal instability and can cause the amp to self destruct.

Unless you drop AMPS across the resistor, you won't be able to have good dynamic regulation.  The bias on a bipolar isn't like a fet, just apply some voltage and the device works.  The bipolar devices actually take current to stay on, especially when the device is being driven hard.  THIS circuit is what makes most biased CB amps IMD generators.

To quote Ian White:  "There are many problems with this simple circuit. The main one is that in order to maintain a constant voltage across D1, the permanent standing current through D1 must be several times higher than the maximum base current drawn by TR1 at the peak of RF drive. This requires an enormous standing current through RV1 and D1 - one ampere or even more. Most designers fail to provide this. Some commercial circuits even use a low-current signal diode such as a lN914 for D1! Bias regulation totally fails when peaks of RF drive create a heavy demand for base current, so this circuit is a real splatter-generator."

Rather than copy / paste a bunch of pages, Ian makes a pretty good presentation on his website, and particularly calls the circuit presented into question as a bad bias design.

http://www.ifwtech.co.uk/g3sek/tr-bias/tr-bias1.htm


To each his own.  Manufacturers have used that design for decades, as has many other "designers".  Doesn't mean it's a good design today, though.  Have I used that design.  YUP!!!!!!  In both single ended as well as push pull designs.   There are better.



For actual designs, spec an plots, comparisons between active and basic bias designs, this guy has a decent write up towards the bottom of his page.

The bias circuit contributed 12 dB to IMD.  That's very significant.

http://www.ok2kkw.com/00003016/bias/bias_new_en.htm


--Shane
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« Reply #10 on: September 12, 2018, 01:10:29 PM »

Phil,


...Unless you drop AMPS across the resistor, you won't be able to have good dynamic regulation...The bipolar devices actually take current to stay on, especially when the device is being driven hard.  THIS circuit is what makes most biased CB amps IMD generators...

...To each his own.  Manufacturers have used that design for decades, as has many other "designers".  Doesn't mean it's a good design today, though.  Have I used that design.  YUP!!!!!!  In both single ended as well as push pull designs.   There are better.

--Shane
KD6VXI

Thanks and I am well aware of bipolar vs. FET biasing requirements, but I thought we were speaking specifically to bipolar biasing in linear amplifiers.

And that was the basis for the simple bias source, i.e., source about 10 times more current than the needed bias current. I never stated it was the best and there are much more complicated designs out there that do much the same thing except they regulate the currents and voltages much more precisely. The design does work as a simple biasing source.

E.g., look at the Ameritron AL-500M which uses bipolar RF transistors. The bias current is sourced by simply adjusting the base voltage of the 2N3055 pass transistors via a voltage divider, sampling temps, crow barring, etc. I.e., The circuit is just a regulated version of the dropping resistor/honkin' diode circuit.  Smiley

The source of IMD in most SS CB amps was the fact they were biasing stages in an approx. ClassBC (more C than B) operating area (in contrast to biasing in the AB1,2 area) in order to squeeze out as much "dirty"   Shocked  power as was possible.


Phil - AC0OB
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« Reply #11 on: September 14, 2018, 05:44:45 AM »

I was speaking to bipolar devices.  The problem is, the diode clamp fed through a resistor and then using a pot to pick the sweet spot falls apart under load.

Unless you drop a few watts across the potentiometer. This is in addition to the amps of current dropped across R1.

A fet doesn't require this, hence my statement pointing the difference out.

The page I posted takes a cb amplifier using the exact circuit you posted, replaces it with an active circuit and the amplifier cleans up in all regards. IMD, saturated power output, etc.

I've used the diode clamp.  It will work.  Im just pointing out that it is by far not the best circuit to use. 



Try this:  simulate the diode clamp, and what does the regulation do when pulling an amp through the wiper of the pot?  Do you maintain voltage regulation?  What's the variance?  Don't have Sim software at work or I'd try.


--Shane
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« Reply #12 on: September 15, 2018, 11:11:58 AM »

...I was speaking to bipolar devices.  The problem is, the diode clamp fed through a resistor and then using a pot to pick the sweet spot falls apart under load...

Not if you use the formula I gave in the attachment.

Rdrop = (Vs - Vfdiode)/(Idiode + Isupply + Ipot)

You have to take into account all current sinks.   Cool


Quote
I've used the diode clamp. It will work. Im just pointing out that it is by far not the best circuit to use.  

Which I what I stated as well and we agree so no need to repeat.   Smiley


Phil - AC0OB

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