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Bias design, amplifier design, filtering etc.

Discussion in 'Amplifiers' started by -=PEAKABOO=-, Dec 4, 2009.

  1. -=PEAKABOO=- Active Member

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    Why not start a discussion about the various bias designs, filtering and just basic theory based around transistorized RF amplifiers.
    Swap ideas, schematics etc. etc.

    I will start off with my first intro into it.

    How many of you guys use a transistor to turn on the bias supply at keyup?



    I have rarely used a relay I normally use something like a 10 amp darlington transistor to control turn on.
     
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  2. HiDef Active Member

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    That scheme lends itself to another very useful scheme. T/R sequencing. Saves chewing up the R.F. relays.
     
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  3. bob85 Supporting Member

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    i have used both methods, im not keen on relays, i used one on the bias boards because it allows grounded base operation when the bias is turned off, the relay need not be used if a transistor switch is used,

    the darlington sounds a good idea, it only needs a little sniff of rectified rf which should not mess up the input vswr,
    a transistor switch is potentially more reliable, if its switching power to a regulated circuit theres no problems with vdrop or thermal effects like you see with none regulated circuits,

    chasing stiff regulated biasing is maybe overkill, its nice to have if maximum linearity is your goal but i cannot hear the difference between that and simpler circuits so long as the bias never dips low enough to cause severe none linearity or switching distortion,

    i have used and modified several different circuits from simple switched pass transistor fed from a regulator to the modified motorola design, i even tried unregulated but thermal tracked circuits, they all work ok but the mot design gives stable bias voltage under any supply voltage/drive levels and pretty stable idle current under a wide range of temparature,
    it can be setup to current limit swinging back towards class c at a certain drive level which may give some degree of protection if the amp is overdriven,
    it also completely avoids the horrible texas star and messenger like problems of having the amp biased too hard in order to maintain a reasonable level of bias under high drive conditions,
    with active regulation the amplifier can run cooler while remaining in the linear region,


    the ultimate bias circuit imho would do the above and automatically compensate for changes in collector voltage on the biased transistors:confused:
     
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  4. dudmuck Active Member

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    bob85: you know your stuff. I admire that!

    ST had a good apnote that describes the problem with bipolars:
    So as you can read. It is more simple for bias to use a mosfet when you must bias an amp for SSB.

    motorola also had a good apnote on it AN860.
     
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  5. bob85 Supporting Member

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    im hoping fets are fairly self regulating, my next mosfet amp will be a minimalist pure class A single ended audio amp with the crudest biasing i have ever seen, as few components as possible.
     
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  6. -=PEAKABOO=- Active Member

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    How are you guys tracking heat in your bias designs?
    Wanna trade schematics through PM's?
     
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  7. ken white Active Member

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    bob85, all FET's are non-linear when used for large stage gains. If you use multiple stages, you should get a pretty linear system. Use enhancement type MOSFET's since they are biased class "A" with no signal.

    Ken
     
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  8. bob85 Supporting Member

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    thanks ken,
    the circuits i am looking at are very simple pure class A zero feedback using a single fet biased for half the supply voltage on the load resistors at idle,
    i love the sound of zero feedback single ended 300b tube amps and dislike the sound of most solid state amps, the single ended fet amp is something to experiment with because i have most of the parts lying around from my hifi days;)
     
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  9. bob85 Supporting Member

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    peakaboo, i dont think i have a schematic, other people may have one i drew them, what type circuit are you interested in?

    for tracking i have used diodes with a flat filed on for better thermal contact, i have also used the base emitter junction of transistors which is what im doing with the rm kl550 base amp at the moment, the factory biasing is a miserable design poorly implemented but it can be made reasonable with a few mods,
    the bias boards based on mot's application notes use a single transistor, the basic circuit is available on communications concepts website, i modified that to give more current, it works better than any other circuit i have experimented with or seen in hf sets and amplifiers,

    on the regulator fed pass setups i modify, i use one diode or transistor to track the finals and one to track the pass transistor both in series to ground from the pass transistor base, it seems to work ok, much better than no tracking,
    the base emitter junction of a second device the same as the pass transistor seems to do the best job of minimising thermal drift in the bias circuit itself,
    this secondary tracking transistor was added to 212's amplifier after it was noted that the bias voltage creeps up as the pass transistor heats up even though one diode is trying to track the finals,

    linearone posted a simple circuit i gave him on here a few years ago, an argument ensued with ccm over why i had two diodes, the suggestion was i dont know how biasing works,
    the answer is it dont work with one diode and it works best in my tests if both diodes are tracking temperature one for the pass device one for the finals, even though hf sets generally only use one diode for tracking in very similar circuits,
    it seems to me that the best tracking would be obtained from using the base emitter junction of a transistor the same as what you are biasing but the thermal mass of a large sensing element will likely slow down the response time,

    is it a ballancing act between tracking accuracy and speed of response hmmm maybe maybe not,
    if junction temperature rises instantaneously then your tracking element is at the mercy of thermal lag,
    the shorter the thermal path between the junction and the junction of the sensing element the faster you get the sensing element hot, long thermal paths and high sensing element mass slow down tracking speed,

    spill the beans boo, whats going on in the 8pill joker amp that was floating around the forums, that and one that dave built are the only decent looking circuits i ever saw in a cb amplifier,
    i looked at it and speculated but only you would know how you did that and how effective it was, if you dont want to describe it on here i will take a pm
     
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  10. Cotton Mouth Member

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    I think this thread is going to be real interesting and hope it will continue as there could be a lot of dam good information for everyone to tune into and learn a lot too as bob85 and PEAK A BOO both know their shit with out a doubt.

    I know I'm staying tuned in and patiently waiting for more information as this is where it all is and begins right here.

    Cotton Mouth.
     
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  11. paws264 Active Member

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    I vote that you don't, it was done in the past. Through someone's infinite wisdom, those "CCM/AMPower/Justin/Mopar/et al." arguments about the 50 ways to apply 6/10 of 1 volt to the base of a transistor did not get transfered to the new forum.

    For the sake of appearances, this is the "AMATEUR RELATED SECTION" of the forum; Chicken-Band Transistor Amplifier builders should peddle their wares in the CB Section.

    .
     
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  12. -=PEAKABOO=- Active Member

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    Bob I used transistors base emitter junction. to track heat. It worked ok but I have since improve a little on the way it tracks. That amplifier was switchable between AB and C I believe.

    I need to try and dig out the hand drawn schematic of that design and pm it to you.
     
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  13. -=PEAKABOO=- Active Member

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    If you would prefer I will not even discuss on the matter.
     
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  14. Moleculo Administrator Staff Member

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    Biasing techniques of HF transistor amps applies to whatever HF band you want to use them on. It's a good learning and sharing discussion. What happened before was that certain individuals didn't understand how to have a discussion without letting it erupt into a flame war.
     
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  15. -=PEAKABOO=- Active Member

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    I agree.
     
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