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Bias design, amplifier design, filtering etc.

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Old 12-03-2009, 10:44 PM
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Default Bias design, amplifier design, filtering etc.


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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Old 12-04-2009, 07:04 AM
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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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Old 12-04-2009, 02:53 PM
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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
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Old 12-04-2009, 05:51 PM
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bob85: you know your stuff. I admire that!

ST had a good apnote that describes the problem with bipolars:
Quote:
Bipolars, on the other hand, have a positive temperature coefficient and are prone to
thermal runaway. The main reason for this is the increase of hFE due to the increase of
temperature. As the device draws more current its temperature rises, hence hFE rises and
even more current is drawn resulting in a further temperature hike. This goes on until the
device fails. Hence, bipolars need elaborate temperature compensation to prevent such
occurrence. MOSFETs, however, are protected against thermal runaway and no
compensation is required.
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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Old 12-05-2009, 01:39 PM
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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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Old 12-05-2009, 02:07 PM
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How are you guys tracking heat in your bias designs?
Wanna trade schematics through PM's?
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Old 12-05-2009, 04:08 PM
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Quote:
Originally Posted by bob85 View Post
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.
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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Old 12-06-2009, 08:59 AM
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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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