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Old 23rd November 2012, 10:21 AM   #11
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FETS are a good idea, and you can bootstrap the drivers to get full rail voltage swing. However at low bias currents the large transconductance will cause massive capacitive current spikes and I don't think that will sound good. BJTs can be biased low and still have reasonably low crossover distortion. I don't know, maybe there is a suitable FET pair.

The only pair I can think of for this is the 2SC4883A/A1859A from Sanken. Paralleling outputs would be a good idea but 4 outputs is already plenty to fit in an altoids tin with room to spare for lytics and such. FET input would allow us to save space on film caps.

Low-bias amplifier design is an art.
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Old 23rd November 2012, 10:41 AM   #12
qusp is offline qusp  Australia
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linear... poetic license? or you are simply referring to a non-switching amp?
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Old 23rd November 2012, 11:37 AM   #13
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How about a current dumping approach with smaller (BD139/140) fast transistors for the crossover region. Or perhaps a CFP output stage.
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Old 23rd November 2012, 12:07 PM   #14
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The problem with a CFP output stage is you have to keep the drivers from turning off. It can be done, but by that time there is little advantage over an ordinary EF.

I have finally found a page on what current dumping is really about:

Current Dumping Article

A current-dumping output stage is by nature a low-gain stage, which means we will have to make up for the gain in other stages, possibly making the amp more complex than it needs to be. Furthermore, the preceding stage will have to have its own quiescent in order to drive in the "off" region, making the whole arrangement somewhat redundant.

In terms of performance, I think an ordinary EF output will work best, .82R emitter resistors gives us 15mA output quiescent, making for 450mW output idle consumption. Can't go too low or emitter resistors cut into voltage swing. Current-dumping may get us lower but is it necessary to sacrifice the performance?
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Old 23rd November 2012, 12:31 PM   #15
bigwill is online now bigwill  United Kingdom
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Originally Posted by keantoken View Post
The problem with a CFP output stage is you have to keep the drivers from turning off. It can be done, but by that time there is little advantage over an ordinary EF.
Is this a problem? It seems fairly common to run CFP structures (including the driver) in Class-AB. You also have the advantage of the driver transistor having some control over Iq instead of relying on the tempurature dependant Vbe drop of the finals
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Old 23rd November 2012, 12:53 PM   #16
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Okay, but what is controlling the driver transistor's Iq? Variations in the driver's Iq will be multiplied in the output's Iq. Depending on the implementation this can be many times worse for a CFP. In a CFP the outputs' Hfe tempco is added to the driver tempco, and Hfe tempco is unique to each transistor type. All this must be designed around. Typically a CFP tempco is larger than an EF tempco for this reason, and much more tedious to compensate.

A CFP output stage could be used, but it would be a tedious hassle, a liability, and will be marginally better than an EF at best.

It would certainly be interesting, in the sense of a learning experience, but personally I think I've learnt most of this one already.

If you have seen all these issues overcome, then I would be eager to discuss the schematics you provide.
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Old 23rd November 2012, 01:08 PM   #17
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Okay, but what is controlling the driver transistor's Iq?
The way I understand it, the output of the CFP goes through an "emitter" resistor to the load, and the driver transistor samples the Iq via its voltage drop. Obviously the temperature will vary this to a degree, but importantly it's not thermally linked to the current drawn by the output transistors. If the output stage decides to draw more current, the driver will turn it off more. I'd imagine there's also a degree of self compensation - if the driver warms up and conducts more, the output transistor will conduct more in turn thus increasing Iq and turning off the driver in rough proportion.

If you parallel output devices then of course you would have "real" emitter resistors from emitter to rail on the big transistors.
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Old 23rd November 2012, 01:19 PM   #18
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That works, but you get the drivers turning on and off.
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Old 23rd November 2012, 01:27 PM   #19
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That works, but you get the drivers turning on and off.
I know, but is that an issue?
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Old 23rd November 2012, 01:35 PM   #20
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As it happens, I have a real circuit breadboarded up working right now with CFP outputs and it seems to work very nicely. The crossover performance seems very good at surprisingly low Iqs. I will draw up the circuit shortly.
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