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Old 9th March 2007, 07:16 AM   #1
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Default Kantishna MK III, experimental MOSFET AMP

This is a MOSFET amplifier I conceived of, trying to make a more powerful MOSFET amplifier that shied away from traditional MOSFET design. Of course it's experimental, so it needs a lot of work. I am hoping to sell a kit later though.

Any chriticisms, commentary or suggestions welcome.

I am a relative newbie to solid state discrete amps. I have an MSEE, but really have only built chip-amps.

I also plan to produce a baby version before the big one. I will post the design as soon as I have it.
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Old 9th March 2007, 08:46 AM   #2
SSassen is offline SSassen  Netherlands
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Almost there, just a few things need changing:

- use a normal current mirror, not your contraption with two diodes and a trimpot.

- use a normal current source, either a transistor biased by two diodes, or something a bit more fancy.

- take a look at the output swing of your driver stage, there's easier/better ways to do this.

- you don't have a compensation cap or Vas stage, might want to look into that.

Best regards,

Sander Sassen
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Old 9th March 2007, 11:48 AM   #3
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Hi, Stahlight,

Your Q10-11-12-13 is interesting, is this current feedback (receiving feedback at their emitors)?

Q12's base should be sitting at negative voltage. Collector of Q27 cannot go negative.
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Old 9th March 2007, 03:33 PM   #4
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Default Re:


Q10-11-12-13 are a sort of long tail pair: they are a circuit similar to a cascode pair intended to provide gain while 'beating' the miller effect. Think of it as a common collector-amplifier tied to a common-base amplifier. The function is similar to a common-emitter amplifier, but does not invert. The operational bandwidth is also much higher.

R8-R12 provide local feedback for the output stage, making it (hopefully) stable with respect to the rest of the amplifier. This may be removed later if not needed, but for now, in theory, the output stage is quite high gain and potentially seems unstable. The stage does receive feedback via emitter, so technically is current feedback?


Only R18-19, Q30-31 form the current mirror. The diodes and trim pot form a voltage seperation for the output stage quiescent current (I figure on using about 0.15 A).

I am not sure what a Vas stage is, can you explain?
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Old 9th March 2007, 11:30 PM   #5
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Hello stahlight,

there are some similaritys to Greg Balls SKA ("simple killer amp")

May be a look to this thread is interesting:


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Old 10th March 2007, 12:59 AM   #6
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This all doesn't look workable to me at all.
First, current mirror seems to be flipped, I guess Q30 should be diode-strapped.
Bases of Q11 and Q13 are connected together to ground, so at least one of them is off.
Then, probably you wanted Q10 and Q12 to have base potential symetrically towards ground, say +-0.9V, which forces Q27 to work at very low Vbc, saturation...
Thiel-Zobel network at output seems flipped too.
I suggest careful and critical analysis and correcting mistakes.
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Old 10th March 2007, 02:35 PM   #7
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This circuit won't work. First of all the stages formed by Q10-Q11 and Q12-Q13 are not biased correctly, so that won't work. Even if they did work, the feedback to the emitters doesn't make sense.

The ouput section seems questionable with common source amplifiers. How are you going to bias them so that there is not excessive quiesent current?

I would suggest you start with something more conventional.
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