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Old 9th March 2011, 03:08 AM   #1
artu is offline artu  Chile
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Default Alternative topologies

Across some threads I've noticed that some people are missing the discussion of alternative topologies, (giving that nothing is completely original ... I hope not). This thread is open to all that are interested about different (not widely used) discrete amplifiers. To open the thread I show a CFA (Current Feedback Amplifier) based on the LH4118. Feel free to post your alternative amp, I am very interested on CFA's due it's unique properties.

Cheers
Arturo
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File Type: png SQUARE_100KHZ.png (17.8 KB, 759 views)
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Old 9th March 2011, 04:25 AM   #2
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Go Figure...

Prominent features:
Cheap BJTs tricked to behave like depletion devices with matched thresholds...
InsideOutAllison - No bootstrap caps...
Underbiased Schottky Square Law Crossing
Sliding bias would be the two emitter drops across R7.

Sorry, lost the original .asc file sometime back.

Might revisit this one sometime, I got ideas for improved emitter input impedance.
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Last edited by kenpeter; 9th March 2011 at 04:37 AM.
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Old 9th March 2011, 04:36 AM   #3
artu is offline artu  Chile
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Default looks good

It will take me some time to understand it (and simulate it) ... a emitter coupled VFA, isn't it?
input impedance should be low I guess ...

Cheers
Arturo

Last edited by artu; 9th March 2011 at 04:45 AM. Reason: corrections
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Old 9th March 2011, 04:48 AM   #4
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Helps to imagine ordinary input topology with 4 JFETs of 0.65V threshold.
OK, these virtual JFETs only have 47K emitter (er.. gate?) impedance...
But they are dirt cheap and come 2 matched in a DIP (4 BJT transistors).

Firstly, R15 tries to raise output bias.
But Q5+Q6 stop it right at the 1.3V required sliding bias...
Never enough to fully turn on all four cold Schottkys at once.
I must emphasize "cold", because hot power devices are never
allowed to shape this crossing. All shaped in overkill TO220
sized diodes that barely dissipate any heat.

The rest appears to be gravy to limit current and other mishaps.
Soft clipper at front end could and should have been done better.
I've learned a bit since drawn this more than a year ago...

Last edited by kenpeter; 9th March 2011 at 05:14 AM.
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Old 9th March 2011, 04:59 AM   #5
artu is offline artu  Chile
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Default fixing clipping ...

I changed the cascoded VAS to 2n5551/2n5401 completely because the clipping is much better than the prior circuit showed. A little lower THD's but nothing significant.

Cheers
Arturo
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Old 9th March 2011, 05:27 AM   #6
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Quote:
Originally Posted by artu View Post
It will take me some time to understand it (and simulate it) ... a emitter coupled VFA, isn't it?
input impedance should be low I guess ...

Cheers
Arturo
You see through 0.65V emitter of Q1 to the base of Q8 in parallel with R3 47K.
So basically 47K per input emitter, 23.5K for both Q1 and Q4.

If we increase the impedance of the 47K pullups (replace w. current sources),
the emitter impedance of Q1+Q4 then begins to look more like Q8+Q9 bases.
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Old 9th March 2011, 05:33 AM   #7
klewis is offline klewis  United States
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Quote:
Originally Posted by artu View Post
Across some threads I've noticed that some people are missing the discussion of alternative topologies, (giving that nothing is completely original ... I hope not). This thread is open to all that are interested about different (not widely used) discrete amplifiers. To open the thread I show a CFA (Current Feedback Amplifier) based on the LH4118. Feel free to post your alternative amp, I am very interested on CFA's due it's unique properties.

Cheers
Arturo
Arturo,

Someday I will build a CFA power amp, keep going... I'm using a CFA opamp for my pre amp, 2000v/us and unmeasurable thd (at least with my equipment).

Ken
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Old 9th March 2011, 05:17 PM   #8
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Hi Kenpeter,
A question: Q8,Q11 and Q9,Q14 drives M1 and M2, isn't the output impedance of this drivers enough low as M1 M2 are showing a capacitance maybe (including Miller) of 1 to 2 nF?, it seems that also Q15 and Q16 (that are limiting the current of the mosfets) also 'steals' some of the driving current. I am understanding OK?

Cheers
Arturo
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Old 9th March 2011, 08:12 PM   #9
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Default About the CFA

Hi Ken, I am finishing a PSU system that will be common to all the amps variations I plan to test (besides 2 'traditional' topology i've build years ago) with instruments and auditions. For me there are 'good' and 'bad' amps with a range in between. I also don't believe on what should be the 'correct' sound (as a musician aficionado), I learned to hear several instruments and voice of different types in different environments (outdoor, at studio, chamber music, my 'music room' ... etc). So it is fair 'learn' hearing different designs and acquire a more broad listening culture, and learn the 'why's' for each case (if it is possible to link electric measures with audition). I am close to retirement so I will have a lot of time to do so.

Returning to the thread topic the CFA shows some nice properties among them a huge stability (f180 at 38MHZ with Dom. pole at 5.11MHZ with gives a PM of 142) , a rapid response to high variations of current demand at the load, also it does'n has 'high impedance' internal nodes much susceptible to parasitics. The output stage topology (diode strings at the buffer) is a well known 'good' sound for me and has a decent distortion with 1.5 OHM output impedance drive, also has the ability to reduce the quiescent AB current from CLASS A at 30 to pure B at 100, ~10W Class A when it is in AB region at 65. I have found that stripping the degeneration resistors (of the VAS driver) also reduces the thermal drift to +/- 15 mv (from 0 to 100), besides it reduces a bit the THD.

Cheers
Arturo
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Last edited by artu; 9th March 2011 at 08:37 PM. Reason: corrections
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Old 9th March 2011, 08:20 PM   #10
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Its a three way battle between push, pull, and sliding bias.
Resulting in two answers that drive the MOSFETs and keep
bias from running away....

Open loop impedance is too high, would not function open.
Low driving impedance only a result of the feedback loops.
There is enough available current to overcome Cgd Miller.

I want to try again with EKV+VDMOS hybrid .subckt per
Bob Cordell's book.
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