Hi,
An externally hosted image should be here but it was not working when we last tested it.
Practical Wireless - Texan Amplifier
http://www.angelfire.com/sd/paulkemble/sound8h.html
rgds, sreten.
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just the E90 plastic box - have a long hanging ES amp project with a PS that scares me and a few different HV fets lying around but little forward motion
vaguely relevant since most sims and single test so far involve op amp front ends - tested a depletion mode Fet cascoding a optoisolater output - is amusing that ES headphone current can be passed by an optoisolator
vaguely relevant since most sims and single test so far involve op amp front ends - tested a depletion mode Fet cascoding a optoisolater output - is amusing that ES headphone current can be passed by an optoisolator
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Have you read the opamp handbook that was edited by W.Jung?
There is a big section on composite amps.
One part you can't apply is the relative gains of the two stages, since most chipamp gains are way above the figure given in the handbook for the second stage of the composite.
One of the nice things about using a transconductance stage (e.g. Howland Current Pump) as the power stage (a la MyRef or MiniRef) is that the open-loop gain of the chipamp can be traded off for bandwidth. In both the MyRef and MiniRef, the transconductance of the power stage is ~1 S, i.e. 1 A/V. To drive 2A into an 8 ohm load requires just about 2V at the input of the Howland - a very easy task for the opamp in the outer loop. The Howland chipamp ends up with a closed-loop BW in the region of a few MHz or greater, which can be compensated down (in many different ways) to about 100 to 200 kHz, closed-loop. Almost any opamp remains stable in the outer loop of the MiniRef - no problems with fast opamps like the LM6172, LT1364, LT1208, etc.
It also takes care of the the different rail voltage capabilities of the opamp and the chipamp - the chipamp can use its typical +/- 30 to 35V rails, while the opamp can use +/- 12V and still have ample headroom to drive the chipamp into clipping.
The same idea can be used with high-voltage driver ICs and discrete power stages like the TDA7250 + power BJTs or LME49810 + power BJTs as the Howland, but I haven't got around to trying them out, mainly due to a lack of good Spice models for the driver ICs.
I think that is the paper I have read (though revisiting is never a bad idea), I recall the Howland current pump in the body of the article.
Thanks
You can read Bob Pease's thoughts on the Howland Current Pump in the National Semiconductor application note, AN-1515. Here it is in TI format: http://www.ti.com/lit/an/snoa474a/snoa474a.pdf
The Howland Current Pump and other four-quadrant current sources open up a can of worms when it comes to stability, as described in above app note.
Tom
Thanks for the app note.
The issues relating to trim and drift are not a factor in composite amps with an inner Howland - I use hand-matched resistors that are matched to better than 0.1%, and the outer loop corrects most of the DC drift issues. About the only thing to watch out for is the capacitance/AC-stability issue, but a variety of compensation schemes exist that work well in practice with very reasonable external capacitance values (say 10 to 100 pF) that this too is not a problem. Of course, it helps to have a good AC simulation model to get it right without much of trial-and-error experimentation.
The issues relating to trim and drift are not a factor in composite amps with an inner Howland - I use hand-matched resistors that are matched to better than 0.1%, and the outer loop corrects most of the DC drift issues. About the only thing to watch out for is the capacitance/AC-stability issue, but a variety of compensation schemes exist that work well in practice with very reasonable external capacitance values (say 10 to 100 pF) that this too is not a problem. Of course, it helps to have a good AC simulation model to get it right without much of trial-and-error experimentation.
Please look at Fig.1 on page 53 for this explanation. The complementary power output Mosfets are operating in the common source configuration. Two consequences emerge. One is the joint connection of their sources is set as common/ground. Secondly this output stage has a voltage gain which develops across the loudspeaker load , and inverts the phase of the signal presented to its jointly connected gates of Mosfets. Play this game by example for M1.
As the AC input signal to the gate of M1 goes positive, current [by convention] flows from the positive end of battery V1, through its drain, and out of its source to common/ground. But, this current must return to the battery via its negative end. So, mentally connect the common/ground at the joint source of Mosfets to the common/ground of the loudspeaker; which is the same electrical point. Now the current flows through the loudspeaker [sings] and back to the battery's negative end where it originated. Next, analyze the current flow from V2 through M2 as the signal presented to the joint gates goes negative instead. It begins flow from common/ground at and through the joint of the Mosfet sources...
Note that overall negative feedback [NFB] from the loudspeaker returns to the Non-Inverting port of the Op Amp. This is so to take into account the inverting voltage gain reaped from the power output stage. Send the NFB line to the inverting port of Op Amp instead, and the whole amp will oscillate due to positive feedback.
Thanks Antoinel for the explanation, it wasn't expected but appreciated (inwas getting there )
The circuit reminds me of a bridge circuit (except in my own h bridge build I used a common drain configuration)
Other Than that its less intuitive (for me to understand) than a CFP or simple follower stage.
)The circuit reminds me of a bridge circuit (except in my own h bridge build I used a common drain configuration)
Other Than that its less intuitive (for me to understand) than a CFP or simple follower stage.
while happy to see Liner Audio flourishing I don't think its appropriate to devote too much diyAudio thread time to circuits not visible to all
there are several threads on floating supply amps, Crown, QSC, Behringer all make cheap PA with this topology
http://www.diyaudio.com/forums/soli...o-floating-supply-amplifiers.html#post2202342
http://www.diyaudio.com/forums/solid-state/200572-inverted-not-inverting-output-stage.html
http://www.diyaudio.com/forums/solid-state/101602-qsc-clones-backwards-amps.html#post1204834[/QUOTE]
there are several threads on floating supply amps, Crown, QSC, Behringer all make cheap PA with this topology
http://www.diyaudio.com/forums/soli...o-floating-supply-amplifiers.html#post2202342
http://www.diyaudio.com/forums/solid-state/200572-inverted-not-inverting-output-stage.html
http://www.diyaudio.com/forums/solid-state/101602-qsc-clones-backwards-amps.html#post1204834[/QUOTE]
Once again, many thanks to all.
Excellent links and pointers from you all, I am very humbled.
For discrete options I have at my disposal:
BD139/140
BC237/557
BC441/641 (OR is it 461?...anyway...)
TIP2955/3055
IRF4905/IRFZ44N
BD237
(Better/faster OP devices may be a good idea, but I think even those old 3055 will do, for now)
So initially I have several ideas that I could try (once I've digested the wealth of info)
CFP stage with BD139/140 with TIP3055/2955. (possibility for real power)
CFP with just BD237 and complimentary device.
I think the IRFs will not get used at all. These are billed as 'TTL driveable' switching HEXFETs and were left over from a motor control H-bridge I built and saturate at probably 8Volts gate drive. Or more accurately below 8V gate drive, Rdson is high enough to create heavy losses - not great for a H Bridge. I am still very much a novice, but I doubt they will be any use.
The grounded bridge concept is interesting, and I'm intrigued enough to want yo build one, even if it is only 2-5 watts.
Now for me: Less dreaming and more reading.
Thanks again.
(if I do anything clever, ill be sure to post here)
Excellent links and pointers from you all, I am very humbled.
For discrete options I have at my disposal:
BD139/140
BC237/557
BC441/641 (OR is it 461?...anyway...)
TIP2955/3055
IRF4905/IRFZ44N
BD237
(Better/faster OP devices may be a good idea, but I think even those old 3055 will do, for now)
So initially I have several ideas that I could try (once I've digested the wealth of info)
CFP stage with BD139/140 with TIP3055/2955. (possibility for real power)
CFP with just BD237 and complimentary device.
I think the IRFs will not get used at all. These are billed as 'TTL driveable' switching HEXFETs and were left over from a motor control H-bridge I built and saturate at probably 8Volts gate drive. Or more accurately below 8V gate drive, Rdson is high enough to create heavy losses - not great for a H Bridge. I am still very much a novice, but I doubt they will be any use.
The grounded bridge concept is interesting, and I'm intrigued enough to want yo build one, even if it is only 2-5 watts.
Now for me: Less dreaming and more reading.
Thanks again.
(if I do anything clever, ill be sure to post here)
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