lateral mosfets vs. bjts in audio amplifier design

[Kevin Aylward]

My NMR and MRI gradient amps use a similar topology... the driver
opamp has its supply rails cascoded to make signal currents, and the
upper and lower power stages are active current mirrors. I'd argue
that adding an opamp per power fet makes things faster, stability
better, and compensation simpler, since the gate/Miller capacitance
disappears...

And your argument would be incorrect I am afraid to say. I have already
outlined these issues recently in another post.

There is no such thing as a free lunch. You cannot achieve better stability
by using extra feedback loops in this manner. The transfer function of the
op-amp and fet combined will cause considerable grief. Only if you roll off
the system much earlier will it be stable.

each fet now looks like a very fast, pF input
capacitance, DC-perfect device, and essentially disappears from the
overall loop dynamics.

Unfortunatly not. Consider a two stage amp circuit, one first stage, being
the conventional gain stage, followed by a second stage connected with a
local loop to make a CLG of unity. Additionally, the second amp connecting
back to the 1st (via a beta network if used) to form the composite. Now
calculate the system loop gain by breaking both feedback connections *at
once* an compare it with the 2 stage system both running open loop. The
transfer functions are identical. So, having the two stages means at least 2
roll offs. That is including an *additional* op-amp (to get better lf
performance) around the mosfet, will generate an additional pole that would
not have been there. Therefore the system is inherently more unstable.

I suggest you actually perform the *detailed* simulations of these type of
circuits.

I point out two circuits

http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp1.jpg
http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp2.jpg

They are similar, but one has a local feedback loop around the output
devices, one doesn't. One has better LF accuracy, one can be stabilised to a
higher UGF. Which is which?

The fundamental trade off here is basic physics, there is an inherent
constraint of H(Power, Accuracy, Speed) = 0, e.g.
http://www.kevinaylward.co.uk/ee/cmospafl/cmospafl.html


You just can't just get better LF performance, and expect to get it all. It
don't work that way. Like,....


When I was young, I prayed and prayed to the Lord to get me a bike. After
many years, and no success, I realised that HE, doesn't work that way...So I
stole the bike, and preyed for his forgiveness...

Now just pile on as many opamp+fet pairs as you
need. DC balance and current sharing become as good as the opamp
offset voltages, microvolts if you like, so fet gate threshold
variations and transfer curves don't matter any more. So use very
small source resistors and cut losses.

For LF, the op-amp approach is very nice, for HF, its not so nice.

My amps often work in pulse mode, when doing chemical NMR. They settle
to PPMs of the target value (which is current, since we're driving
gradient coils) in 10's of microseconds.

This is very slow, by about 2 orders of magnitude at least. At this slow
speed, op-amps are probably a good choice. You just clobber with a big cap.
By fast, I meant at the < 100ns level.

Kevin Aylward
www.kevinaylward.co.uk

 

[Kevin Aylward]

I was delighted with the consistency of my D Series of bipolar amps.
No trim pots at all. Designed out in the design process. In fact the
entire amp had not one trimpot. That means faster production and
final test plus a cost reduction.

It would be a good idea if those lateral mosfet makers had current sense
transistors so that a current mirrored, push pull, source follower can be
done.

Kevin Aylward
[email protected]
www.kevinaylward.co.uk

 

[Jamie]

Jamie wrote:




I'm quite sure that JF is far too out of touch with the technology currently
used in the best audio amplification.

Also, it has to be economic to manufacture. That's easily half of it.

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
that being the case, then it must be shit you're making!
I think any beginner can accomplish that.

Graham

http://webpages.charter.net/jamie_5"

 

[Eeyore]

LOL!!
the crap is getting deep!

You clearly haven't the tiniest clue about the economics of high-volume manufacturing.
Or the importance of pcb layout. Not to mention the cost savings of offshore
manufacturing too.

One product of mine (in its various channel sizes) sold over 100,000 units.

Graham

 

[Eeyore]

It would be a good idea if those lateral mosfet makers had current sense
transistors so that a current mirrored, push pull, source follower can be
done.

Motorola did a mosfet with a current sense terminal. Doubtless only N-channel
though.

Graham

 

[Eeyore]

I point out two circuits

http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp1.jpg
http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp2.jpg

They are similar, but one has a local feedback loop around the output
devices, one doesn't.

I'm a great believer in local feedback. Linearises stages nicely and improves HF
and phase response. If you need to, you can make up some lost overall gain with
jellybean transistors (or ICs) at the front end.

Graham

 

[Eeyore]

Exactly. I'm behaving professionally. Nor do I want to let all the Chinese see
how to do it right.


You haven't the skills and experience in this field.

Ain't gonna work.

I'm sure you're very good at other things.

Graham

 

[Eeyore]

I've seen plenty of data sheets for devices in volume production that still say
PRELIMINARY on them. So ?

Graham

 

[Eeyore]

You would think they already have a know it all janitor.

That's a bit low Michael. The fact of the matter is that after something of
a drought (partly for health reasons) it's damn monsoon out there right now.
I don't know how I can satisfy both clients who both have very interesting
respectively moderately big and huge projects on right now.

Plus my back's still fucked.

Graham

 

[Eeyore]

But it's FUN !

Where would you place the threshold (and what harmonic structure) of audible
THD ?

Graham

 

[Eeyore]

I wish you every success.

It also has to fit industry standard form factors. A 3u 19" rack is considered big
for a 2kW amplifier for example, must be as lightweight as possible and must have
certain input processing requirements such as anti-clip circuitry, balanced inputs of
course and user selectable high pass filters to name a few.

Graham

 

[Eeyore]

Not if the "additional" pole is at a much higher frequency than the
previous pole inherent to the capacitances of a heap of fets driven by
some wimpy resistive source. Increasing the bandwidth of the output
stage - the serious speed problem - by, say, 20:1 has got to help the
overall loop.

Just buffering Ciss helps a ton.

There's no free lunch here: we're adding GBW, and paying for it. But
not much, since opamps are cheap.

I confess I do that kind of thing. Just not put op-amps round the actual output
devices myself so far but it sounds interesting. They'd have to be damn fast
though.

Graham

 

[Eeyore]

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
that being the case, then it must be shit you're making!
I think any beginner can accomplish that.

As ever you're quite clueless.

What do you a PCB / chassis mount professional audio XLR 3 pin gold-plated
connector should cost for example ?

Or an NE5532 ?


Graham

 

[[email protected]]

"Damon Hill"




** It can be very useful to have a digital storage scope when measuring THD
residuals.

Most digital scopes have an "averaging" feature, so as long as you have the
time-base locked to the fundamental, the harmonic residual signal will add
to itself as many times as the scope allows while noise and any AC supply
harmonics tend to disappear from the trace as they are not correlated to the
fundamental sine wave.

Nowadays, many folk like to use a PC with a 24/96 sound card and FFT to do a
spectrum analysis -  makes harmonics stand out like dogs balls.


** Better ask someone like Halcro ( an Aussie manufacturer).

http://www.halcro.com/home.asp

They have been making a fortune selling $40,000 amps with 0.0005 % THD to
New York's fattest & dopiest Jews.

......   Phil

Or just feed the residual to a DSA. While it is interesting to see
what harmonics are present, looking at the residual relative to the
source is probably more useful. For instance, if the residual error
gets large at the crossover, you know where to tweak. The residual
could also get large at the peaks if the output stage is not beefy, or
some current source is out of compliance on large swings.

 

[Eeyore]

Just sticking a unity-gain buffer, like an LT1010, in front of the
gate helps a lot.

I've used discrete Class A, with AB capability to get that node really slewing.

But then, you may as well use an opamp and close the
local loop.

I just have this feeling there's a snag here somewhere. Can't place it yet, ah maybe
slew rate from negative cut-off back to conduction would cause a small dead band (in
time terms).

Graham

 

[Eeyore]

its nt as low as the crap you've been spreading in these audio
threads.

I don't do crap, especially not in audio. Do you want some references btw - one
would be from a guy with a PhD and a Chartered Phsysicist, the other from the
Directors of the company who make the best audio to digital and back converters in
the world.

Big deal. I'm sill 100% disabled, and I had a palsy in my good eye
four months ago that left me effectively blind. I still only have
limited use of that eye, after three months of not being able to see out
of it. I have sores on my legs that aren't healing, and I am scheduled
for some surgery to remove something from my right lower eyelid in about
two weeks. It has been growing rather fast, and I have to wait and see
if it will cause more problems. I have tripped, and banged into so many
things while blind that I am too sore to do much of anything.

I have said before that have every sympathy for you. There's hardly anything
practical I can do though.

Graham

 

[Eeyore]

This is what I meant. If you can't stand the heat, quit shoveling on
the burning coal.

I am a true audio expert Michael, John Fields is not. Each to their own.

Graham

 

[Eeyore]

Or just feed the residual to a DSA. While it is interesting to see
what harmonics are present, looking at the residual relative to the
source is probably more useful. For instance, if the residual error
gets large at the crossover, you know where to tweak. The residual
could also get large at the peaks if the output stage is not beefy, or
some current source is out of compliance on large swings.

The crossover area is the critical one and indeed as you say is also the harmonic
element. If it gets nasty near full power, get a bigger amp !

Graham

 

[Eeyore]

---
It depends:

http://www.google.com/search?source...q=harmonic+distortion+threshold+of+audibility

JF

I was interested in YOUR opinion though.

Graham

 

[Eeyore]

But I have given him a helping hand by recommending the very best stage for the
job. The complementary emitter follower biased into full Class A. More efficient
than his resistive load circuit too.

Graham