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Old 19th March 2013, 07:46 PM   #261
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Originally Posted by Bob Cordell View Post
In some ways, the MOSFETs are the opposite of BJTs. In my 1983 paper, I coined the term "transconductance droop" to describe the class-AB crossover shortcoming of MOSFETs. Later, I believe Doug coined the term "gm-doubling" to describe the crossover problem one runs into when over-biasing a BJT output stage.
Not me, gov.

The term has been around for longer than I've been in the business. I object to it, because I have never thought it sensible to describe output stage behaviour in terms of transconductance when what you are really trying to build is a unity-gain voltage amplifier. I disagreed deeply with Ed Cherry on this.

The term I proposed (not entirely seriously) was "gain-deficit-halving" which caught on about as well as you would expect it to.
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Old 19th March 2013, 07:55 PM   #262
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Originally Posted by kgrlee View Post
Back to suggestions for the next edition ..

I'd like to make a plea that the various schematics be checked for missing details that might lead to self destruction on overload. My particular beef is the emitter follower driving the VAS.
(all references to the 4th edition

eg fig 6.16 : TR12's collector current is unlimited on overload so will kill TR4 on HF overload. A simple 'cure' is a resistor in TR12 collector.

Some of the circuits shown are OK. eg fig 5.24 where Q9 protects both Q10 & Q11. I feel this point needs to be stressed ad nauseum as the emitter follower is an essential part of "Blameless".
I agree that this important point should have been emphasised more in the past. It has its own section in the new edition.
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Old 19th March 2013, 07:59 PM   #263
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I would very much like something about the drive of lateral mosfets, gate-stoppers, oscillation, use of RF-beads, ideal current for class A-B operation. thermal tracking considerations regarding bias spreading.
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Old 19th March 2013, 08:02 PM   #264
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Originally Posted by DouglasSelf View Post
Hi Bob

I'm sorry, but I cannot get enthusiastic about such a vague acronym. Three letters just aren't enough to convey the information.

I could settle for TOIMC, (output-inclusive) so we can also use TIIMC. (input-inclusive)
Doug,

I am not trying to get you to be enthusiastic about the term TMC. I'm hoping you'll refrain from re-naming what most others have been using for more than 3 years now.

TMC is no more vague than TPC. Two pole compensation. Let's see, how many different architectures can yield two pole compensation? TPC by itself certainly does not imply a variation of Miller compensation, yet we generally understand TPC to mean a variation of Miller compensation wherein the feedback capacitor is split and loaded to ground at the center with a resistor. BUT, one could certainly have two-pole compensation using only lag compensation networks. Anyway, I hope you get my point and don't insist on inventing your own name for TMC and adding confusion.

Out of respect for you, I chose to reinforce the use of the term "gm doubling" in my book, even though one could make a case that it is also vague to the less-experienced. I happen to think it is appropriately descriptive. TLAs (Three Letter Acronyms) are convenient, but often necessarily vague to the inexperienced.

Cheers,

Bob
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Old 19th March 2013, 08:09 PM   #265
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Quote:
Originally Posted by Bob Cordell View Post
Out of respect for you, I chose to reinforce the use of the term "gm doubling" in my book, even though one could make a case that it is also vague to the less-experienced. I happen to think it is appropriately descriptive. TLAs (Three Letter Acronyms) are convenient, but often necessarily vague to the inexperienced.

Cheers, Bob
As I said in my post just above, (#261) I did not invent the term "gm doubling" and I do not like it.
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Old 19th March 2013, 08:18 PM   #266
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Originally Posted by DouglasSelf View Post
Not me, gov.

The term has been around for longer than I've been in the business. I object to it, because I have never thought it sensible to describe output stage behaviour in terms of transconductance when what you are really trying to build is a unity-gain voltage amplifier. I disagreed deeply with Ed Cherry on this.

The term I proposed (not entirely seriously) was "gain-deficit-halving" which caught on about as well as you would expect it to.
Hi Doug,

Golly! I could swear the first time I came across it was from you! I've been in the business since 1970, but of course that dates me to the point where my memory may not be as good as it used to be :-).

Both views of the output stage are important, but I think it is important to keep in mind that the departure from unity gain in an emitter follower is directly related to transconductance, thus the importance of transconductance. Variation in transconductance of the stage translates to variation in dynamic output resistance of the stage, and that in turn results in gain variation in the stage when it is loaded. Guess I'm with Ed Cherry on that one (I get rather uncomfortable on those rare occasions when I find myself on the opposite side of the fence from that brilliant man).

For someone who has done such a good job of explaining static crossover distortion, I am surprized that you don't seem to rate transconductance more importantly in the output stage.

Cheers,
Bob
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Old 19th March 2013, 08:39 PM   #267
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Originally Posted by DouglasSelf View Post
No. Variable Vgs, transconductance...not helpful. You know where you are with a BJT.
Hi

I fundamentally agree but if some sort of Vgs 'adjustment' is made by some sort of error correction this problem can be mitigated to the point it is no longer a problem.
Of course there are some other issues with mosfets that have to be overcome as well such as the effects of Cgd.
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Old 19th March 2013, 09:06 PM   #268
wahab is offline wahab  Algeria
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Some attention should be given to laterals fets , namely
the Hitachi/Renesas , theses are extremely convenient
devices.

Their lower transconductance compared to Toshiba s verticals
or Hexfets is not a problem as shown by National s app note
for their integrated driver , distorsion is not substancialy
higher than the Toshiba while being notably better than
IRFs.
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Old 20th March 2013, 02:12 AM   #269
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Dear Mr.Self.
pics posted I've found in a rather old book, 1983 to be exact.
It does look like TMC, although the purpose is(was) to design fast settling hybrid op amp...
Being sufficiently ripened I am just curious about the history of analog design.
Image #3 shows the circuit to be placed instead of compensation capacitor Coc1 @ Fig.1.
Boxes on Fig.3 show input stage (left) and VAS (right)
Fig.2 shows open loop gain.
Sincerely,
Alex
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Old 20th March 2013, 06:10 AM   #270
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Default pure Cherry

.. if I may be so bold as to beg another addition to the next (or future) edition ... though I'm certain Mr. Self will not have time to investigate this to the thoroughness he will want to bring to the subject.

This is to compare TMC and other heresies with pure Cherry as God intended. As an example of what can be achieved using this Holy technique on stone age topologies, I offer post #2321 at

Discrete Opamp Open Design

Further descriptions are at posts #2060 2062 & 2183 on that thread.

This started out as an improved 990 OPA; ie a "Blameless" variant.

Self and practically everyone else have focussed on the VAS having low Zo. This requires the output stage to be a 'perfect' follower.

Alas, the departures from this normally lead to xover and nasty high order harmonics. Self deals thoroughly with these departures and shows that if this approach is taken, it is essential to control Iq to very fine limits.

But what if we current drive the output stage. Then even if the B of the outputs are seriously different, the distortion products (including BJT B reduction with current and yukky Vgs behaviour of MOSFETs) become very low order. Iq is now quite non-critical.

This is why I spurn heretical versions like TMC. These hang evil bits on the critical high Z VAS o/p and even use our valuable feedback to make the output stage Gm more evil.

This is the basis of Cherry compensation. He shows the reduced sensitivity to gm in Feedback, Sensitivity and Stability of Audio Power Amplifiers which I regard as essential reading for guru & pseudo guru alike.

Cherry compensation also maximises the feedback around the most evil stage; the output.

Some people (including Guru Self & even Great Guru Baxandall) have problems getting pure Cherry stable.

I wonder if both these worthies (excuse my use of the 3rd party, Mr. Self) have tried Cherry's own solution to this; an emitter resistor on the VAS (R14 in #2321). If a current mirror is used on the i/p transconductance stage, this does not reduce the open loop gain. My own experience is that Cherry's collector base caps on the driver (evil cos they connect to the VAS output) may not be necessary either.
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Is this guy pontificating through the wrong orifice?

No. I haven't tried this circuit for real. I'm presently a real beach bum and a SPICE newbie. But I have tried pure Cherry on a "Blameless" topology circa 1990. And the SPICE model in #2321 does the things I expect to see in real lifefrom Jurassic experience.

I feel the advantages of pure Cherry are so great as to be worth a lot of effort dealing (with perhaps mythical) instability. To see crossover @ full power 20kHz disappear with less than 10mA Iq, leaving only 2nd & 3rd, was very satisfying.
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I also think when Mr. Self starts investigating a topology that doesn't rely on tightly controlled and small Gm, he will take a less jaundiced view of MOSFETs.

In my case, Cherry compensation opened up the use of evil vertical MOSFETs instead of the Hitachi lateral MOSFETs I favoured in dem days for their nicer 'gm' properties.

Last edited by kgrlee; 20th March 2013 at 06:16 AM.
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