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Old 5th December 2012, 11:12 PM   #2471
Struth is offline Struth  Canada
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Hi Guys

Having only skimmed through Slone's high-power audio amp book, the chapter of interest there is actually a verbatim copy of a Motorola app note (AN1308), showing how to use the 3281/1302 BJT pairs to build a 100W and 200W amp. I have a copy of the app note, written by Andrew Hefley, an audio consultant, and in these designs complimentary diff-amps are used but they are resistively loaded.

I assume the problematic circuit is from Randy's project book?

One detail that I find to be completely over-blown and dripping with historic lethargy is the predictability of VAS current. From their ealiest writings in Wireless World, Self, Linsley-Hood, Baxandall and others have parroted the idea that the VAS current should be essentially solid and unchanging, hence the evolution from a fixed R, to a bootstrapped R, to an active current source load for the VAS. Push-pull VASs are denounced for not having a steady idle current, which is nonsense in a properly designed circuit.

My impression of why the predictable steady current is desired is the coincident desire to cling to a single-BJT bias regulator. It has been demonstrated in more than one place by more than one designer that slightly more complex bias spreaders exhibit the flat voltage control versus current that one needs particularly with a push-pull VAS. Most bias circuits in modern amps are slightly more complex anyway, with additional tempco constants built in, many making use of a second BJT. If these complexities are allowed, why is a "better" bias regulator not okay?

Some of the solutions found in the other threads linked above, provide elegant methods to produce predictable VAS currents when using current mirrors in comp-diff amps. There is a basic compromise that is hardly mentioned or glossed over inasmuch as emitter degeneration must be used if device matching is not to remain critical. The added Rs contribute noise, as do the mirrors themselves, so going fully complimentary with mirrors and current sources and anything else that can be duplicated seems like _a_ solution, but so is retaining the simpler circuit and matching the BJTs. Either way, slew rate is far higher than required for audio, so not much is lost except excess.

Have fun
Kevin O'Connor
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Old 6th December 2012, 09:01 AM   #2472
AndrewT is offline AndrewT  Scotland
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Originally Posted by Struth View Post
..........................My impression of why the predictable steady current is desired is the coincident desire to cling to a single-BJT bias regulator. It has been demonstrated in more than one place by more than one designer that slightly more complex bias spreaders exhibit the flat voltage control versus current that one needs particularly with a push-pull VAS. Most bias circuits in modern amps are slightly more complex anyway, with additional tempco constants built in, many making use of a second BJT. If these complexities are allowed, why is a "better" bias regulator not okay?........................
I think you are chasing the wrong horse.
The Vbe multiplier is basically a constant DC voltage source combined with a near zero AC impedance.
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Old 6th December 2012, 09:45 AM   #2473
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Hi Kevin,

Are you suggesting that the troubles with Randy's amp can be solved by using a more stable OPS bias generator, i.e. one that is insensitive to a variable VAS current?
If so, then I disagree. The trouble with Randy's VAS is that it is not just a bit variable, rather it's undefined, totally. It can range from zero to a destructive high current. A better bias generator will not solve these issues.

Cheers,
E.
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Old 6th December 2012, 02:48 PM   #2474
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A Vbe multiplier impedance follows the diodic impedance rule. At 10mA it's about 3.3R, at 1mA it's about 33R. Or 27R or thereabouts. This is multiplied by the factor of the multiplier. A 2.4V multiplier for instance will have an impedance of 13.2R. If the transistor has an Hfe of 100 and a 470R BE resistor is used, Ib adds about another 3.5R to this.

If we consider a 10% drift in bias significant, then we know it takes about 4.4mV to cause a 10% shift for a single diode. 8.8mV for a complementary EF output stage not counting emitter resistors. Say .22R degeneration at 120mA bias (.26mV). This means the output stage needs an extra 2.6mV for a 10% bias drift.

For a Vbe multiplier biased at 10mA, 2.4V with an Hfe of 100 and a BE resistor of 470R, with a complimentary EF output stage biased at 120mA with .22R degen, It would take only an excess 680uA through the VAS to raise output stage bias by 10%. With a bog standard 2Q CCS, a 10% reduction would be achieved if the current sensing transistor was heated 22C above normal, which would be somewhat extraordinary. In a 56V dual rail bootstrapped VAS however, a 3.7V rail drift would be enough.

I think this helps in getting some perspective on the issue.

If this isn't good enough, consider a CFP Vbe multiplier.

In amps where the VAS current source is not very good, which includes most bootstrap amps, the variation through the Vbe multiplier can be much more than 680uA at loud passages. This can affect crossover behavior.

In complimentary VAS amps with a double EF output stage, the driver Ib will go much more than 680uA and if you think about it this current is necessarily subtracted from the Vbe multiplier bias current. This effect can artificially lower the "oliver null point" because the multiplier's bias current peaks sharply during crossover.

So it can be important to have an effective lytic bypass across the Vbe multiplier if you want to have the expected crossover behavior.

Last edited by keantoken; 6th December 2012 at 02:53 PM.
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Old 6th December 2012, 03:36 PM   #2475
Struth is offline Struth  Canada
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Hi Guys

Check the corral. Self showed quite clearly in figs. 7-19 and.15-34 (of his power amp design book, 2009) how adding one R to the simple BJT bias reg improves its output voltage stability, and how a CFP bias reg performs far better than the singleton, respectively. I showed this in TUT2 (1997) in fig.4-37. It is plain to see that a not much more complex bias regulator provides superior performance to the single BJT everyone clings to. Keantoken's description also shows how a single BJT just does not have the power to behave like a voltage source - at least not a good enough voltage source to be used as a proper bias regulator.

My post had nothing to do with whether such a bis reg could "save" the current-mirror loaded comp-diff front end - it can't. But... a proper bias reg that actually acts like a voltage source allows the use of push-pull VAS with its varying current output and all of both their other benefits.

Self and Cordell both spent much time trying to stabilise Vq, and Self made a pretty convincing demonstration of the importance of this design facet as a means to minimise overall distortion.

As I said, the goal to have a simplistic bias reg has often been a part of thrifty UK design, and this ripples into the rest of the design decisions, justifying them and resulting in an overall economical design with reasonable performance. Performance can always be improved no matter what strides we make, and it seems that "reasonable" is a moving target. My assertion is just that the economy that drove the initial decisions back in the 1960s have resulted in a dogma that is quite ripe now four decades later.

Have fun
Kevin O'Connor
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Old 6th December 2012, 04:05 PM   #2476
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Yes, that collector resistor needs to be equal to the impedance of the bias generator. I have not actually seen anyone calculate the impedance of the bias generator, they just use the value Self used. I've been told that it's for temperature compensation. If the value of this resistor is chosen right, both an increase and decrease in bias generator Iq will result in a falling bias.

The resistor can be calculated from:

Rc=(.026*(Rcb/Rbe+1))/Ic+(Rb/Hfe)

Where Rb is the resistance seen by the transistor's base, IE Rcb and Rbe in parallel. Rbb should be added to this if you know it. Rbb can be obtained from Cordell's SPICE models for many transistors.

This will not apply to strange bias generators such as CFPs or that have an extra diode in the divider or at the emitter.
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Old 6th December 2012, 04:42 PM   #2477
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Quote:
Originally Posted by Struth View Post
Hi Guys
....
My post had nothing to do with whether such a bis reg could "save" the current-mirror loaded comp-diff front end - it can't. But... a proper bias reg that actually acts like a voltage source allows the use of push-pull VAS with its varying current output and all of both their other benefits.
...
Have fun
Kevin O'Connor
Okay, agreed. But aren't we drifting away from the original subject, raised by Olivier? That subject has nothing to do with bias generators.
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Old 6th December 2012, 04:55 PM   #2478
Struth is offline Struth  Canada
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Hi Guys

It seems to me that one of the other threads linked a few posts back has the fix for the VAS idle current issue for CM-loaded comp-diffs. There are probably other solutions but even you thought the idea was elegant.

My suggestion for a long overdue review of the notion that a constant or near-constant VAS idle current is "preferred" does seem related to the present discussion. A lot of respected designers parroted that same old line, casting a pall over push-pull VASs of all kinds for truly no good reason. I think if one dispenses with the VAS constant current dogma, one is freed to use push-pull VAS, push-pull diffs, better bias regulators, and all kinds of circuit variations that hold benefits as yet not fully explored.

The predictability of VAS idle current when CM-loaded comp-diffs are used seems mostly accepted. The various means suggested to fix the problem each have their pros and cons, as compromise of some sort is inherent in all design. So, you make a choice between balancing diff currents using matched devices or with CM loads. You choose between resistor noise or BJT noise. You might look at a common-mode current source load option at the expense of a few more parts. You choose between absolutely predictable VAS current or letting it land where it may. The list goes on.

I think one of the simplest solutions is to use the Wilson CM with fourth BJT added, as this sets both diff collectors at the same potential - or at least the collector in question has a set voltage. A VAS with emitter resistor then has a set base voltage to work against, with R being adjusted for the desired current. So, another compromise, doubling the BJT count for the mirrors...

Speaking of getting away from the original topic, shouldn't we be talking about how great Bob's book is? It has a few holes and some repetition but otherwise is very fine!
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Last edited by Struth; 6th December 2012 at 05:22 PM.
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Old 6th December 2012, 06:37 PM   #2479
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Quote:
Originally Posted by Struth View Post
Hi Guys
...
I think one of the simplest solutions is to use the Wilson CM with fourth BJT added, as this sets both diff collectors at the same potential - or at least the collector in question has a set voltage. A VAS with emitter resistor then has a set base voltage to work against, with R being adjusted for the desired current. So, another compromise, doubling the BJT count for the mirrors...
...
Hi Kevin,

This 'simplest solution' considerably reduces the VAS gain. Is that what you want?

Cheers,
E.
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Old 6th December 2012, 07:18 PM   #2480
Struth is offline Struth  Canada
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Hi Guys

Edmond, you certainly know that there are a multitude of ways to increase VAS gain. Is your query meant to be rhetorical?

It just seemed to me that the problem in defining the VAS current with the CM in place for comp diff inputs depended too much on exact Vbe levels in the operating circuit, and therefore some means of providing a less device-dependent output voltage was needed. The four-device Wilson CM does that, as far as I see, as would simply adding a diode to the two-device CM.

Poor VAS current definition is abated when the approximately 2Vbe of the mirror is weighed against a single Vbe for the VAS input, with the difference taken up by an emitter resistor. If you wanted to push the circuit back towards criticality, it is obvious that a Darlington VAS with 2Vbe input matches the 2Vbe CM output. But the devices all carry different currents at different temperatures, so their exact junction voltages will not be defined and you are back to the original problem.

You may have a subconscious agenda to keep the voltage loss in this circuit section as low as possible in order to maximise utility of the supply rails and thus attain maximum power? Most designers have that in mind even when it is not at the front of their mind. Personally I don't worry about it as low-loss output stages are easy to devise, and it is not always necessary to have maximum VAS gain to achieve very low distortion - it may be if this is the only stage you expect to get gain from.

I also don't worry about supply utility in that way as I don't run my amps to the point of clipping or anywhere near it. Most music signals live way below the full output of most amps, unless the amp is only capable of a watt or two.

Have fun
Kevin O'Connor
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Last edited by Struth; 6th December 2012 at 07:21 PM.
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