Bob Cordell Interview: Negative Feedback

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#63
Re: Re: Re: Re: Re: Re: Re: Re: Perspective on Open Loop Bandwidth Comparisons

andy_c said:


Hi Heinz,

Good to see you back!

I suspected that an inverting configuration would also have the same effect. One thing that concerns me about inverting mode is that if the input is open for whatever reason, there's a unity gain situation, which could lead to oscillations. This worries me, so I figure non-inverting is a safe solution.

BTW, thanks again for your circuit for biasing the diodes for clipping. You probably noticed it in my file from the other forum.
Click to expand...

Thank you, I see it, its a really impressive big circuit... and I wait for AS´s convergence solution 😉

Yes, with a open input oscillations can occur if one use e.g. a opamp wich is compensated for a gain more than one.
In this case a resistor from the summing point to ground will help!

Heinz!
 
#70
darkfenriz said:

Now I use tripple darlington output stage based on high Hfe transistors (2n5401/2n5551, 2sa1837/2sc4793, 2sa1943/2sc5200), so input impedance of output stage is around 10Mohm probably with all those nonlinear mechanisms, but is paralleled with 67kohm most linear electrical device around.
Which one would you prefer, input impedance being of comparable value?
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But all those non-linear mechanisms are still there, and the VAS will still need to drive them. The only difference adding a resistor will make is that the VAS will then have to drive the resistor as well.
Also, unless you are running a really low current VAS, 67k would be negligible loading anyway. 750uA peak will drive a 67k load to over 50V. In fact, if your amplifier has any frequency compensation done at the VAS stage (either miller or feedback or shunt), the loading of the VAS from that at high frequencies will be a couple of orders of magnitude greater than the loading provided by a 67k resistor.
 
#72
Hi Ingrast
---If a sufficiently high open loop gain can be attained within practical stability constraints to achieve this level of performance, then the native nonlinearities can be considered to be adequately tamed, and may be this is what your mentioned papers made reference to.---
That's it. We are in agreement.
 
#76
This principle also gives another nice extra bonus. The output transistors are always working in constant Vce (can be set about 5-10V), in all output voltages. Constant Vce is nice, isn't it 😀 Considering Early effect.
Bonus #2, since the Vce can be set small, we only need few transistors for big output rating.
 
#79
G.Kleinschmidt said:




But all those non-linear mechanisms are still there, and the VAS will still need to drive them. The only difference adding a resistor will make is that the VAS will then have to drive the resistor as well.
Also, unless you are running a really low current VAS, 67k would be negligible loading anyway. 750uA peak will drive a 67k load to over 50V. In fact, if your amplifier has any frequency compensation done at the VAS stage (either miller or feedback or shunt), the loading of the VAS from that at high frequencies will be a couple of orders of magnitude greater than the loading provided by a 67k resistor.
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If Zin of output stage is in order of Mohms than nonlinear current driving it is neglegible compared to linear current driving a resistor and, yes, a smallish lag capacitor.
 
#80
john_ellis said:
Hi all

I'd like to pick up on John Curl's point about all sorts of dynamic distortions.

I was influenced (I admit it) by Otala's TIM work when that came out but I now think that his approach is unsound as I have designed some amps with low distortion and wide open loop gain which don't appear to TIM.

However, I agree with dynamic distortions being a problem other than slew rate. What seems to be overlooked quite often is the gain linearity of a bipolar - I've mentioned this in the 50W amp threads. With a high open loop gain it seems possible to me to use good old 2N3055's (actually no, old RCA 2N3055's are only available as collectors items) and still build a <0.01% distortion amp.

MOSFET's present a conundrum. While their static linearity might be streets better than a bipolar, (at least in terms of Id:Vg/Vb) they are temperature sensitive, and it is necessary to consider the dynamic linearity. A MOSFET's gain could perhaps almost halve between room temp and (say) 100C.

Of course most music is a series of pulses, and in both bipolars and MOSFETS, the dynamic thermal impedance is far lower than the static thermal impedance. In other words, short pulses don't affect the temperature much. What this does mean is that if we are going to ensure that simulations are correct, we need to have a thermal model as well as an electrical one. And this introduces any dynamic changes in crossover distortion, gain non-linearity, etc. etc.

Meanwhile, it seems to me that dynamic gain variations can only be handled effectively with large feedback. I am not convinced yet that local feedback can provide enough to do this, but possibly the latest bipolars from ON Semi and Toshiba etc (MLL3281 etc) might allow progress in this direction because of their superb linearity.

Roll on SPICE-Thermal!

cheers
John
Click to expand...

You raise some good points and questions, John.

First, I think when we refer to "dynamic" distortions there are at least two classes of them that we need to distinguish.

The first class, which I think is referred to more often, is dynamic in regard to the rate of change of the signal. So things like TIM and other high-frequency nonlinearities fall into this category. This is what Otala is testing for with the DIM test. Similarly, this is also being tested for with tests like THD-20 and CCIF.

The second class, and perhaps equally important, is "dynamic" distortions and effects in regard to thermal dynamic activity, where, for example, the temperature of different components changes as a function of the program material, and with thermal time constants. I don't think this one gets as much attention, but I worry about it a lot in regard to output stages. It is obviously much more difficult to simulate or measure. I have usually thought of this mostly in terms of the damage done by fluctuations of the Class AB bias point (as discussed before), but your point about device gm variations, especially with MOSFETs, is also to watch out for.

I would say that so far, I have not seen these thermal variations in MOSFET gm to be a problem. These thermal variations, by their very nature, will be slow - milliseconds at the fastest - so in large part it is not difficult for feedback to correct them - at least in a simplified point of view. BUT, if they cause output stage bias to go far from the optimal, that will create crossover distortion, which is HARD for NFB to correct.

Cheers,
Bob
 
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