[woody]

Zitat:Original geschrieben von woody

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Ich kopiers mal (Bauteilbezeichner angepasst):

Zitat:OMC Ordinary Miller Compensation
TMC Transitional Miller Compensation
TPC Two Pole (Miller) Compensation

Siehe (bei Bedarf) hierzu: Cordell, Kapitel 9, - p. 171ff

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Zitat:Fig.5 shows a simple, but fully complementary symmetrical implementation, which also incorporates two Baxandall super pairs. If single transistors were used instead of super pairs, the low distortion of the IPS would have been completely swamped by the distortion of them.
Although the current gain of the TIS is just 0dB, the gain-bandwidth-product of the stages enclosed by the Miller loop is still too high. Therefore, a so called PLIL compensation (C3 & R17) is added to stabilize the Miller loop. See John Ellis for more information on this subject.
By the way, many other configurations also need additional compensation to tame the Miller loop.
Since the VAS has been replaced by a cascode having a current gain of just 1x, another problem arises: Any load (including C6) at the output of the TIS will also been 'seen' by the IPS. At first sight not much seems to be gained by this circuit. But... this is where TMC comes into play (and shines!), as it moves the capacitive load from the TIS to the OPS, well, at least at frequencies of interest. In this way (differential) collector currents of the IPS are greatly reduced. Compared to TPC at 20kHz by a factor of about 20 and compared to OMC by a factor of about 4 (of course, depending on circuit details).
For the same reason THD20k is much lower with TMC: 51ppb, leaving TPC (C6 & C7 not swapped) in the dust with an embarrassing 1.3ppm.
It should be noticed that the current gain and gm are still rather low. Ignoring second order effects (finite beta, etc.) they are approximately: gm ~= 2 / ( RE + VT / Ic )
= 2 / ( 10 + 26 / 2.5) = 98 mA/V (simulated: 94 mA/V)
Iout / Ifb ~= Rfb*gm = 200*0.098 = 19.6 (simulated: 17.4).
Therefore, it's highly recommended to compensate for the lack of gain by means of a pre-driver.

[...]

A welcome side effect of a TIS gain of just 1x is that you don't need a CMCL to define the quiescent current of the TIS (that saves 4 to 6 transistors). Instead, it's defined by the tail current of the IPS. More precisely, it's just equal to the tail current.
On the other hand, the limited gain has a marked effect on the effectiveness of TMC in reducing the distortion of the OPS: As the reduction is based on increasing the loop gain, it is only effective if there is enough gain 'in reserve'. Without a 'surplus' of gain the distortion from the OPS will not or hardly be lowered by means of TMC.
Another point of concern is an increase in THD when the inputs of the IPS don't 'see' equal impedances. If, for example, R15=0 then THD20k = 212ppb and if R15=400 then THD20k = 123ppb. These issues will be remedied in subsequent implementations.

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Zitat:Some Specifications
THD20k at Vi = 1.47V-pk: 61ppb, mostly 2nd and 3rd harmonics and independent of Ri (within certain limits, of course).
THD200k at Vi = 1.47V-pk: 3.1ppm.
Slew Rate without input filter: max. 500V/us.
Max. output voltage at node out: +/- 47.5V;
Max. voltage at node A: +49V. Thus almost rail to rail