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Diagram comparing output circuits

Problems with inverted Darlington circuits
The following are the reasons why these outstanding circuits are not used more commonly:
A)
It is extremely difficult to maintain thermal stability for bias current.
B)
Each stage has its own gain, so it is easy for a slight oscillation to arise in the phase margin, and thus sophisticated mounting technology is required.
A)
With a normal Darlington system, the variation direction of the base voltage (Vbe) due to temperature is always the same, so total temperature compensation can be achieved at once by attaching a temperature compensation transistor to a heat sink equipped with an output level transistor. However, with an inverted Darlington, there is inverted amplification, so the direction of variation is reversed between the previous and later levels, and compensation cannot be achieved with the same method. For this reason, temperature compensation is conducted on two levels (as shown in the diagram) with special temperature compensation for the first driver, and separate temperature compensation for subsequent levels. With an inverted Darlington in particular, the most critical point is stabilization of the first level, and this almost completely determines the bias current on the output level. Therefore the bond for first level temperature compensation transistor was strengthened by mounting, together with the 1st driver, to an aluminum heat radiator which has a small thermal time constant.
Diagram of temperature compensation for power amp section
B)
The diagram shows analysis, with a simulator, of a 2-level Darlington and 2-level inverted Darlington circuit. A particularly striking difference can be seen in the output impedance and its phase characteristic. In an ordinary Darlington, the phase (in the high range in particular) fluctuates in both the + and - direction due to bypass current, so there is only a small amount of total phase shift near the bypass current which is actually used. With an inverted Darlington, however, there is only a shift in the + direction, so the phase margin is small and there is greater tendency to oscillate. The diagram shows a simulation of the 3-level inverted system used in this case, but the amount of phase shift increases, and furthermore there is a greater tendency to oscillate. As can be seen from the diagram, there is an output impedance peak at 20-30MHz, and it is crucial to control this phase shift skillfully.
You can use an impedance for phase correction at the base of the output level transistor. However, with an ordinary air-core coil, Q is high, and there are problems with diving and high-frequency fusion, so we use ferrite beads in the jumper wire. These ferrite beads have extremely high magnetic permeability at low frequencies, high loss in the 20-30MHz band whose phase is to be shifted, and low Q. The jumper wire is simply threaded with ferrite beads, so there is total shorting in the audible range, and there is no degradation of sound quality.