Opamp based Amp with current driven VAS

[moschfet]

That's an idea for an opamp-powered amplifier with current-driven VAS. The simulated values are not bad, so far.

Especially since the amplifier is operated (simulated) only at 26.2mA total. 5mA for the power transistors.

 

[abraxalito]

The 3k3 resistors in the power rails of the opamp look to be a problem. At the nominal quiescent current (2.6mA) they'll be dropping 8.6V each. The opamp will still operate at such reduced supply but its output voltage swing will be severely constrained.

 

[moschfet]

Yes, the resistor forms a constant current source with Q1. This prevents the currents from shooting through when overdriven.
The instability occurs when I increase the bias above Q9.

The pdf shows the last circuit.

 

[UltimateX86]

A folded cascode nice

 

[sgrossklass]

Using opamps with high rail impedance always is kinda risky business. That's just the opposite of rail bypassing, so you can probably guess that stability issues are not unlikely. Things may work in sim but not in real life, or fail miserably with a different opamp type.