Modeling differential input pair (audio amplifier)

[materials_guy]

v

[dannyf]

The beta will change with bias Ic. So without a defined amplification factor, your current gain would change as well.

[GK]

The amp design is very cleaver. Rather than the typical VAS, the input stage is followed by a current amplifier (CAS?). The only thing that comes close to this is THS4120 - see attached.

It is the first time I have seen people doing this in audio - JLH1969 may be the only exception.

       

This amplifier isn't very "cleaver" at all. It is in fact very rudimentary circuit completely typical of the era. That diode is just a lame and relatively ineffectual attempt to compensate for the VAS transistors Vbe and force current balance in the long tail pair. The diode doesn't change the fundamental operation of the 2nd stage - the stage is still in every sense a "VAS".

The "2nd" stage of the THS4120 is hardly unique. It is known as a "folded cascode" and has been used in integrated op-amps for decades (the AD797 is the most popular "audio" op-amp to use the topology). It is in no way analogous in operation to the amplifier circuit presented here. The folded cascode "VAS" simply mirrors the long tail pair signal current, transposed onto a higher value quiescent current. It is in fact a common-base stage that mitigates the Miller effect and actually has a signal current gain of one. It is for the latter reason that the combined 1st and 2nd stages of amplifiers using the topology are often lumped together and referred to as a single gm/gain stage/block.