Direct-coupled amplifier circuit

[tca]

I was browsing the google patents site and found this direct-coupled amplifier circuit. The circuit is this



Is there other way of getting a direct coupled output from a class A amplifier with out dynamical changing the bias of the amplifier? Comments on the schematic?

Cheers.

[Cliff Schecht]

This schematic is what you typically see in direct coupled amplifiers, that is some sort of output DC servo circuit that feeds information from the output back into the input. While this isn't necessarily a standard way of doing such a circuit, you rarely see audio DC coupled output stages without some sort of correcting circuit. The problem is every amplifier (tube, BJT, MOS) will have some sort of output offset voltage associated with it. There are many reasons for this (input bias conditions, non-matched components, non-infinite open loop gain in a negative feedback systems, etc) but regardless, there is almost always SOME DC at the output if it isn't corrected for.

Sorry no real comments on the circuit itself other than it's a neat output stage and gives you both good current drive into a resistive load (common drain amplifier) and some voltage gain as well (common source amplifier). Definitely useful in a budget oriented design where you can skimp on a gain stage by using something like this instead to get both voltage gain and power drive.

[tca]

This schematic is what you typically see in direct coupled amplifiers, that is some sort of output DC servo circuit that feeds information from the output back into the input.

I've done a few research and found exactly that! An opamp is usually used to do the job.Thanks.

I wonder why isn't used more in stomboxes? 

[Cliff Schecht]

We don't need the bass response offered by direct coupling or the added complexity that your typical DC servo circuit requires. Low E on a guitar tuned standard is only 82 Hz which doesn't require lows down in the Hz range. Furthermore all of our circuits operate around 4.5V bias point (for a 9V supply) which means we have to block DC anyways before we send signal to the next circuit (especially a tube preamp where the input is usually direct coupled). If we ran off of dual supplies and our signals were centered around ground (0V) then a DC coupled circuit *might* make sense, but I usually avoid extra complexities like this unless I have a specific reason to do so.

FWIW the hifi guys are nuts about this (direct coupling) because of the extremely low bass response you can get. Also you avoid any distortion added by capacitors in the signal path which some golden eared experts say they can hear. At a certain point you start getting into voodoo electronics and could probably do an entire PhD thesis over the psychoacoustic phenomena that is found in audiophool grade capacitors but nonetheless, the circuits are still super cool and a good study in both clever biasing schemes and DC feedback loops..