Somehow CCS implementations became a theme of the year 2011 for me. This little scribble is here just to make sure I do not forget a nice topology I came across. Apparently this one has been invented by someone else, not me.Here's the original source where I saw it for the first time:
|Shadow Constant-Current Source a.k.a. "Compliant CCS" by John Broskie|
The top MOSFET is configured in what I call a compliant constant-current source, as it is not set to any fixed amount of current flow, rather it adjusts to match the idle current through the bottom MOSFET and sets the DC division of the B+ voltage to 50% at the output. As far as I know, I am the only one championing this topology, which is odd, as this circuit works wonderfully well and is quite simple. (c) John Broskie @ TCJ
I tend to agree with Mr. Broskie's statement that it's odd not to see this topology being deployed in handful of audio designs. Possibly I have an explanation to this oddity: most of electronic engineers are trained to think in the "voltage domain" and can only manage currents via the literal application of the Ohm's law. Understanding this topology and (what is even more demanding) - appropriate application of it to a design would require somewhat orthogonal "current domain" mindset. I must admit that tube designs train this "current" thinking better than OpAmps 🙂
As of today I am thinking of utilizing this "Compliant CCS" topology in the voltage amplification stage of my still-coming-soon "HotFET Headphone Amplifier". There the Compliant CCS could indeed simplify things. Though it may not be recognizable in my schematic from the first glance. Here it is presented in its virgin beauty in the hope that nobody could blame me in a plagiary later on.