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Re: I fixed a bias problem in my Mosfet follower (experiment #1 continued)

Posted by kurt s on June 30, 2003 at 20:28:38

In Reply to: Re: I fixed a bias problem in my Mosfet follower (experiment #1 continued) posted by mfc on June 30, 2003 at 20:02:34:

"How lightly loaded are the output stage CFs in a Solid State
amplifier with respect to this discussion?
Since CFs are used as output stages, and speakers are the load,
how well behaved can this arrangement be?"

Ever hear about the "Super Linear Cathode Follower" that Allen Wright uses? I don't know if he invented it or he just coined the name, but it's pretty neat.

First, much nonlinearity in cathode followers occurs because the plate current changes with changing Vg (and Vk). To combat that, a constant current source is used as the load for maximum cathode resistance. Second, since Vpk is also moving, there is distortion with Vg and Vk moving. To combat that, you add a "cascoded" stage from the Vk output and run another cathode follower back up to the plate voltage. In this arrangement as Vk moves, Vp moves with it as it follows Vk dynamically (with a level shift of course). Then you have a situation where Vpk and Ip are both constant with varying Vg and Vk. This forces Vgk to be constant. In other words, the original cathode follower never moves from its central bias point on any parameter with the signal, to a first order approximation!

The SLCF sounds very transparent so long as the load is not difficult and doesn't upset the constant currents and voltages around the cathode follower. The more you load it down, the less constant these operating points can be, same as before. I have heard an amp where it had a CF modified to be a SLCF and the difference was very nice.

It is also interesting to note that a FET and a pentode do not change behavior much with changing Vpk or Vds when operating in the constant current region (saturation region). So it does not need the plate voltage to be held constant with respect to its cathode like a triode (or BJT). What I am suggesting is that good power MOSFETs have that one advantage over BJTs, and pentodes over triodes. Maybe MOSFETs are better than BJTs as followers despite the fact that there is a tighter voltage control of Vbe over a Vgs for this one reason.

I seem to recall a schematic where someone used BJT followers on the output of an amp and used a cascoded BJT to hold Vce constant for increased linearity.

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