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

Posted by mfc (A) on June 30, 2003 at 22:18:21

In Reply to: 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:

Hi Kurt,

> 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!

I'm thinking in terms of transfer curves and feedback these days.
Just a mode to be in and kind of interesting for now. What I've
seen in doing some hi-res simulations is that signal swing relative
to a transfer curve is very important in determining the linearity
of the stage (neglecting feedback for the moment). This seems to
be because transfer curves are linear in their middle part, and
non-linear at their extremes.

What you are talking about above in terms of transfer curves and
feedback is that the operating point can be very well confined so
that a signal won't cause much movement up and down the transfer
curve. Ideally, it would stay at a single point! This should make
for a very linear stage I would bet (without trying it). I might
have to try investigating the SLCF.

What my previous message was saying and the very interesting thing
that clicked with me is to think about a CF in terms of feedback...
Picture a simple CF. It has an unbypassed cathode resistor...
this causes feedback to occur. To get as near to feedback=0 as you
can get with a CF, replace the cathode resistor with a current
source. In this case there still is a feedback mechanism. It is the
load. In fact, the load is now *the primary* feedback mechanism.
Now if the load isn't well behaved (reactive), then feedback won't
occur at the 180 degrees like it is supposed to and won't be well
behaved either. This would cause problems.

Other types of stages (like a common cathode, or transformer) have
their feedback determining component more isolated from the load.

This leads me to wonder about Solid state output stages that use
followers in the power output stage connected to a load (speaker)
that isn't well behaved at all. If the load is now the primary
feedback determining part of the circuit, and a load isn't well
behaved, then this would lead to problems, unless the supporting
circuitry can hold the devices so that they don't swing much over
their transfer curves.

This post is made possible by the generous support of people like you and our sponsors:

Kimber Kable

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