Single ended amplier

[Mark Hennessy]

The power FETS behave like resistors with a minimum on resistance, which can be low enough that there is no real "saturation" in the sense of valves or bipolar.

Indeed, I've already said this (emphasis added):

The one thing I'm not sure about - because of my lack of experience with valves - is just how "saturated" a valve can be. With bipolar transistors, transistors, it's pretty easy at a first and even a second look (though to do saturation properly gets surprisingly complex, but it's mercifully rare to need to go there). With MOS-FETs and JFETs it's even easier because the channel is basically resistive. But honestly, I'd be guessing when it comes to valves.

In that same post, I went on to say:

Obviously that, plus whatever is across the cathode resistor (assuming it's held still by a bypass cap) determines how low the anode can swing.

So it's completely clear that when I used the word "saturated" (in quotes), I was talking about the available output voltage swing from the anode (or collector, or drain). Specifically, how low it can go.

The exact mechanism inside the device is interesting in its own right, but is tangential to the point we were trying to get across (how to choose the standing DC voltage or current).

It's also worth saying that "saturation" is one of the terms used in the recording industry when over-driving something like an analogue tape machine or a "tube" stage for artistic effect.

So, we've answered the question about valves - 20-40V approx - and I certainly plan to read up on this for my own edification. But I'm left feeling guilty for asking the question because it's confused the thread, and the voltage isn't even relevant to the OP because he's using a transformer anyway! All I was trying to do was explain the thought-process behind setting DC conditions, using a resistive anode load for simplicity - which I hope has helped in the end.

So, by all means start a separate thread on this - I think it could be a worthwhile conversation. But can we please leave the subject alone in this thread? Remember - we're trying to help someone learn