Originally Posted by KimLaroux
Wow thanks for the clear and informative post. Really clears things up. Source follower topology really confused me, as I'm used to use MOSFET as a switch to power electric motors and such.
That makes a lot of sens now. The gate voltage sets the limit, so that the source will never rise over Vg-Vgs(th). How clever!
The idea in this amp is to adjust the bias so that the source never goes higher than the heater voltage, or not too much higher. Raising the gate voltage will raise the source voltage, which in turn will make the heaters pull more current. So there's really no way to adjust Vgs, since it's a property of the MOSFET itself, and the heater will always "push" the source voltage until Vgs(th) starves it. If I got that right, then there's simply no reason to raise the gate voltage, as it does not give the MOSFET any more headroom. Sure it will allow more current for the output, but I think 150mA is enough for most headphones, right?
When I finally get the time to continue working on my amp, I think I'll just revert the bias to what it was so that the source sits around 15v. Damn, to think that I blew a nice Sylvania tube for no reason.
Again, thanks the_equalizer, really appreciated.
Originally Posted by Goobley
Yeah this is really great, it's helping me learn so much more than I expected from a simple amp project, I'm sure this info will be useful one day (having had no formal training).
Thank you. I'm glad you found the post clear and informative. This piece of understanding eluded me for so long that I actually have a feeling of relief now. And to think it was so obvious right from the start. For me too, aside from the musical pleasure it has brought me, building it and tweaking the amp has taught me a lot.
I've kept thinking about the source-follower configuration and doing analysis and simulation trying to understand some other aspects of the output stage. For instance, KimLaroux mentions headroom. At first I also thought that the output's stage headroom depended directly and completely on Vgs. I thought that since the drain is sitting at 48V and the source at 13V then the gate voltage should be right in the middle of that range, that is 13 + (48-13)/2 = 30.5V, then that would give a headroom of 17.5V peak-to-peak and set the quiescent point of the MOSFET right in the middle of class A... Oh, how mistaken I was! I was trying to understand a source-follower (or common drain) as if it were a common source amplifier...
However, armed with what we now understand of the behavior of Vgs in the source follower configuration we can more clearly see the limits of voltage swing available to the output MOSFET.
On the negative swing side we have the situation where no voltage is dropped across the source load, 0 volts. All the supply voltage is dropped across the MOSFET. Since the quiescent (no signal) voltage drop across the load is ~13 volts it follows that the input signal at the gate can go to ~ -13 volts. Probably a little more since the source-follower's voltage gain is slightly less than 1.
On the positive side the theoretical limit of positive swing would be when Vg reaches the supply voltage; when it 'hits the positive rail'. Since the quiescent gate voltage is ~17 volts, it follows the maximum positive swing is ~31 volts. Probably somewhat less since as the source voltage rises following the gate voltage, the drain-source voltage decreases dramatically and it'll reach a region of non-linearity or saturation (given the Vds, the MOSFET cannot let more current through to support an increase in Vs, or support it linearly) before the gate gets to 48 V.
As you can see the headroom is assymetrical and if you read the literature on source-followers (or tube cathode followers for that matter) other biasing schemes are required to allow the MOSFET to swing symetrically; such as a negative supply connected to the source load, or a 'bootstrapping' resistor.
Please note that the above discussion assumes the ONLY load is the MOSFET source load, in the SSMH that is the tube heater. The moment you hook up a capacitor to take the AC and apply it to the headphones load, the available headroom alters quite significantly. I've run some simulations with 32 ohm resistive loads and the negative and positive swings are considerably diminished. I don't have the exact numbers handy but it was something like +15 to -5 V ! This is logical since, from the alternating current point of view, the load looks like 23 ohms (84 ohm and 32 ohm in parallel). As expected, higher impedance headphone loads have less noticeable effects.
Being able to swing 'only -5 V' with a 32 ohm headphone load is probably a bit of a moot point given that even Mr. Millett himself mentions that listening to a 3V RMS signal in a pair of Grados will cause permanent hearing damage. But sill, understanding the inner working and limitations of the SSMH output stage will definitely help in understanding how the amp will interact with difficult loads (like planar headphones) and/or to modify it to improve it's performance.
Edited by the_equalizer - 11/12/12 at 11:52am