Looks like the "super-complementary" stage had some use in Bryston amplifiers:
http://www.bryston.com/PDF/Schematic...SCHEMATICS.pdf
(Sorry for posting transistorized links on a tube forum.)
"Alex Pogossov" wrote in message ...
"patrick-turner" wrote in message ...
I've posted up an additional schematic for a Hybrid amp for 39Watts class A which has EL34 driving complementary N&P source followers helped along by common source P & N mosfets which I have not yet ried, but which looks good in theory.
Those wanting to simulate the schematic, or actually try it out by building it may like to post criticism here.
See Fig 16 at the bottom of the page at http://www.turneraudio.com.au/solids...ube-input.html
Patrick Turner.
Criticism? Critique? I do not think these are the correct terms. How can a stamp collector criticise a coin collector? Each one would think that his hobby is great, but the hobby of the other person is utter waste of time. We are free willed people and each of us is fully entitled to go insane in a particular and unique way. Therefore we should not judge or criticise which is an infringement on the free agency. Instead of "criticisig" I will make some comments.
1) The whole concept of using a tube to drive MOSFETs is ridiculous becaude it kills the greatest advantage of a solid-state design -- DC coupling and deep feedback. In your circuit you have two (!) differentiating capacitors in the feedback loop: one to the screen, the other to the MOSFET gates. Therefore low-F transients will be horrible. DC offset on the output is not compensated and needs trimming. What happens to the output when the tube is warming up or hot pulled out of socket? An enormous cone ripping pop!
2) This monster uses heavy iron as a 40H choke and other variants even interstage paraphase transformers. Who in the right mind would use solid-state compact devices combined with this monstrosity? Why not make a proper well desidned DC stabilised solid-state amplifier? In class A everything will sound terrific. Just use large heatsinks and more fans.
Now concerning your "quartet" of MOSFETozzi.
A MOSFET has a unique feature -- a quadratic transfer function. Consider a MOSFET with zero threshold and with I = v^2. At 1Amp bias it will have S=2A/V. Consider the MOSFET is biased to 1 Amp by 1 Volt of gate bias.
In the top leg of the push-pull stage output current Ip = (1 + x)^2 = 1 + 2x + x^2, where x is AC component of drive voltage.
For the bottom leg In = (1 - x)^2 = 1 - 2x + x^2.
After combining (subtracting) currents Ip - In the load current will be Iload = 4x.
In other words, in class A, second harmonics are TOTALLY cancelled, and the total effective transconductance is 4A/V. This is a uniwue feature of a MOSFET -- TOTAL cancellation of distortion. Neither tubes with their law of 3/2 not BJTs with their exponential law can do that miracle. This is why several generations of audio designers have been phooled into thinking that MOSFETs are superior to BJTs.
Now consider your approach of taking a current from one MOSFET, converting it into voltage (by 2.2R resistor) and feeding as a drive to the gate of the next MOSFET. Here you violated your own rule of thumb: "Never use a distorted signal from one power device to drive another power device". What you virtually suggest is to use a distorted drain current of the first MOSFET to drive the second one. You add quadratic distortion on top of another quadratic distortion. Now the transfer function will be of the fourth order.
Some maths:
Let us suppose we use the above ideal MOSFETs and using a resistor of 1Ohm, derive drive voltage for the next MOSFET.
Then for the top leg: Ip = ( 1+ x^2)^2 = 1 + 4x + 6x^2 + 4x^3 + x^4. We can see that now we have all the harmonics up to the fourth order. (Note: in this simplified case the transconductance gets doubled. In your circuit you use a AC divider to bring it down for equal current sharing.)
Similarly for the bottom leg: In = 1 - 4x + 6x^2 - 4x^3 + x^4.
Now when you combine (subtract) both legs, the load current Iload = 8x + 8x^3. The additional members of the "qartet" will nicely produce 50% of the third harmonic (at full power)! Note also that this 50% refers to the current. Since the stage works as follower, voltage distortion is reduced by the internal NFB -- the closer the gain to unity, the less is the voltage distortion.
I thought you intended to use a quartet to equalise differences between P and N MOSFETs which though called "complementary" are not exactly so. If I understand correctly you attemp was to make top and bottom legs identical by using both genders in each leg.
Well, there exists a proper solution for that -- super-complementary follower:
http://www.valveradio.net/en/home/au...ary-stage.html
However it is not popular because even if top and bottom side are different say by 20% in the transconductance (because of inferiority of P-MOSFETs), the residual asymmetry will be almost completely defeated by the deep global NFB in a solid state amp. In your case where the NFB is perhaps not more than 20dB, symmetry becomes far more important, even in class A.
Perhaps the concept described in the above link will be of more benefit to your case than to any one's else.
Regards,
Alex