Reinventing the N-channel wheel?

[Tube_Dude]

Originally posted by Eva
Currently, I have a working prototype and this is how it looks:

It looks like a Hovercraft , with the fan at the rear side...

Now seriously , a complex schematic as yours , in a breadboard without oscillation is a accomplishment ...

[Eva]

Breadboards are evil. The prototype has several compensation capacitors (a total of 7, I think) whose optimum value may change a bit for the PCB version. Anyway, by employing proper layout techniques (the stuff I learn everyday from my SMPS prototypes) stable amplifiers may be built into breadboards, even HEXFET CFPs.

Note that the output devices, the current sense resistors, the output zobel and the supply decoupling capacitor (a single 10uF one between +Vcc and -Vcc since the circuit does not have a power ground connection) are all placed very close to the output devices and inserted in the same breadboard rows. Also, the area of the loops into which high currents flow have been reduced to the minimum, and the last stage of the gate drivers is just in front of the output devices. On the other hand, the area of small signal loops carrying less than 5mA is irrelevant.

By the way, I have replaced two resistors by current sources and I have added RC networks to reduce the gain at HF. As a result, CFP feedback is much stronger at audio frequencies and I can no longer measure any substantial bias current change between 15ºC and 60ºC heatsink temperature. The drawback is that each gate drive cell now requires 5 small signal transistors and 6 or 8 1N4148 diodes, but indeed the gates are well controlled

[ilimzn]

I was just reading your design goals and SOA issues on bipolars... have you ever considered bipolar-MOS cascodes in the output stage? Dimensioning it right makes it possible to use the bipolars up to their maximum current, but there are issues to solve, like providing proper paths to charge/discharge the gate capacitance and collector capacitance of the MOSFET. Not obviously, this combo has less rail loss than MOSFETs alone, because the cascode is self-bootstrapping, and only the BJTs define rail loss.

[Eva]

I have rediscovered a nasty pitfall about CFPs... The circuit produces one small 1uS cross-conduction spike each time it comes out of deep clipping

Fortunately, the zero current crossing is fine.


ilimzn: could you draw it?

[X.G.]

hi,eva




I had build a pre amp and a power amp which have your output topo basing on Janpanese Kenata circuit.the sound is nice.

the output topo can protect itself when the ouput is short!!!

I plan to design and try N-MOSFET diff + super pair + N-MOSFET output power amp.

good lucky

X.G.

[Ultima Thule]

Hi Eva,

are you going to use Source resistors at OP?

Cheers Michael

[Eva]

Yes, I'm using resistors in the current version, but they are not exactly source resistors but rather current sense resistors. The prototype now looks more or less like that:




It's quite weird but each gate drive cell asks for different frequency compensation, at least in the breadboard. The positive cell has less phase margin, maybe due to the current mirror, while the negative cell is faster (probably due to the folded cascode).

[ilimzn]

Originally posted by Eva
[BJT-MOS cascode output
ilimzn: could you draw it?

Here is an example - of course, this one uses 'complementary' MOSFETs and BJTs.
Note that Cbc and Cgs, drawn in the pic, are actually the internal capacitances of the BJT and MOSFET. For LAPT transistros such as 2SC2922, 2SC5200 etc, Cbc is not small, neither is reverse collector leakage, but even so, typical Cgs of a MOSFET in the position as drawn will overwhelm both, essentially there is no path for the Cgs to discharge or the MOSFET remains 'stuck' on, and the cacode does not work.
Because of this you have to add Rg and/or Rds as shown in the lower part of the drawing.
The cascode voltage sources can of course be capacitors in a bootstrap arrangement. The voltage on them defines the maximum voltage the BJTs will see across C-E, and in order to be inside the SOA and get good gain, and accounting for the required Cgs of the MOSFET, this will usually be about 10-20V. As long as this voltage is at least somewhat below 20V, the MOSFETs do not need extra gate protection zeners, and can be driven into Rdson region, by the gate voltages being bootstrapped over the rail voltages, so you get very little rail loss.
Even so, gate zeners can be used on the MOSFETs to provide current limiting. A remote way to short the MOSFET gate to source can be used to switch on/off the output stage completely, eliminating the use of relays for output protection.
As amp power goes up, over about 35V per rail, most dissipation has to be endured by the MOSFET, and they can easily be paralleled for that, or more current. Since gate drive is not a problem (it's driven from the output of the amp where current is abundant) very large MOSFETs can easily be used.

[Eva]

I have tried the prototype with +-24V rails and it works fine, so what we have now is a nice but complex 60W/4ohm N-channel amplifier. I'm considering trying other output devices that I have handy, like IRF520, IRF540 and IRFZ44, just to know how they behave before going into higher rails. I also have a few IRFP450 and IRFP460, but these are TO-247 and don't fit to the breadboard. Would a single pair of IRF640 survive driving a 4 ohm reactive load (a bass horn) with +-40V rails provided enough cooling? (I'm afraid we are going to know it soon).


ilimzn:

That's a smart way to take advantage of the dissipation capabilities of MOS devices without messing with gate drive circuits and frequency stability issues as I did

[CBS240]

What if you flipped the mosfets around and used seperate drive circuits(diff + EF/w current source; mosfet as output, -fb from drain) for each one, + & -? Similar to guru's SKA, with a DC offset to set the Vce of the BJT. This way you could get rail to rail and have it "regulated" as far as the BJT is concerned. if you can keep it stable...

Add a lower rail voltage and a couple of Schottkys and you have class G.