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Old 25th August 2006, 09:37 AM   #1
Stee is offline Stee  Italy
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Default Toroidal Output

hello
see this simple hexfet (IRFP240/9140) magnificator
with a standard PSU toroidal transformer (110+110/25+25)
sense is 2V and bias 4.25V (classA)

NOTE: sources are fixed on ground
--> for super-linearity put a resistor on each source (0.5/1.0 ohm)
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Old 25th August 2006, 11:09 AM   #2
sajti is offline sajti  Hungary
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I'm not sure, that it works. But I'm sure that Your FETs will be dead in very short time, due the thermal runaway...

sajti
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Old 25th August 2006, 01:34 PM   #3
ilimzn is offline ilimzn  Croatia
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The idea is OK in principle but only in principle.

1) The biassing implementation should be changed. As it is it would only work for lateral MOSFETs. VMOS/HEXFET/TFET and similar architectures exhibit a POSITIVE Id coefficient with temperature rise, at constant Vgs. Of course, it is reduced by adding source resistors, but this also reduces GM. The resistor + pot in series between the MOSFET gates should be replaced by a Vbe or, even better, Vgs multiplier. For a Vgs multiplier, a small MOSFET will work better than a larger TO220 one if a low current through it is desired. That being said, the 1M resistors providing bias are needlessly high - 100k would give you 50k resistve input impedance, which is standard, yet provide a useful current through a Vbe or Vgs multiplier for proper bias setting. The input 47k can be replaced by a large value (1M) and the input caps need to be increased substantially (this has other benefits, see below).

2) The power supply to the bias setting resistors should be well filtered, to prevent power upply hum and noise from being injected into the amp input.

3) The input impedance of the amp is almost entirely capacitive, in parallel with about 500k (as it is drawn now). This is actually the least contributing factor as the capacitances in question are rather high, and not very linear. Cgs is about 1.2nF for each MOSFET and Cgd is about 160pF, but this gets multiplied via Miller effect by the gain as seen from the drain pin. All in all, we are talking about some 506nF of input capacitance with negligible resistiove component as the sources are grounded. This requires a preamp capable of relatively high current output, and certainly low output impedance.

4) Finally, there must be a means to set DC balance. Even though the input is capacitively coupled, and the output inductively, so no DC can pass through the amp, any DC current imbalance in the promary will result in a DC flux in the transformer, wich will saturate the transformer core more or less successfully, depending on the type of the core. Certainly, this schematic will not work very well with toroids as they are especially sensitive to saturation. A DC offse is practically guaranteed as the IRFP9140 and 240 have slightly different treshold voltages at the given bias current.

5) Output impedance will be rather high, this circuit is the equivalent of a pentode tube amp. It would be quite simple to add a minimum of feedback through the MOSFET sources - the input sensitivity would drop, but so would the output impedance. A few turns of wire around the core would suffice.
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Old 28th August 2006, 09:53 AM   #4
Stee is offline Stee  Italy
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Default modify of Hexfet Magnificator

ciao
thanks a lot for your suggest
i will try to re-design, test and pubblicate the new-one
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Old 28th August 2006, 10:53 AM   #5
ilimzn is offline ilimzn  Croatia
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Sorry, the time alloted for editing ran out - under (3) it should say 5-6nF of input capacitance...
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Old 28th August 2006, 11:51 AM   #6
Stee is offline Stee  Italy
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can you write a little schematic of definitive layout?

that i can pubblicate on my website like a joint-adventure


www.esafono.it

...also i have another idea (I'm an italian)
may be need a little transformer for input

www.audiophonics.com

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