ClassB+a mosfet amplifier

U130421

Disabled Account
2015-12-16 2:00 pm
This amp makes use of dynamic expander character of mosfet in classB mode to deliver ultra fresh sound with opposite phase odd order harmonics.
Warning , the sound of this amplifier is addictive .
 

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Lateral MOSFETs are self-stabilizing above a fairly small current value, but VFETs may have this threshold of thermal stability way up at tens of amps - basically they need thermal compensation typically.


There is a point of inflection in the response curves below which the tempco is negative, and where this is matters.
https://www.onsemi.cn/PowerSolutions/document/AND8199-D.PDF
 

U130421

Disabled Account
2015-12-16 2:00 pm
Can I use IRFP260 for power mosfets?
This mosfet has more than twice the input capacitance but less than twice in transconductance , which results slower response. It needs to redesign with lowering the gate resistors 220 ohms to maybe 180 ohms and increasing the CCS . The bias current needs to be determined by listening for fatigue free.
The irfz44 is very popular and costs very very cheap. It is identical to irfp150 but with lower Vceo,60v and Pdmax, 150w.
 
I had a go at simulating this topology with a few variations. I find the feedback network resistors R4/R6 can be increased in value by at least an order of magnitude, the gate biasing resistors R3/R7 can be increased and the constant current source reduced to match, again by at least an order of magnitude, and this seems to remove a strong peak in the frequency response at HF.



The distortion residual with negative feedback looks like mainly crossover distortion to me - certainly not the 3rd harmonic of the open loop.
 

U130421

Disabled Account
2015-12-16 2:00 pm
To design this amp you need to understand how it functions . The driver made of Rush cascade and CCS pours it's power to the load via zener , 220 ohms and two diodes . The current is sensed by the resistors and command the mosfet to pour it's current to the load and unload the class a driver. This type of amp is immune to crossover distortion as the class a amp fills the blanks while the outputs need time to switch on. On simulator with outputs biased just on cut off with leakage current has no crossover distortion at 100mw. But in real world the gate voltage of the outputs do not cutoff so sharply bellow 20ma . I could reduce it down to 5ma by 17ma on CCS , the sound is exactly the same as biased 120ma . Only by listening several days that 20ma bias by 20ma CCS I got the best fatigue free listening.
 
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indra1

Member
2010-11-05 6:44 am
Bogor
DLH has a fatal output mosfets cross conduction stating at input voltages ~> 1.5V RMS. Please check your circuit, I strongly suspect it has the same inherent flaw. I proposed addition of 1k8 to base of NPN, T5 in your case to contain the issue.

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