I have heard from some ppl in here that lateral mosfets dont need the source resistors.
Is this true or a false claim ?😕
Is this true or a false claim ?😕
true. degeneration or summing resistors are not required as local feedback for thermal issues do not exist with lateral MOS. Do make sure to match the set for Ids, though.
All my 2SK135/j50's are from an old mono 800w pa amp that was scrapped so im pretty sure their matched sets.
I have built up both 2pair and 4pair lateral output mosFET amplifiers.
In all cases (4 amplifiers) I matched laterals for Vgs @ planned output bias current.
Some of my matched sets were closer than others.
All were fitted with source resistors and these were very closely matched so that any Id measurements taken later would have some meaning.
I found with my level of Vgs matching, that all the Laterals pulled different Id when the Vgs were of necessity identical as they are fed from the same Drivers. This confirms that Pq of all the devices were different. The Tc of all the devices were different. The spare temperature de-rated SOAR available to drive the reactive load were different for each device from it's partners.
I can absolutely assure everyone that the biggest advantage of not fitting Source Resistors is that the builder never knows that the output devices are operating at different currents and different junction temperatures.
There-in lies the advantage. If one cannot measure the errors ,then in the head of the listener/builder there can be no errors.
edit.
I forgot to mention the single pair Laterals (versions of FetZillas) I have built. Three with source resistors and one without source resistors. All these FetZillas have an unusual output offset variation. It never holds steady. Offset wanders continuously over a range of +-1.5mV for Source resistor versions and slightly worse for no Source resistor.
In all cases (4 amplifiers) I matched laterals for Vgs @ planned output bias current.
Some of my matched sets were closer than others.
All were fitted with source resistors and these were very closely matched so that any Id measurements taken later would have some meaning.
I found with my level of Vgs matching, that all the Laterals pulled different Id when the Vgs were of necessity identical as they are fed from the same Drivers. This confirms that Pq of all the devices were different. The Tc of all the devices were different. The spare temperature de-rated SOAR available to drive the reactive load were different for each device from it's partners.
I can absolutely assure everyone that the biggest advantage of not fitting Source Resistors is that the builder never knows that the output devices are operating at different currents and different junction temperatures.
There-in lies the advantage. If one cannot measure the errors ,then in the head of the listener/builder there can be no errors.
edit.
I forgot to mention the single pair Laterals (versions of FetZillas) I have built. Three with source resistors and one without source resistors. All these FetZillas have an unusual output offset variation. It never holds steady. Offset wanders continuously over a range of +-1.5mV for Source resistor versions and slightly worse for no Source resistor.
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AndrewT is totally right.
Having source resistors also aid with current sharing and provide a means to incorporate VI limiter circuits (not saying they're good or bad, please don't debate that here). The only downside of having these resistors are a slight loss in power and to a lesser impact PCB space.
Having said that however, if my amp uses just 1 lateral FET output pair, I'd go without source resistors and rely on rail fuses.
Having source resistors also aid with current sharing and provide a means to incorporate VI limiter circuits (not saying they're good or bad, please don't debate that here). The only downside of having these resistors are a slight loss in power and to a lesser impact PCB space.
Having said that however, if my amp uses just 1 lateral FET output pair, I'd go without source resistors and rely on rail fuses.
I also have made amps with and without source resitors. Neither showed any advantage or detrimental effect, so I am not voting for the one or the other.
However, the lateral mosfets is a varying resistance and placing another resistance in series makes no difference really, you still end with a bunch of different resistors in parallel, one will pass more current than another under dynamic conditions.
However, the lateral mosfets is a varying resistance and placing another resistance in series makes no difference really, you still end with a bunch of different resistors in parallel, one will pass more current than another under dynamic conditions.
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