"anyone want to explain how 'N' channel amps work without a 'P' transistor?"
Generally, they don't. I assume you refer to "quasi-complementary" topology: The NPN transistor is actually a part of PNP Sziklai pair. Some amplifiers with interstage transformers might not need the other polarity transistor either because the transformer acts as a phase inverter.
Generally, they don't. I assume you refer to "quasi-complementary" topology: The NPN transistor is actually a part of PNP Sziklai pair. Some amplifiers with interstage transformers might not need the other polarity transistor either because the transformer acts as a phase inverter.
I have have attached an SOAR plot of a modified Sugden P128.big jim said:
It shows the Vce for various loads.
The original used 2 pair of 2sj81/k225 rated at 120Vds0.
The supply rails run @ about +-70Vdc when biased to 150mA and rise to over +-80Vdc when unbiased, transformer regulation is atrocious. These voltages are 6% higher when the mains supply is @ 254Vac.
I realise these are FETs, but the important point to extract from the graph is the highest Vds is only just over 100Vdc. Similar measurements/modelling would come from a BJT stage. I believe that many manufacturers are aware of this and know that devices do not usually breakdown due to high Vce. Again look at the loadlines closest to the SOAR limits. Vds of about 60 to 70V is the problem area for reactive loads i.e. near the Vrail voltage when high currents can be passing. But, even lower critical Vds/Vce for lower value loads.
I do not have any other examples to hand but I have seen other schematics with similar Vce0 and high supply rails, they are not that uncommon. Even so it's not something I would advocate, but then I now design for SOAR having been enlightened by others on this Forum.
ps.
this FET amp is sold as an 8ohm and 4ohm capable design that is generally reliable from a reputable manufacturer. I have looked at the 2pair SOAR and I would never ask it to work into 4r0 (resistive) and yet it has been tested by the press in it's day into low loads and no reports of failure were declared. As a 4pair, I consider it to be a good 6ohm capable amplifier that could drive 4 to 8ohm as well as 8ohm.
Attachments
Vce
Transistor Vce0 is not only selected based on the power supply rail voltages, but can also be selected based on the maximum theoretical voltage that can appear across them. The latter can be significantly lower than the supply rails.
Whereas some manufacturers may choose to cut it fine (using under-spec'ed parts), as a DIYer one has the choice to build a circuit that is inherently more reliable. Why would one consciously choose to compromise the quality of a design you have full control over?
Transistor Vce0 is not only selected based on the power supply rail voltages, but can also be selected based on the maximum theoretical voltage that can appear across them. The latter can be significantly lower than the supply rails.
Whereas some manufacturers may choose to cut it fine (using under-spec'ed parts), as a DIYer one has the choice to build a circuit that is inherently more reliable. Why would one consciously choose to compromise the quality of a design you have full control over?
Re: Vce
I completely agree with the sentiment in your posting. Why compromise reliability when it could be very easy to avoid the riskof unreliability?
Put this another way, there will be few if any transistors that pass the SOAR requirement and yet fail the Vce0 requirement.
I find that looking at current ability at Vce=Vrail voltage gives much more useful design information than selecting by Vce0 ever does.
Hi Shaun,Shaun said:
I completely agree with the sentiment in your posting. Why compromise reliability when it could be very easy to avoid the riskof unreliability?
but, using Vce0 as one of the important selection criteria does NOT guarantee reliability.
SOAR design demands much more from the performance of the semiconductors and virtually guarantees that the Vce0 requirement is met.
Put this another way, there will be few if any transistors that pass the SOAR requirement and yet fail the Vce0 requirement.
I find that looking at current ability at Vce=Vrail voltage gives much more useful design information than selecting by Vce0 ever does.
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