Rollover - A Decent Amplifier with JFET in and MOSFET out

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Joined 2005
#1
Uses LSK189 for input.
Buy at mouser.com: https://www2.mouser.com/ProductDetail/Linear-Integrated-Systems/LSK189-TO-92-3L-BK?qs=T%2BzbugeAwjjUrS49wFak%2Bw==
Uses ECX10N20 and ECX10P20 for output.
Buy for example at: https://gb.profusion.uk/uk/audio/transist/lateral-mosfets

The input JFET pair can be other JFET, too.
Bias in output is set to 300mA.
THD is 0.00020% with the LSK189.

Rollover LSK189_07.jpg
 
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#5
What's the deal with the unequal gate stoppers?
Click to expand...

minek123

Post in thread 'LatFet Amp Based on Philips AH578'

>tcd1963

1) Exicons have Zeners built in. No need for gate protection Zeners here.
There were several discussions about this here on DIY, and it was confirmed several times already.

2) The role of caps on GS is to equalize input capacitance on both devices (P and N).
Usually N device has lower capacitance than P device, so some designers ADD extra cap on N device to make them equal.
Example: Rod Elliot's P101.
If capacitances are not equal - that may cause oscillations.

Another way of dealing with this issue, is by using DIFFERENT gate stoppers, using formula RC = constant, where R is...
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#10
I guess that's why all the IRFP(9)240-based builds I have filling various rooms with music haven't blown up yet. 😀 So I wasn't completely wrong in my initial statement, since the difference in capacitance is negligible if you stick to "bog standard" FETs. Still learning all the finer nuances of component selection...
 
#13
Is it stable? You have a differential input stage loaded with a current mirror which is driving another differential stage which is driving yet another differential stage with a current mirror load.

I can only echo Rod Elliott’s words: “I suspect that it could be a cow to stabilise in a real amplifier circuit”
 
#14
I have some comments on the schematic
In the simulation what level and frequency is the THD figure for?
My guess is that with 3 differential amplifier stages and an output stage loop stability might be problematic. So I agree with the previous post by PMA.
Also simulation can only take the design so far. The currents in the first and second differential amplifiers are identical which seems unrealistic. I guess it's assuming identical transistors which is only an approximation to real behaviour. It might worth adding some offsets and modifying some of the transistor models to see the effect.
 
#15
Multisim is not very reliable in detecting amplifier instability. That's what I got for your scheme. Of course, it can be argued that there are different models. If you place the diagram in Multisim format, then I will extract the netlist and calculate the diagram in Qspice with your models. I can calculate the loop gain in the high-signal mode in Qspice.

1749209673955.png
 

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#17
The number of stages worries me. Stability will be a problem. What are the gain and phase margins? I once worked on amplifiers for a company called ILP in the UK. Three stages were essential in order to achieve low harmonic distortion figures for 1kHz, full power output. The quiescent current had to be zero or very close to it as there were no means to adjust it. At low volume you could hear the crossover distortion. I never heard one complaint. I'm of the opinion, though, that stabilty margin is important in listening tests.
 
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