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Old 28th April 2014, 12:41 PM   #11
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by Ranchu32 View Post
Mooly, would you please explain the function of the gate stops and your reasoning for the large value? The stoppers on quasi's nmos design are only 27R. Thanks
Higher values will "play safe" and make the design more tolerant of any compromised layout and so on. That was my reasonlng. It removes a potential source of trouble for you.

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Originally Posted by Ranchu32 View Post
Bone - thanks for the reference to Nelson Pass' project. Reading this article and studying several other schematics from this vintage, I believe the performance will be inadequate without a bootstrap on the VAS.

All - updated schematic attached. Is this workable? How to I go about optimizing the values of R10, R12 and R13?

Any thoughts on how best do I determine the required bias voltage?

thanks
Yes, its workable although it may need need more refinement to get the very best from it.
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Old 28th April 2014, 12:43 PM   #12
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Oh yes, nearly forgot

Bias. it would be much better (more thermally stable) with a proper vgs multiplier in place of the diodes. You could just copy a normal bjt transistor one (one NPN transistor, one resistor and one preset) and use that. The transistor must be in thermal contact with the heatsink.
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Old 28th April 2014, 01:23 PM   #13
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R14 is wrong, it should be between the output node and emitter/drain node.

R10 and 13 should be the same value. 470 as you have is a good starting point. 220 is also a good starting point for R10.

You will need an oscilloscope or simulator to optimise the values.
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Old 28th April 2014, 01:42 PM   #14
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This works at a basic level. It needs the bias network adding
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Old 28th April 2014, 02:56 PM   #15
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With these, always a good idea to look at what the circuit is doing open loop as well. Just so you know what goes into the sausage
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Old 28th April 2014, 04:19 PM   #16
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MOSFETs are finicky things, they will oscillate just if you look at them funny... in addition to gate stoppers I have included additional footprints on all my latest pcb's that use MOSFETs at the output to include an SMT zobel network between gate and drain. A series R and C. You don't have to populate the pads, but they are there if needed or if you end up trying a particular MOSFET that's more finicky than the others. Put some nice rail caps close to the FETs on the pcb too.

Depending on which MOSFET you want to add gate protection zener diodes - most MOSFETs need them.

I would also allow space for lead compensation, a small cap from VAS collector to the feedback node.

I'd add a resistor (e.g. 82R) in the power rails with cap to ground, to isolate the front end from the power output stage. This is fairly common, to avoid unintentional feedback via the power rails.

Sorry - I added so many parts !!!!
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Old 28th April 2014, 06:33 PM   #17
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Although not a quasi, if you look up International Rectifier Application Note AN-948 you will find a nice and simple worked example of a HEXFET amp together with a simple board layout.
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Old 28th April 2014, 06:56 PM   #18
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I agree with Mooly's suggestion for the LTP tail resistor (your R4, his R2) which shoudl be 22K not 12K. This does cut the current through each transistor in half but it allows the collector load resistor to be twice as large in value which helps push up the open loop gain. Although less current means less slew rate, you can ameliorate somewhat this issue if you keep Cdom small by adding the lead compensation cap I mentioned already.
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Old 29th April 2014, 01:57 AM   #19
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Quote:
Originally Posted by Mooly View Post
This works at a basic level. It needs the bias network adding
Very generous of you to take the time to simulate this. Any chance you would send me the files to play with?

Quote:
Originally Posted by Bigun View Post
MOSFETs are finicky things, they will oscillate just if you look at them funny... in addition to gate stoppers I have included additional footprints on all my latest pcb's that use MOSFETs at the output to include an SMT zobel network between gate and drain. A series R and C. You don't have to populate the pads, but they are there if needed or if you end up trying a particular MOSFET that's more finicky than the others. Put some nice rail caps close to the FETs on the pcb too.

Depending on which MOSFET you want to add gate protection zener diodes - most MOSFETs need them.

I would also allow space for lead compensation, a small cap from VAS collector to the feedback node.

I'd add a resistor (e.g. 82R) in the power rails with cap to ground, to isolate the front end from the power output stage. This is fairly common, to avoid unintentional feedback via the power rails.

Sorry - I added so many parts !!!!
I can see you are conspiring to turn my simple amplifier into a complicated beast

The prospect of adding the 4 passive devices (even pads) for the zobel networks makes me very unhappy indeed I need to read more about about the causes and remedies for parasitic oscillation in MOSFETs.

Bob Cordell published a paper discussing the issue on his website here: CordellAudio.com - A MOSFET Power Amplifier with Error Correction I'm hopeful that with only a single pair of outputs, the relatively high gate stopper resistances in this design, close decoupling around the source/drain and careful PCB layout will save the day. But I will take your suggestion under advisement and may incorporate it in the PCB layout after further research.

I plan to put place rail decoupling caps as close to the MOSFETs - as large and as close as practical. My aim is to lay out a 2ch amplifier on a single 10x10cm board. Ideally I'd like the bulk filter caps onboard (say a 4,700uF cap per rail per channel) but if space constraints dictate otherwise, I'll move the bulk caps offboard and put some 100-220uF decoupling caps there instead.

I'll give serious consideration to the gate-source protection diodes, thanks for the suggestion. I'm a little perplexed that the large majority of MOSFET designs here (including Quasi's popular Nmos project) omit this protection.

On a different note, you might be pleased to know I've decided to give the LTP a fighting chance by adding a modest RC filter to the +ve rail between the VAS and the tail resistor.

Quote:
Originally Posted by Mooly View Post
Although not a quasi, if you look up International Rectifier Application Note AN-948 you will find a nice and simple worked example of a HEXFET amp together with a simple board layout.
Thanks again Mooly. This is almost exactly what I want to build (substituting a quasi output stage for the complimentary pairs).

Quote:
Originally Posted by Bigun View Post
I agree with Mooly's suggestion for the LTP tail resistor (your R4, his R2) which shoudl be 22K not 12K. This does cut the current through each transistor in half but it allows the collector load resistor to be twice as large in value which helps push up the open loop gain. Although less current means less slew rate, you can ameliorate somewhat this issue if you keep Cdom small by adding the lead compensation cap I mentioned already.
Sounds like good advice to me and I'll adopt the 22k for the tail resistor. Mooly has also increased the feedback network attenuation in his Spice model to give a more reasonable closed-loop gain - something that was on my "to do" list - noted and adopted.

I'm interested in playing with the Cdom value and experimenting with lead comp once I can get this thing into Spice.
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Old 29th April 2014, 06:22 AM   #20
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Default DMMT5401

I will likely follow in Gareth's footsteps and lay this out using a combination of SMT and TH devices.

I stumbled across this part in a SOT-32-6 outline and thought it would be ideal for the LTP since it comprises two "super matched" 5401 NPN transistors in a single package. A quick search here revealed that AKSA have used this in some of their designs.

http://www.diodes.com/datasheets/ds30437.pdf

Are there any downsides apart from the reduced Pd, which won't be an issue for me in this application. Frankly I'm surprised this device isn't more widely used considering how important a close match is for the diff amp transistors.
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