How to make a low distortion lateral MOSFET amplifier? - Page 14 - diyAudio
 How to make a low distortion lateral MOSFET amplifier?
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diyAudio Member

Join Date: Jul 2004
Location: Scottish Borders
Quote:
 Originally Posted by scopeboy Well, presumably this RC network is supposed to have some relation to the pulsed SOA vs. DC SOA of the device.
I agree.
Just reduce the delay.
Check the DC values of your detection and trigger.
Then see how fast it reacts to step function overloads when a reactive load is attached to the output.

Another thought. The protection resistor is required to go near to saturation to effectively limit the voltage at the input to the output stages. This requires significant current. The 10k may be a bit high to allow that current to flow. What about 1k or 2k?

Have you found the solution to the oscillation? Remember that the protection transistor starts to turn on long before Vbe reaches 600mV. It passes big current @ 500mVbe and significant current @ 400mVbe and becomes insignificant below ~300mVbe.
__________________
regards Andrew T.
Sent from my desktop computer using a keyboard

Last edited by AndrewT; 5th November 2013 at 11:24 AM.

diyAudio Member

Join Date: Nov 2012
Location: Lisbon
Thank you very much AndrewT and scopeboy I really appreciate your help,

I've changed the circuit to become stable and limit the current like it's supposed to do, but to achieve this I've to use the RC network with R=100R and c = 10n, I can't use larger resistor or capacitor values without having problems (oscillations or a bad current limit threshold). Can you help me here?
I should use larger values like 1k and 10n, but using this values, I've got oscillations during the current limiting action and that's weird.

Best regards,
Daniel
Attached Images
 amp7_tmc_hb5_bd11_prot.jpg (224.5 KB, 130 views)
Attached Files
 Cordell Models.txt (19.2 KB, 3 views) BC3x7_40 Models.txt (1.2 KB, 3 views) Amp7_tmc_hb5_s_bd11_prot.asc (27.5 KB, 7 views)

 6th November 2013, 09:13 AM #133 diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders Can you do a stability analysis around that protection loop? That may reveal where compensation can or should be added without affecting normal audio performance. __________________ regards Andrew T. Sent from my desktop computer using a keyboard
 6th November 2013, 09:16 AM #134 diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders What effect can the simulator show (for maximum peak output current) for fast signals compared to the DC results. Did you try 100us pulse, 10us pulse, 1us pulse, 0.1us pulse? __________________ regards Andrew T. Sent from my desktop computer using a keyboard
 7th November 2013, 03:24 AM #135 diyAudio Member   Join Date: Nov 2012 Location: Lisbon Hi AndrewT thank you for helping me, How I can perform a stability analysis around the protection loop? Could you help me please? I've attached the pictures of the amplifier overdriven with a 100us, 10us and 1us square wave with the output shorted to ground, with higher frequencies the signal is so distorted that you can't see anything, even with a period of 1us the signal is terrible. For 10us there's some oscillations, with 100us I've evidence of overshoot. What I should modify in the VI limiter to cancel this oscillation and overshoot? With larger base to emitter capacitors I've got more oscillations, even with a square wave with a period of 100us. PS: I've modified the TMC network to something that I've called BTMC (bridged transitional Miller compensation), that's based on a technic used in TPC to cancel the peaking at audio frequencies (1-20kHz) that accompanies two pole compensation schemes, and to improve phase margin at those frequencies ( I have values of phase between 5 and 20 degrees with TMC at 10kHz and I think that with TPC is similar), one of the solutions to this problem was proposed by Bob Cordell and consists in adding a small valued (1-10pF) capacitor between the IPS/TCS inverting output and the output of the VAS/TIS, this capacitor should be 10 times or more lower than the equivalent Miller dominant pole capacitor. The disadvantages are an increase in THD1, but remains much less than the THD1 produced by "conventional" dominant pole compensation, the THD20 remains almost the same with only a slight increase. The advantage as I said previously is more stability at lower frequencies, I think that's not good to have peakings and low phase margin at audio frequencies with a real world speaker, the Barkhausen criterion for self sustaining oscillation is not met but if for some reason the feedback becomes positive you can have burst oscillations that could damage the speakers. Well I don't know if that's good or not, but in simulation this little change in the TMC compensation scheme seems to improve gain and phase margins, what do you think about that, it's a good idea? Maybe I'm over worried about the lack of phase margin and the little peaking at the audio frequencies that accompanies two pole compensation schemes, but maybe I shouldn't, there are already lots of good amplifiers with this compensation schemes TPC and TMC and they are stable and perform really well, so maybe the problem is only in my mind. I'm sorry I can't attach the files I will attach them later the diyaudio server is having some problem with the file uploader. Best regards, Daniel
diyAudio Member

Join Date: Jul 2004
Location: Scottish Borders
Quote:
 Originally Posted by danny92 How I can perform a stability analysis around the protection loop? Could you help me please?
Sorry I can't help with simulation techniques. I am not a simulator guru. I can't even repeat a procedure I implemented the previous day.

I am suggesting a scenario that the Sim Gurus may help you to find solutions.

BTW,
I did not say test into a shorted output !!!! That is a silly VI test. All that needs is for the amplifier not to blow up !!!!

Test into a sensible reactive load. If you are designing for 100W (40Vpk) into 8ohms reactive, then the amplifier should be able to drive 4r0 easily and preferably drive 2r3 without the peak voltage dropping significantly.
__________________
regards Andrew T.
Sent from my desktop computer using a keyboard

 8th November 2013, 08:46 PM #137 diyAudio Member   Join Date: Nov 2012 Location: Lisbon Hi everyone, thank you very much for your great help, AndrewT, I will put my question about the VI limiter stability tests in the Software forum of diyaudio. What can be considered an acceptable inductance value to use in the series inductor to simulate the real speaker, maybe 1-10uH or more? I've one more question to do if you don't mind, why TMC open loop seems to have the 0dB point at 2-3 MHz and the dominant pole compensation has it at 500kHz, there's any problem in having the 0 dB point at 2-3 MHz? It was told me that's not normal, that I should use 500kHz, what do you think about that? I also want to know your opinions about "BTMC", can you read my last post, please. Best regards, Daniel Last edited by danny92; 8th November 2013 at 08:52 PM.
diyAudio Member

Join Date: Aug 2012
Location: Ontario
Hi Guys,

Daniel, find attached a representation of a 2-way speaker system with a cross-over, 1 pole for woofer, 2-pole for tweeter. A project i have been working on.
Based on Bob Cordell's PA book information and some stuff I added from the speaker guts and a mod to the tweeter.
Hope it helps you out. If you want the .asc I can post it as well.

Rick
Attached Files
 Altec lansing 3 xover.pdf (12.3 KB, 19 views)

 9th November 2013, 05:22 PM #139 diyAudio Member   Join Date: Nov 2012 Location: Lisbon Hi rsavas, thank you for your help, Do you make diy speakers? That seems really interesting. Could you send the .asc file, please, I'm curious about that speaker representation. Best regards, Daniel
diyAudio Member

Join Date: Aug 2012
Location: Ontario
LTspice speaker simulation.

Quote:
 Do you make diy speakers?
A few for my own use.
I posted this thread, but know one replied, I guess I am asking for too much.
Finding the correct woofer as replacement for an existing box
Sure, see attached. I know the tweeter equivalent ckt is overly simplified. The woofer I need to plot Z vs Freq to see what that looks like. Obviously every speaker type, has its own Z curves. First time I have done this, so lots of work to do to refine the speaker models.

Enjoy
Rick
Attached Files
 Altec lansing 3 xover.asc (2.7 KB, 9 views)

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