even more lateral fets

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ed
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#61 Re: even more lateral fets

Post by ed »

I'm now at a crossroads.

In spice I can't find any simple improvement over my emitter follower with a current mirror as shown in post #49. Notwithstanding my senior moment of feeding a non centred square wave, the spice output for the whole circuit looks ok ish with proper square wave input.

so...do I spend more money on new boards, just add the components in a rat's nest to the existing boards, or....sell the heatsinks and chokes thinking why do I need another amplifier....the thrill of the chase has lost it's shine I think.

the final circuit without the zeners connected(still creeping up on that) the LT1113 is meant to be a drop in for OPA2134 that I have and was the closest model I could find...
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#62 Re: even more lateral fets

Post by ed »

to elaborate on the issue of matching in the current mirror. I got most of my background from the analog tutorials and the apnote for the current mirror under an emitter follower recommends ssm2212. I couldn't find a spice model at the time so just used a pair of ubiquitous 3904s thinking it would be close enough. I've since found the spice model for ssm2212 so will amend the diagram...

the bummer is that the ssm2212 is only available in soic format so if I do continue with this it will almost certainly need new boards, or a breakout that I can cobble onto the existing boards....

I realise this is not flavour of the month on this forum....but I am grateful for the input, so thanks again.
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#63 Re: even more lateral fets

Post by Nick »

I would agree with Mike on the current source. Not sure why using a current mirror would be preferable to just a simple two transistor current source, or just a jfet CCS. Unless I guess you want the current to vary with the supply rails which the mirror will do.

Unless there is some reading I should look at for what the mirror is better for?
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#64 Re: even more lateral fets

Post by Nick »

Just had a thought about the gate protection diodes. While I agree its unlikely that the gate will be driven in such a way that causes a problem, the inductor on the source would let me imagine things happening on the speaker terminals either going open circuit or short causing a fast change in current through the inductor inducing a high voltage on the source exceeding the Vgs value.
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#65 Re: even more lateral fets

Post by simon »

I understand so little of what you're experimenting with ed I probably shouldn't be posting, but I feel your pain!

I don't know why you're having these problems but looking at Mofo4 there looks to be a little more complexity than I have, around the PS and gain stage, but as per my email reply a couple of weeks ago I haven't had any problems with a standard opamp gain stage in front of the DIYAudio MoFo pcb. I do have the 100R resistor on the output of the LME49720, but I also have a separate power supply for the opamp. I've had the opamp stage remote from the MoFo amp connected by interconnect, and on the same board directly connected, both without issue.

If it's any help I have a spare pcb for the opamp gain stage you're welcome to if you wanted to knock it up to see what happens.
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#66 Re: even more lateral fets

Post by ed »

Nick wrote: Thu May 04, 2023 2:18 pm I would agree with Mike on the current source. Not sure why using a current mirror would be preferable to just a simple two transistor current source, or just a jfet CCS. Unless I guess you want the current to vary with the supply rails which the mirror will do.

Unless there is some reading I should look at for what the mirror is better for?
Is the current mirror preferable? I'm certainly not qualified to say. I used a current mirror because it was the example used in the analog apnote.
I've googled one method verses the other and so far haven't found any info that sheds any light.

@Simon..many thanks for the offer but I'm fast running out of enthusiasm for this project. It turned into a learning excercise after I discovered the capacitive load problem and couldn't fathom why my first version had worked. Having reflected, I'm now unsure why I need another amplifier.
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#67 Re: even more lateral fets

Post by Nick »

Is the current mirror preferable?
I don't know. My only point was that the current was set by the voltage across the resistor and that voltage (and so the current) would vary with the supply voltage (and any ripple). A simple ring of two CCS like MIke suggested the voltage across the sense resistor is set by Vbe so I would expect to have much better PSRR.
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#68 Re: even more lateral fets

Post by ed »

I think this will be safe now...
it's still relatively simple
please shout if I've missed something...I do seem to be missing some obvious things lately.
also..can some kind soul confirm that q2 and q3 don't have to be matched and could be common or garden npn
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#69 Re: even more lateral fets

Post by Nick »

I would put the back to back zeners between G and S, as it is it wont protect the device from voltage generated by the choke, or for that matter you may want to generate more that 15v on the output without exceeding the Vgs limit.
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#70 Re: even more lateral fets

Post by Mike H »

Nick wrote: Wed May 03, 2023 11:39 pm Well to be picky it's the Vbe of Q2/R2.
Sorry, typo! :oops:

What he said Image

Image
 
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#71 Re: even more lateral fets

Post by Mike H »

ed wrote: Thu May 04, 2023 9:40 am so...do I spend more money on new boards, just add the components in a rat's nest to the existing boards, or....
This is why I mostly use plain matrix board – stripboard if I must, but that can be a bit of a pain, cutting all those tracks, then cocking it up by cutting in the wrong place.... or missing one out...

Back in the distant mists of time I used to make my own boards, DAYS were spent painstakingly drawing tracks onto freshly degreased (with Vim, always worked well) single-sided copper laminate board with PCB marker pen, then etched with ferric chloride (with attendant chances of chemical accidents), laboriously drilling all the holes with a 1mm drill and trying not to break it, finally build the thing then find out the circuit is a load of bollocks. :D
....the thrill of the chase has lost it's shine I think.
I know the feeling! :D
 
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#72 Re: even more lateral fets

Post by ed »

Mike H wrote: Fri May 05, 2023 9:01 pm
This is why I mostly use plain matrix board – stripboard if I must, but that can be a bit of a pain, cutting all those tracks, then cocking it up by cutting in the wrong place.... or missing one out...
yeah but no but....

I thought I was being modern by giving up the paxolin and matrix board and twin and earth but in the end I don't look forward to any of it now...oh waily waily...I'm doomed. snigger
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#73 Re: even more lateral fets

Post by Mike H »

ed wrote: Fri May 05, 2023 2:59 pm I think this will be safe now...
it's still relatively simple
please shout if I've missed something...I do seem to be missing some obvious things lately.
also..can some kind soul confirm that q2 and q3 don't have to be matched and could be common or garden npn
Looks OK to me, only occurs to me maybe a wee small value capacitor between op-amp o/p and its inverting '—' i.p, like 22 – 100 pF or something like that, if it needs it. I only mention it because once I designed a supply regulator as an op-amp followed by a darlington pair, it was HF unstable if no load after the power transistor.

I'd want way more current through Q2, because we can, as it is, it seems very low, that won't drive the MOSFET gate I don't think, 62Ω for R4 will get 10 mA. I'd do that, see how it goes. Q2 & Q3 could also be 2N3904 if that's what you've got.

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#74 Re: even more lateral fets

Post by ed »

thanks Mike it does help...
although, after reading all the TI ap notes I still don't understand why I need drive current to overcome the GS charging capacitance at low frequencies like audio. I am content with my lack of understanding though.

I've amended the diagram to take R4 down to 68R and R3 down as well. R4 at 11ma and R3 at 2ma. all looks good sin and square.

I originally had 15pf between output and negative in on the original diagram. This 15pf has now moved to the output of the buffer. Is it this capacitor that you were referring to. If so, do you think I can just move it from the output of the buffer to the output of the opamp or would it be better to put another cap in?. As you may have guessed, I don't fully understand it's purpose.
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#75 Re: even more lateral fets

Post by Nick »

Just seen the CCS question. Yep any t will be fine with ok V and I rating.
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