Nelson & Charles,
your recent exchange was the most extremely ineresting, informative, curteous and entertaining. congratulations and thankyou.
Regards
13th Duke of Wymbourne
ps. is cascoding a source follower really beneficial?
your recent exchange was the most extremely ineresting, informative, curteous and entertaining. congratulations and thankyou.
Regards
13th Duke of Wymbourne
ps. is cascoding a source follower really beneficial?
13DoW said:
In a source follower configuration, vertical MOSFETs will have a relatively high and relatively non-linear input capacitance. This will cause relatively large levels of high frequency distortion unless you take counter measures.
One of the counter-measures you can take is cascoding. Cascoding will hold the Vds to a fixed low value. The "fixed" part will essentially banish the non-linear part of the input capacitance. But the "low" part means that the input capacitance will be even higher. Furthermore cascoding reduces the available output signal swing, necessitating higher rail voltages for a give power level.
So just like everything else in life, cascoding a vertical MOSFET output stage results in tradeoffs.
Lateral MOSFETs in a source follower configuration have a relatively low and relatively linear input capacitance. You can get good measured performance from them without the use of feedback or cascodes. But their transconductance is significantly lower. To get equal low-frequency performance to vertical MOSFETs requires at least 5x the number of output devices. This drives up the cost of the amplifier considerably.
So just like everything else in life, using multiple lateral MOSFET output devices results in tradeoffs.
Hi Subwo1,
thanks for the reply. I had not thought of that, makes sense that the final result you want is voltage at the gate after the cap and not just composite voltage of cap plus parasitic MOSFET capacitance.
Could this be related to another effect I noticed some time ago and never got a grip on? - I noticed that in a standard high pass filter (cap in series and R to ground at the +in of a FET opamp, OPA2134) , I got a loss of apparent dynamics if the cap was to large. No matter what kind or brand of cap I tried, if the series cap was over 1 uF, it was noticeable. This was an interstage filter after a DAC I/V to get rid of DC offset, so no additional cable capacitance effects. I asked a question in these forums back then but only got an explanation for C to ground driving problems (obvious case of concern).
MBK
thanks for the reply. I had not thought of that, makes sense that the final result you want is voltage at the gate after the cap and not just composite voltage of cap plus parasitic MOSFET capacitance.
Could this be related to another effect I noticed some time ago and never got a grip on? - I noticed that in a standard high pass filter (cap in series and R to ground at the +in of a FET opamp, OPA2134) , I got a loss of apparent dynamics if the cap was to large. No matter what kind or brand of cap I tried, if the series cap was over 1 uF, it was noticeable. This was an interstage filter after a DAC I/V to get rid of DC offset, so no additional cable capacitance effects. I asked a question in these forums back then but only got an explanation for C to ground driving problems (obvious case of concern).
MBK
MBK,
I wonder if there is something going on with the physical size of the larger cap picking up interference or some anomoly like that with your circuit. Otherwise, I cannot explain it either.
I wonder if there is something going on with the physical size of the larger cap picking up interference or some anomoly like that with your circuit. Otherwise, I cannot explain it either.
Actually I thought of it this way: A capacitor is a charge reservoir. A large capacitor is a large charge reservoir. The construction is (ideally) a huge plate of conductive material with no resistance. So if a capacitor is very large, say infinite 😉 , it is no better than a short circuit until the repulsion due to the opposite plat "kicks in". Or in other words, to charge even a real life capacitor, the source has to fulfill an infinite dC/dt at t0, basically, it has to have an infinite slew rate. The charge curve then tapers off very quickly, but in essence it is an hyperbole. Now there is finite DCR in a cap and other real life considerations. But I just wondered whether that "almost a short circuit" (endless reservoir / bottomless pit for electrons...) during the first nanoseconds or so of the charge cycle of a fairly large cap, could have caused this loss in dynamics. Could be that that particular op amp could cope with a certain amount of "impossible requirement" but not beyond, and that this limit was about 1 uF. Then again, the coupling caps used in some class A designs are up to 1000 uF. OTOH those are not driven by op amps with lots of NFB either.
Hey, maybe I just imagined it, or maybe the lower cutoff of the HP with a larger cap actually made for a cleaner sound, but I interpreted it as "lack of dynamics" (distorted and "phat" bass often sounds more impressive than real and clean bass for instance).
MBK
Hey, maybe I just imagined it, or maybe the lower cutoff of the HP with a larger cap actually made for a cleaner sound, but I interpreted it as "lack of dynamics" (distorted and "phat" bass often sounds more impressive than real and clean bass for instance).
MBK
very large, say infinite
"the opposite plat "kicks in".
Man........ I definitely would say it has kicked in! Why couldn't I have gotten some of that for my post surgery pain? Either this lost something in the translation or this is truly cosmic at some level I likely won't reach in this lifetime.
If the doors of perception were cleansed everything would appear to man as it is: Infinite.
--William Blake, The Marriage of Heaven and Hell
"the opposite plat "kicks in".
Man........ I definitely would say it has kicked in! Why couldn't I have gotten some of that for my post surgery pain? Either this lost something in the translation or this is truly cosmic at some level I likely won't reach in this lifetime.
If the doors of perception were cleansed everything would appear to man as it is: Infinite.
--William Blake, The Marriage of Heaven and Hell
Fred,
In the begining there was sound, the sound was good, till it became Infinite. Now that it's Infinite I just can't get charged up maybe my slew rate is to low. Now that the end is near I wondered whether that "almost a short circuit" (endless reservoir / bottomless pit for electrons...)
Maybe the Vicodin is wearing off are you feeling better.🙂
In the begining there was sound, the sound was good, till it became Infinite. Now that it's Infinite I just can't get charged up maybe my slew rate is to low. Now that the end is near I wondered whether that "almost a short circuit" (endless reservoir / bottomless pit for electrons...)
Maybe the Vicodin is wearing off are you feeling better.🙂
Now you dissapoint me jam, you have already posted that
image before. Maybe your harddisk with images isn't infinite
as I thought, after all? 😉
image before. Maybe your harddisk with images isn't infinite
as I thought, after all? 😉
With friends like that who needs Darvocet
I am purchasing mail order sensitivity training courses for both of you.....
Jam, That is exactly what it feels like except for with a two by four shoved through my nostrils, in both axis.
Jim , thanks for not telling them about me searching for my spoon in the bottom of my Vietnamese soup with my chop sticks for a full five minutes, while dining under the gentle sway of Vicodin. I really thought I was being fairly low key about it all until the waitress came over and said " Hey mister!...... maybe you need fork to eat noodles in soup?"
Even my damn surgeon who talked me into this adventure, replied "Okay....... it looks like Darvocet is not your friend " after I showed him the 5 new stiches in my head from meeting a door frame at 2 in the morning during a leisurly stroll about the house in search of god knows what while under " the less narcotic effect from Darocet" as he described it when prescribing it. I can drive my car on Viicodin (you might not want to ride with me however ) and I can't even walk with the less narcotic Darvocet........ and I still hurt like hell when I took it.
Everyone is a comedian. I guess I might as well steal the chance from one of you for this oldy .
Does your face hurt?
No....... why?
It's killing me!
Don't try any of this at home kids (or adults)........
I am purchasing mail order sensitivity training courses for both of you.....
Jam, That is exactly what it feels like except for with a two by four shoved through my nostrils, in both axis.
Jim , thanks for not telling them about me searching for my spoon in the bottom of my Vietnamese soup with my chop sticks for a full five minutes, while dining under the gentle sway of Vicodin. I really thought I was being fairly low key about it all until the waitress came over and said " Hey mister!...... maybe you need fork to eat noodles in soup?"
Even my damn surgeon who talked me into this adventure, replied "Okay....... it looks like Darvocet is not your friend " after I showed him the 5 new stiches in my head from meeting a door frame at 2 in the morning during a leisurly stroll about the house in search of god knows what while under " the less narcotic effect from Darocet" as he described it when prescribing it. I can drive my car on Viicodin (you might not want to ride with me however ) and I can't even walk with the less narcotic Darvocet........ and I still hurt like hell when I took it.
Everyone is a comedian. I guess I might as well steal the chance from one of you for this oldy .
Does your face hurt?
No....... why?
It's killing me!
Don't try any of this at home kids (or adults)........
That's just........ wrong.
Great half of my face hurts, I'm on Narcotics, and you throw that one at me........ Only the fact that I can't sleep for more 20 minutes will spare me from a nightmare that could take years of therapy to undo!
YOU'RE FREAKING ME OUT MAN.
Mercy! No mas. No mas.
jam said:
Great half of my face hurts, I'm on Narcotics, and you throw that one at me........ Only the fact that I can't sleep for more 20 minutes will spare me from a nightmare that could take years of therapy to undo!
YOU'RE FREAKING ME OUT MAN.
Mercy! No mas. No mas.
I apologize if my obtuse semantics were lost on the cognoscenti (in spe). In other words, sorry if I asked a question you couldn't understand .
I like to imagine things in simple models. A simple textbook model of a cap is, two plates that act as temporary charge reservoirs, and separated by a gap. As one plate gets charged by a source("accumulates electrons"), the charge so accumulated "pushes away" the electrons of the opposing plate. Real capacitors have a finite capacity for charge, the capacitance C. If a resistor is added in the circuit, the charging process is slowed down and we get the RC constant to characterize it.
Now, if one charge reservoir (capacitor plate) was infinitely large, it would act as if it was a short circuit to earth. And to make any voltage appear across this infinite reservoir, you'd need a voltage source with infinite capability. "Slew rate" was the wrong word I guess. It's a thought experiment, dudes!! Lighten up.
So, the above made me wonder whether when a real cap is charged, there was an (infinitesimal) time span after applying voltage, during which the source would see the cap as if it was this infinitely large charge reservoir, regardless of any resistor in series, and would therefore see a short circuit to earth for a small fraction of time. And THAT *would* matter for current drive and slew rate considerations.
MBK
Oh yeah, for the incredulous re: large coupling caps, here is one that uses 3300 uF:
http://headwize2.powerpill.org/projects/showproj.php?file=balkishan1_prj.htm
. I like to imagine things in simple models. A simple textbook model of a cap is, two plates that act as temporary charge reservoirs, and separated by a gap. As one plate gets charged by a source("accumulates electrons"), the charge so accumulated "pushes away" the electrons of the opposing plate. Real capacitors have a finite capacity for charge, the capacitance C. If a resistor is added in the circuit, the charging process is slowed down and we get the RC constant to characterize it.
Now, if one charge reservoir (capacitor plate) was infinitely large, it would act as if it was a short circuit to earth. And to make any voltage appear across this infinite reservoir, you'd need a voltage source with infinite capability. "Slew rate" was the wrong word I guess. It's a thought experiment, dudes!! Lighten up.
So, the above made me wonder whether when a real cap is charged, there was an (infinitesimal) time span after applying voltage, during which the source would see the cap as if it was this infinitely large charge reservoir, regardless of any resistor in series, and would therefore see a short circuit to earth for a small fraction of time. And THAT *would* matter for current drive and slew rate considerations.
MBK
Oh yeah, for the incredulous re: large coupling caps, here is one that uses 3300 uF:
http://headwize2.powerpill.org/projects/showproj.php?file=balkishan1_prj.htm
Old Blue Eyes is Back??? Please let it Be so......
"I apologize if my obtuse semantics were lost on the cognoscenti"
Not at all, you even got the plural form of cognoscente correct and I can find no fault with your semantics. In fact your asault on those in the nose (so to speak) , with cognesenti may even suggest a bit of subliminal humor. I looking at some of the words that can be made from the letters........ got in snit not go sonic nose scent.........
I knows! You are tired about hearing about my nose! Me too........
Why do those initials look so familiar? MBK....... MK........ wait a minute......... I couldn't be that lucky and it must be the pain killer clouding my mind. Could it be?
And still, no end is near;
He just won't face the final curtain.
My friend, I'll say it clear,
Mike states his case, and knows he's certain..........
Is that my old buddy Mike trying to pull a fast one on the undisputed master of DIYaudio multiple personality disorders? No it couldn't be! Its against the rules and would be way too obivious........ why you might even say as plain as the nose on my face....... and Mike wouldn't do that. I do so miss him though and would welcome him back with open arms for another Sinatra tribute.
Jim,
If you knew how close that is to reality, you might even feel remorse...... nah....... probably not. Warm saline washes are part of the recommended post surgery regime. My wife decided I be must getting better today. I asked her why. She said the last few times I have gone to wash my nose, I didn't scream OH MY GOD!!!!! when I saw what came out. I was honestly starting to wonder if maybe my surgeon somehow dropped his car keys in there when he was working on me. I am quite sure that my eyes don't bug out like that though.
"I apologize if my obtuse semantics were lost on the cognoscenti"
Not at all, you even got the plural form of cognoscente correct and I can find no fault with your semantics. In fact your asault on those in the nose (so to speak) , with cognesenti may even suggest a bit of subliminal humor. I looking at some of the words that can be made from the letters........ got in snit not go sonic nose scent.........
I knows! You are tired about hearing about my nose! Me too........
Why do those initials look so familiar? MBK....... MK........ wait a minute......... I couldn't be that lucky and it must be the pain killer clouding my mind. Could it be?
And still, no end is near;
He just won't face the final curtain.
My friend, I'll say it clear,
Mike states his case, and knows he's certain..........
Is that my old buddy Mike trying to pull a fast one on the undisputed master of DIYaudio multiple personality disorders? No it couldn't be! Its against the rules and would be way too obivious........ why you might even say as plain as the nose on my face....... and Mike wouldn't do that. I do so miss him though and would welcome him back with open arms for another Sinatra tribute.
Jim,
If you knew how close that is to reality, you might even feel remorse...... nah....... probably not. Warm saline washes are part of the recommended post surgery regime. My wife decided I be must getting better today. I asked her why. She said the last few times I have gone to wash my nose, I didn't scream OH MY GOD!!!!! when I saw what came out. I was honestly starting to wonder if maybe my surgeon somehow dropped his car keys in there when he was working on me. I am quite sure that my eyes don't bug out like that though.
- Status
- Not open for further replies.