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Old 7th May 2005, 11:34 PM   #11
ilimzn is offline ilimzn  Croatia
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I have a few ideas but that will have to wait till I get back home...
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Old 7th May 2005, 11:54 PM   #12
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hi Kanwar
another things:
1. Your PSRR would be ugly... it may even not work correctly with a bit variable rails
2. Is it open loop current source or voltage source?
Most N-ch amps are current sources, 'cause you change Vgs of both mosfets.
In your amp upper mosfet seems to play as a follower (voltage source) and lower as current source. It cannot work (in class AB/B at least), 'couse I and V may be not in phase, I mean positive V, negative I or otherwise, which can never happen in your amp.

Just thoughts...it's quite late though, I may be wrong..

regards
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Old 8th May 2005, 06:14 AM   #13
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Quote:
Originally posted by darkfenriz
hi Kanwar
another things:
1. Your PSRR would be ugly... it may even not work correctly with a bit variable rails
2. Is it open loop current source or voltage source?
Most N-ch amps are current sources, 'cause you change Vgs of both mosfets.
In your amp upper mosfet seems to play as a follower (voltage source) and lower as current source. It cannot work (in class AB/B at least), 'couse I and V may be not in phase, I mean positive V, negative I or otherwise, which can never happen in your amp.

Just thoughts...it's quite late though, I may be wrong..

regards
Hello DarkfenriZ,
How would you say that PSRR willnot be that good, I simply donot see any reasons for this , If you really see , then intimate me with reasonableness.
The upper mosfet works as Source Follower and lower mosfet works as Common Source amplifier, whose gates are controlled by string master differentials which in turn act as current sources as well.
Kindly get some good sleep and think again in the morning and attack with some more robustness............
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Old 8th May 2005, 11:12 AM   #14
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hallo Kanwar
now I am awaken but still before morning cofee
so let's think it over again:
1. As the second stage you use a long tailed pair and the tail current is fixed, right?
OK. and the voltage driving output mosfets is produced by drop on collector resistrors of LTP. For simplicity let's analyse what's going on when idle, no input, Vout=0.
Voltage drop on collector resistors is :
Rc*tail current/2
Voltage on the gate of upper mosfet=+Vrail-Rc*tail current/2
Voltage on the gate of lower mosfet=-(-Vrail)-Rc*tail current/2
So voltages on gates are depending on rails voltages making the amp either balancing between class A and C or have low PSRR.

2.
Quote:
The upper mosfet works as Source Follower and lower mosfet works as Common Source amplifier,
Apart from reactive load issues, source follower needs voltage swing on gate equal (of even a bit bigger) to output V swing.
Common source amplifier needs just little voltage swing to make the channel on and off. And it is hard to predict the proportion...
regards
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Old 8th May 2005, 11:58 AM   #15
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Quote:
Originally posted by darkfenriz
hallo Kanwar
now I am awaken but still before morning cofee
so let's think it over again:
1. As the second stage you use a long tailed pair and the tail current is fixed, right?
OK. and the voltage driving output mosfets is produced by drop on collector resistrors of LTP. For simplicity let's analyse what's going on when idle, no input, Vout=0.
Voltage drop on collector resistors is :
Rc*tail current/2
Voltage on the gate of upper mosfet=+Vrail-Rc*tail current/2
Voltage on the gate of lower mosfet=-(-Vrail)-Rc*tail current/2
So voltages on gates are depending on rails voltages making the amp either balancing between class A and C or have low PSRR.

regards

Hi DarkfenriZ,
I see your point regarding biasing string, to overcome this one could implement constant current sources in place of upper Rc of both mosfets, to ensure good PSRR. Thanks for making the circuit better.

regards,
Kanwar
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Old 8th May 2005, 12:04 PM   #16
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Now kindly give your type solution , so that the circuit could be made even better.
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Old 8th May 2005, 12:34 PM   #17
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like this? maybe
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Old 8th May 2005, 12:48 PM   #18
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I like your phenomatic CCS very much , going to simulate it,,,,,,,
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Old 8th May 2005, 01:05 PM   #19
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I have updated the schematic to the newer version.

I liked the response very much especially when driving from variable rail fluctuations.

Thanks DarkfenriZ for the valuable input.
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Old 8th May 2005, 07:04 PM   #20
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the pleasure is mine
I think the last thing to do is to modify the CCS of second LTP to achieve thermal stability.
probably 2-transistor-CCS with one on the main heatsink will do...
or small-signal mosfet in case transistor over-compensates
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