Hi Rahul,
The 1/16th loss is only possible if you drive too low impedance load with single pair of mosfets...
BTW with single pair of mosfets you dont even need the source resistors, thus the rail loss would be eliminated virtually...
Yes, if you want to drive 2 ohms load you need to parallel upto 8 pairs of IRFP260 with source resistors and again the rail loss would be under 1 volt max.....which is also an exceptional case....
regards,
K a n w a r
The 1/16th loss is only possible if you drive too low impedance load with single pair of mosfets...
BTW with single pair of mosfets you dont even need the source resistors, thus the rail loss would be eliminated virtually...
Yes, if you want to drive 2 ohms load you need to parallel upto 8 pairs of IRFP260 with source resistors and again the rail loss would be under 1 volt max.....which is also an exceptional case....
regards,
K a n w a r
Workhorse said:
Hello Kanwar,
Nice design, hats off!
I particularly like your bootstrap circuit to enable the gate drive to swing above the supply voltage.
I would however split the resistor from supply to the zeners in two equal half value resistors and connect the bootstrap capacitor to that point. I am a bit uneasy to see that the bootstart capacitor connects zeners to the output, no hard facts just a concern.
Jan didden
janneman said:
Hi,
Amazingly JAN do you really like my design, Thanks very much for nice compliments...
The reason for using the Zener along with bootstrap capacitor is very much straight forward...you can use a high MFD but a low voltage capacitor which again offers low cost solution as well which in my case is 1000MFD/25V, rather than using like 50V expensive capacitor....
Cheers,
K a n w a r
Workhorse said:
Hi,
Amazingly JAN do you really like my design, Thanks very much for nice compliments...
The reason for using the Zener along with bootstrap capacitor is very much straight forward...you can use a high MFD but a low voltage capacitor which again offers low cost solution as well which in my case is 1000MFD/25V, rather than using like 50V expensive capacitor....
Cheers,
K a n w a r
Yes it may well work the way it is, I haven't tried it this way. I really like those bootstrap circuits, they became almost forgotten when people started to use current sources to feed Vas stages etc. But the capability to swing the driver supplies above the supply voltage is almost free and gives a real improvement in dynamic headroom.
Jan Didden
Workhorse said:
Yes Thats why my circuit swings Rail to Rail because of these Bootstrap circuit...
Dynamic Headroom is much greater than other designs...
Now I am planning to insert VI limiting into it....
K a n w a r
Carefull now, the driver swings above supply but the output will not really swing rail-rail, you lose some volts across the source resistors and also across the D-S resistance. But you'r getting close.
Jan Didden
Hi Jan,
The IRFP250N Mosfet has 75mOhm RDS...When loading to 8 ohms load the voltage drop would be around 0.6V max....
If the Mosfet(s) in parallel configuration are matched well in 10% tolerance than one can eliminate the source resistors as well, further reducing the rail loss...
Lets say parallel 8 pairs yeild effective RDS=0.075/8=0.0093ohms
Loading into 2 ohms load gives us 0.23V rail loss, isn't than its
a R2R amp....
K a n w a r
The IRFP250N Mosfet has 75mOhm RDS...When loading to 8 ohms load the voltage drop would be around 0.6V max....
If the Mosfet(s) in parallel configuration are matched well in 10% tolerance than one can eliminate the source resistors as well, further reducing the rail loss...
Lets say parallel 8 pairs yeild effective RDS=0.075/8=0.0093ohms
Loading into 2 ohms load gives us 0.23V rail loss, isn't than its
a R2R amp....
K a n w a r
Hi Andrew,
Why you think its high.....
The 47K resistor drops 47 volts accross it as the current through differential pair is 1mA...500uA per device...
The voltage of operation is +90Volts w.r.t. positive rail thereby leaving at least 40V accross CCS transistor, which IMO is sufficient enough for proper operation....The resistor also keeps the CCS transistor dissipation to low value....
K a n w a r
Why you think its high.....
The 47K resistor drops 47 volts accross it as the current through differential pair is 1mA...500uA per device...
The voltage of operation is +90Volts w.r.t. positive rail thereby leaving at least 40V accross CCS transistor, which IMO is sufficient enough for proper operation....The resistor also keeps the CCS transistor dissipation to low value....
K a n w a r
Thanks Workhorse,
indeed it does. Obviously based on the high rail voltages, not a solution I would have thought of.
Why do I think it is unusually high:- well most commentators/designers adopt zero to 330r as LTP emitter resistors. I cannot recall any circuit with 1k0 or greater and your choice exceeds the conventional by a factor of 142 times.
Does this create a noise issue?
How about a zener referenced low voltage supply to the LTP tail?
and then how would a cascode loaded LTP sound in comparison?
indeed it does. Obviously based on the high rail voltages, not a solution I would have thought of.
Why do I think it is unusually high:- well most commentators/designers adopt zero to 330r as LTP emitter resistors. I cannot recall any circuit with 1k0 or greater and your choice exceeds the conventional by a factor of 142 times.
Does this create a noise issue?
How about a zener referenced low voltage supply to the LTP tail?
and then how would a cascode loaded LTP sound in comparison?
Workhorse said:
Yeah yeah yeah
. Just as long as you don't tell me that you use 8 pairs of mosfets to save 2V on the transformer secundary.But seriously, it's a mattaer of semantics. Your output cannot swing rail to rail, and I don't think any output circuit can. But the bootstrap will let you get very, very close.
Jan Didden
Eva said:
But Eva, it may be splitting hairs but in theory you can never get at the rails, there is always some loss, no? There's always some Vds required to operate. I'm talking about the output getting to the supply rails.
The highly marketed rail-to-rail opamps get close, but not exactly there. But again, I don't want to split hairs.
Jan Didden
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