Op amp class A biasing.

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#22
classd4sure said:



Hi,

To what end?

I'm just guessing you would do that with the intention of using the NPN in class A and biasing off the PNP, however it doesn't seem to make sense if the NPN has -20dB less PSRR.
Click to expand...


You should bias the follower stage to class A, whatever kind of transistor it is.

If you have 20dB difference in PSRR, you should work on that problem first although implementing a ccs would be a good solution for that problem too.

Uli

:nod: :nod: :nod:
 
#23
uli said:



You should bias the follower stage to class A, whatever kind of transistor it is.

If you have 20dB difference in PSRR, you should work on that problem first although implementing a ccs would be a good solution for that problem too.

Uli

:nod: :nod: :nod:
Click to expand...


Fair enough, but you say "..if you have..work on that problem first" as though it's unusual to have -20dB on one rail, from what I've seen, it's almost a given for any op amp.

CCS all around?
 
#28
Re: if your

uli said:
psu is an issue in the circuit I would work on that.

For example the OP2134 (which I like very much) has no different specs for plus and minus rails.

Uli

:nod: :nod: :nod:
Click to expand...
uli, I'm afraid the difference is 30dB! between V- and V+..... but what does these figures mean if your power supply voltage has a good quality? I think the whole debate forgot about this? I think we never got a real answer on this issue. I think also that this class A is a bit overrated and is a typcial internet rumour more than an important circuit.
 
#29
Good point.

If you took the measures of a very ..... stiff? /clean supply, then I guess you could just concentrate on the best output device to use, and EF would make sense, but in the case of complimentary, NPN.

Never any simple answers now are there?

Is it overrated? Good question, maybe not for a 741 🙂 Maybe for a high quality audio op amp.
 
#30
Re: Re: if your

classd4sure said:
It does if you're looking in the right place, to my shock even the uncompensated opa637 has this symptom.

http://focus.ti.com/lit/ds/symlink/opa2134.pdf

page 5 "POWER SUPPLY AND COMMON-MODE REJECTION

vs FREQUENCY"
Click to expand...

oops!
youre right!


peranders said:

uli, I'm afraid the difference is 30dB! between V- and V+..... but what does these figures mean if your power supply voltage has a good quality? I think the whole debate forgot about this? I think we never got a real answer on this issue. I think also that this class A is a bit overrated and is a typcial internet rumour more than an important circuit.
Click to expand...


Again I agree, biasing class A is just icing the cake when everything else is perfect (->PSRR etc.)

Uli

:nod: :nod: :nod:
 
#34
My two cents

I have used both the 5532, 5534, OP27 , OP37, OPA627, OPA637, and a number of AD***/LT**** opamps.
Both in standard topology as forced in class A.

The 5532 may be an Oldy, but for the money it is still a good opamp, same goes for 5534.
Personally i like the Burr Brown sound, BB in all types sound way different from other manufacturers.
A PMI OP27 differs in sound from a BB OP27, that i noticed some 15 years ago when i started with PMI types on a pre, blew them when fiddling with the power regulation and replaced them by BB types, as my DIY friend did from first go.

Powersupply definitely has an effect, as it always has.
I have used OPA627's forced in class A on a pre with a battery powersupply, as with any class A amplifier circuit differences in the powersupply make a difference in sound.
But on any amplifier, AB or A, Grey Rollins wrote some lines on it on another thread stating the same.

I rebuilt two 4 channel class A car amplifiers i use in my Van.
The original KIA4558s i first replaced by OPA2134's, the TIP transistors by Sanken 1216/2922(16 of them).
Later i made a good deal on a hundred OPA627/OPA637's, placed a conversion PCB above the Dual opamp pads in able to use single opamps on the amplifiers.
As car amplifiers make use of switched powersupplies(which i am not a big fan of) the original car battery i replaced by two batteries with higher Amp*hour ratings, fitted under the truck.
On the battery lines are two 1Farad caps, internally a great number of 1000uF/ 4700uF EYF Roedersteins were added, the toroids swapped for larger ones, the switching transistors beefed up.

Both the 2134 and the OPA637 i later used were forced in class A, the sound benefitted much by any powersupply improvement.
Class A car amplifiers are sold as such, in reality the class A percentage is not that impressive.
The amplifiers in my van have been thermally coupled to the roof of the car, the wind over the railing on the raised roof actually cools the amplifiers.
Because of it i was able to raise the class A level, to an extent where sound quality suffered because the powersupply could not handle the load, so the bias had to come down.

My conclusion:
The Burr Brown OPA627 and OPA637 rank highest in integrated audiocircuits, even on switched powersupply car amplifiers.
Forcing opamps in class A the way Carlos described is an improvement, but it is not a wonderdrug.
Put a high level audio opamp in a crap amplifier, force it in class A, you will still have a crap amplifier.
Using an opamp such as the OPA627 also means paying extra attention to every other part of the amplifier.
Force the opamp in class A and even more attention for the entire layout is needed.

I started with a simple Opamp switch, and added a few resitors.
In the end i had to rebuild the entire amplifier, the entire powersupply, had to use several powerlines the size of my D..., even every powerline needed a separate Shaffner filter.
No work, no Play !:clown:
 
#35
Is this schematic correct to bias opamp in class A by draw 0.2mA current
 

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