so doing some tests i ran across a huge question that i cannnot answer yet
so in a class ab amp the input is usally a diff pair
so the diff amp amplifies the input and if its a npn diff pair the output of the diff pair so between GND and the postive rail
this output is usually fed to a drive which supplies enough current for the output stage
the output stage class B biased on so it is class ab
my question here is
where is the signal getting centered at GND so both the pnp and npn are used in the output stage
my ciruit right now the output from the diff pair is between GND and postive rail thorugh the driver to the class ab power output stage and only the top half is only on
where should the signal be centered at GND and how is this accomplished
?
so in a class ab amp the input is usally a diff pair
so the diff amp amplifies the input and if its a npn diff pair the output of the diff pair so between GND and the postive rail
this output is usually fed to a drive which supplies enough current for the output stage
the output stage class B biased on so it is class ab
my question here is
where is the signal getting centered at GND so both the pnp and npn are used in the output stage
my ciruit right now the output from the diff pair is between GND and postive rail thorugh the driver to the class ab power output stage and only the top half is only on
where should the signal be centered at GND and how is this accomplished
?
Hi !
A classic ClassAB-Design consist of three stages:
1, The Inputstage (using the diffpair)
2, The VAS (VoltageAmplificationStage)
3, The Outputstage (CurrentAmplification)
The answer to your question is the VAS.
The output from the diffpair is very low, normally < 1Volt
This signal is fed into the VAS, centering the voltage and
amplifying it to the full voltageswing. Just take a look
into existing schematics, you will find 2 "complementary" transistors
that are "connected" with their collectors, having their emitters
to the supplyvoltagerails with a small resistor (mostly ~150ohm)
Michael
A classic ClassAB-Design consist of three stages:
1, The Inputstage (using the diffpair)
2, The VAS (VoltageAmplificationStage)
3, The Outputstage (CurrentAmplification)
The answer to your question is the VAS.
The output from the diffpair is very low, normally < 1Volt
This signal is fed into the VAS, centering the voltage and
amplifying it to the full voltageswing. Just take a look
into existing schematics, you will find 2 "complementary" transistors
that are "connected" with their collectors, having their emitters
to the supplyvoltagerails with a small resistor (mostly ~150ohm)
Michael
MikeB said:
The resistor sets the balance of the differential pair.
Input into the VAS is current.
The resistor cannot convert current to voltage as it has the
VAS base emitter diode effectively in parallel, this makes it
a virtual earth point, so linear voltage swing is not possible.
sreten.
Tube_Dude said:
As the input impedance of the VAS is 10k ...10 times greater than the driving impedance (the LTP load resistor are usually ~1k ) we must talk of voltage drive.
The some as the input of the LTP ...it need current to work but we don't say it is current drived ....
Jesus,
This is hardly the thread to get technical and argue simple
details with someone who knows what they talking about.
Most of the frequency range current drive is into Cdom.
And trying to arrange voltage swing is pointless.
sreten.roofingboom - If I understand your real question correctly, I'm not sure it's been answered yet. The signal is "centered" with reference to the output stage by means of the collector of the VAS stage in the top half using everything it "sees" electrically as it's load. That means that the complimentary npn trannie on the static bottom half as well as the whole output stage AND it's speaker load is the load to that upper collector. This load for most practical purposes is quite high in impedance. It is this load that the current from the upper pnp VAS transistor collector sees, and what makes "floating" voltages occur, keeping the levels betwixt the + and - power supplies. Further, if no speaker load were connected, voltage swings would still occur, but no current would be drawn from the output stage(emitter follower). If a load was connected, then the swings would be referenced to ground, thus "centering" it. (???) Now, some amp topologies have a resistor connected from that collector/output node to ground, in which case it's meant to be the primary load that the upper pnp's collector sees,..in which case most of the current will go through it forming the "centered" voltage. So, the first case describes a floating voltage (at least within that node), and the resistor case describes a voltage referenced to ground. Hope that helps, starting out in discrete amp design is a bit overwhelming I'll have to admit. If I misunderstood you,..then,...never mind.......
"this output is usually fed to a drive which supplies enough current for the output stage"
This would be the VAS stage- in between the diff. input and the output stage. Do not confuse the the actual output transistors with any "driver" transistors (also in an emitter follower mode,.... usually..). You know, posting a circuit would really, really be helpful here. Both the VAS stage and output stage are commonly composed of so-called complimentary transistors. They are found by looking at data books,..something that you should become rather familiar with if you are going to do this type of work. Also, reading some books on amp design would be rather helpful as well, and becoming familiar with any and all circuits you come across,..(i.e. looking to see what others have done). There has been much done on the Leach amp, perhaps starting there would help.
This would be the VAS stage- in between the diff. input and the output stage. Do not confuse the the actual output transistors with any "driver" transistors (also in an emitter follower mode,.... usually..). You know, posting a circuit would really, really be helpful here. Both the VAS stage and output stage are commonly composed of so-called complimentary transistors. They are found by looking at data books,..something that you should become rather familiar with if you are going to do this type of work. Also, reading some books on amp design would be rather helpful as well, and becoming familiar with any and all circuits you come across,..(i.e. looking to see what others have done). There has been much done on the Leach amp, perhaps starting there would help.
so when i usally look at schematics
the input stage is the diff pair
thte next stage is the VAS? usally a npn/pnp depending on what kind of input stage?
and the output is complmentary pairs or darlingonton pairs.
my question is what is the VAS? is it the single transistor after the diff pair ?
the input stage is the diff pair
thte next stage is the VAS? usally a npn/pnp depending on what kind of input stage?
and the output is complmentary pairs or darlingonton pairs.
my question is what is the VAS? is it the single transistor after the diff pair ?
roofingboom said:
Power Amplifier Design Guidelines by Rod Elliott gives a good descrition of how the stages of a typical class ab amp work. Also see some of his projects for more info.
Doug Self's website details the interaction/problems between the various stages of a class ab amp. (although as i write this, the server seems to be down). His book is more comprehensive.
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