Hi folks,
today a bit more theoretical than usual.
Given that amplifiers are delicately compensated for a choice of bandwidth, slew rate, overshoot, one is (should) be concerned a bit when substituting transistors. Especially when working on an older design and original parts are no longer available (for comparison) and one does not want/is able to rework the compensation.
Think of a general 3 stage amp (input differential, VAS, outputstage without gain).
Since speed (slew rate) is determined by the VAS and not by the output stage I come to my first thought:
Assume there is no feedback taken from the output stage back (to whereever), substituting a 30MHz bjt for a 3MHz bjt in the gainless output stage should not affect stability, nor other parameters?
Next, a bit more tricky: reworking the VAS. The 15MHz 2N5416 is maybe a nice part, but close to 40 years old. Replace that old guy with say a 200MHz 2SA1360.
Given that the frequency compensation around the VAS is done properly, I would believe that this should also not affect stability.
Why?
As far as I understand Bode-analysis, the poles and zeros of the loop gain determine stability (+ compensation), thus as long as the new transistor has a similar gain, stability should be conserved, since the gain is anyway 0 after a certain frequency. Of course this assumes comparable parasitic capacitances and means also that VAS slew rate is unchanged by this substitution. Only remaining advantage slightly lower (non-linear) Cob (?) and better linearity.
Any inputs greatly appreciated guys!
Heat up the solid state section here, so that we have a nice discussion! Benefits all, it's always hard to get elaborate replies on such questions.
Have fun, Hannes
------------------
To name the beast, I'm talking about the Cello Encore amp, schematic is
http://www.nasotec.co.kr/acecart/bin/boarddata/DataRoom/114.pdf
discussion was here
http://www.diyaudio.com/forums/showthread.php?s=&threadid=116237&highlight=
In case you want to dig a bit into the workings of this amp, you can use Steven's enlightening analysis as starting point
voltage clamp
http://www.diyaudio.com/forums/showthread.php?postid=1420539#post1420539
http://www.diyaudio.com/forums/showthread.php?postid=1421533#post1421533
current mirror in VAS, cascode
http://www.diyaudio.com/forums/showthread.php?postid=1421545#post1421545
output stage
http://www.diyaudio.com/forums/showthread.php?postid=1424115#post1424115
simulation of that amp is a bit wonky, so better rely on general principles.
today a bit more theoretical than usual.
Given that amplifiers are delicately compensated for a choice of bandwidth, slew rate, overshoot, one is (should) be concerned a bit when substituting transistors. Especially when working on an older design and original parts are no longer available (for comparison) and one does not want/is able to rework the compensation.
Think of a general 3 stage amp (input differential, VAS, outputstage without gain).
Since speed (slew rate) is determined by the VAS and not by the output stage I come to my first thought:
Assume there is no feedback taken from the output stage back (to whereever), substituting a 30MHz bjt for a 3MHz bjt in the gainless output stage should not affect stability, nor other parameters?
Next, a bit more tricky: reworking the VAS. The 15MHz 2N5416 is maybe a nice part, but close to 40 years old. Replace that old guy with say a 200MHz 2SA1360.
Given that the frequency compensation around the VAS is done properly, I would believe that this should also not affect stability.
Why?
As far as I understand Bode-analysis, the poles and zeros of the loop gain determine stability (+ compensation), thus as long as the new transistor has a similar gain, stability should be conserved, since the gain is anyway 0 after a certain frequency. Of course this assumes comparable parasitic capacitances and means also that VAS slew rate is unchanged by this substitution. Only remaining advantage slightly lower (non-linear) Cob (?) and better linearity.
Any inputs greatly appreciated guys!
Heat up the solid state section here, so that we have a nice discussion! Benefits all, it's always hard to get elaborate replies on such questions.
Have fun, Hannes
------------------
To name the beast, I'm talking about the Cello Encore amp, schematic is
http://www.nasotec.co.kr/acecart/bin/boarddata/DataRoom/114.pdf
discussion was here
http://www.diyaudio.com/forums/showthread.php?s=&threadid=116237&highlight=
In case you want to dig a bit into the workings of this amp, you can use Steven's enlightening analysis as starting point
voltage clamp
http://www.diyaudio.com/forums/showthread.php?postid=1420539#post1420539
http://www.diyaudio.com/forums/showthread.php?postid=1421533#post1421533
current mirror in VAS, cascode
http://www.diyaudio.com/forums/showthread.php?postid=1421545#post1421545
output stage
http://www.diyaudio.com/forums/showthread.php?postid=1424115#post1424115
simulation of that amp is a bit wonky, so better rely on general principles.