O.K. I'll give it another try.
If your nonlinear vbe/base-current relationship gives an error of 20% for instance it will translate into 20% voltage error at the collector of a common-emitter stage (you are of course free to take linearising measures like local feedback or drive the base by a current source etc.... it is even recommended to do so of course, but I'd like to compare the topologies "naked" here).
If you now take an emitter follower, the same 20 percent error of the vbe/base-current relationship will translate into less than about 0.2 Volts absolute error at it's emitter. Given an amp that swings +- 30 volts the absolute error of the common emitter stage (i.e. VAS) would be 6 Volts approx for the aforementioned error of 20 % and this is more than an order of magnitude larger than the 0.2 Volts for the emitter follower.
This is of course a simplistic way of looking at it. Another way of looking at it is regarding an emitter follower as a stage that uses 100 % (intrinsic) voltage feedback (also a simplistic view admittedly).
As I already mentioned, N.P. developed some amps that took NFB from the driver stages (or even the VAS ?). He said that they measured about the same as if feedback had been taken from the output but they sounded better !
If you don't take feeedback from the output stage, your amp is less prone to load dependant oscillations but it will be somewhat less linear. So one has to decide which is the better compromise.
Regards
Charles
Traderbam,
No, I understand what you are saying but I'm not confussed as there is a subtle difference in this case.
Impedence is a measure of frequency dependent "resistance" of a current flowing into a load whereas I'm talking about current flowing out of a load back into an amplifier which is back EMF. thus there will be a voltage differential between what the amp is trying to put out vs the back EMF from the load at the point where the NFB starts its return to the front end. This can't be good!
Pro,
Thanks for the reference. At first glance (I download at work and read at home) it is pretty much along the lines I am talking about.
Regards
No, I understand what you are saying but I'm not confussed as there is a subtle difference in this case.
Impedence is a measure of frequency dependent "resistance" of a current flowing into a load whereas I'm talking about current flowing out of a load back into an amplifier which is back EMF. thus there will be a voltage differential between what the amp is trying to put out vs the back EMF from the load at the point where the NFB starts its return to the front end. This can't be good!
Pro,
Thanks for the reference. At first glance (I download at work and read at home) it is pretty much along the lines I am talking about.
Regards
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