I have read with some interesting comments on the high output impedance of output stages using common emitter or common source circuits. These of course use the collector or drain as the output instead of the emitter or source.
One person commented that nobody uses the common source configuration. This of course is an error in thinking. All of my amplifier designs for example use common source configuration. Naturally the output impedance of these and designs that may be similar circuits are going to have higher output impedances than those using common drain configuration. This rise in output impedance is primarily due to the fact that the common source configuration does not use 100 percent negative self-feedback in the output stage that a source or emitter follower configuration has. The great benefit of using a common source configuration in a amplifier that drives a loudspeaker is that you gain the ability to control the output impedance via a selected amount of negative feedback around the stage. This is because the restriction of 100 percent self-negative feedback is gone. This has some major benefits when one wishes to achieve maximum performance when driving loudspeakers..
Many persons have been lead to believe that an audio power amplifier must have very low output impedance and thus able to provide high damping factors. This line of thinking is incorrect in my opinion since it totally disregards what it takes to make loudspeakers perform the best. There are designs that actually include positive current feedback to achieve near zero output impedance over a good portion of their operating range.
If one has the ability to adjust the output impedance while listening to music played though your favorite loudspeakers you may find that output impedances between .2 and .5 ohms actually provide the best sound quality. And also that low feedback factors around the output stage in the order of 6 to 15 Db are best. This should provide damping factors in the order of 12 to 20 and is similar to what a tube amplifier would provide. Feedback factors of 6 to 15 Db cannot be achieved with circuits that use a source follower in the output stage since they run at close to 100 percent negative feedback already.
The source follower output stage is more popular for two main reasons. It is easy to use and second it has lower distortion. Unfortunately the low distortion and thus its linearity is provided by 100 percent negative feedback. Considering that negative feedback is undesirable in such large amounts makes me wonder why the configuration is used at all to drive loudspeakers.
For every one who is seeking the sound of a tube amplifer in a solid state design the first thing you have to get rid of is the source follower in the output stage. Then adjust the negative feedback around the output stage to reasonable values.
John Fassotte
Alaskan Audio
One person commented that nobody uses the common source configuration. This of course is an error in thinking. All of my amplifier designs for example use common source configuration. Naturally the output impedance of these and designs that may be similar circuits are going to have higher output impedances than those using common drain configuration. This rise in output impedance is primarily due to the fact that the common source configuration does not use 100 percent negative self-feedback in the output stage that a source or emitter follower configuration has. The great benefit of using a common source configuration in a amplifier that drives a loudspeaker is that you gain the ability to control the output impedance via a selected amount of negative feedback around the stage. This is because the restriction of 100 percent self-negative feedback is gone. This has some major benefits when one wishes to achieve maximum performance when driving loudspeakers..
Many persons have been lead to believe that an audio power amplifier must have very low output impedance and thus able to provide high damping factors. This line of thinking is incorrect in my opinion since it totally disregards what it takes to make loudspeakers perform the best. There are designs that actually include positive current feedback to achieve near zero output impedance over a good portion of their operating range.
If one has the ability to adjust the output impedance while listening to music played though your favorite loudspeakers you may find that output impedances between .2 and .5 ohms actually provide the best sound quality. And also that low feedback factors around the output stage in the order of 6 to 15 Db are best. This should provide damping factors in the order of 12 to 20 and is similar to what a tube amplifier would provide. Feedback factors of 6 to 15 Db cannot be achieved with circuits that use a source follower in the output stage since they run at close to 100 percent negative feedback already.
The source follower output stage is more popular for two main reasons. It is easy to use and second it has lower distortion. Unfortunately the low distortion and thus its linearity is provided by 100 percent negative feedback. Considering that negative feedback is undesirable in such large amounts makes me wonder why the configuration is used at all to drive loudspeakers.
For every one who is seeking the sound of a tube amplifer in a solid state design the first thing you have to get rid of is the source follower in the output stage. Then adjust the negative feedback around the output stage to reasonable values.
John Fassotte
Alaskan Audio