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It really has nothing at all to do with BJTs or MOSFETS. The damping factor of the amplifier is purely a function of its ability to regulate a load, which is directly tied to the amount of feedback being applied at a given frequency.Either output device can be employed into a design that features extremely high regulation (damping). That said, I'd still take MOSFETs over BJT simply due to the overall reliability differences between the two devices.
Hi Pete, , which mosfet would you take lateral or vertical.. and why?... btw bjts are reliable too if you put a resistor at their emitter...
BJTs can be reliable, but it's the 2nd breakdown that makes them less desirable to me as an output device. That and they are more difficult to drive than MOSFETs.I've played with a lot of vertical MOSFETs over the years and am very comfortable with them. Haven't had the chance to play with any lateral types.
Pete,what do you mean by 2nd breakdown(thermal runway?),that's not a problem(resistor at the emitter and run them cool),laterals are negative temperature coefficient, similar beasts to verticals but more robust when it comes to failure
2nd breakdown isn't quite thermal runaway.https://en.wikipedia.org/wiki/Safe_operating_areaUsing multiple BJTs definitely reduces the likelihood of 2nd breakdown.
Pete, when I said run them cool,that's what I meant soa with no other soa protection,I have had only one failure in over 20 years and that was from different brand fake bjt...
Back to mosfets,I run them with bigger currents,this translates to bigger damping,seems an inherent damping of the device,and I can hear it with bass,both lateral and verticals have similar effect...
Damping is just another word for regulation. How well does the output stay within the theoretical gain of the amplifier as the load impedance varies.
You are exactly correct. Damping, which is directly related to output impedance, is set by the amount of global negative feedback, among other things.If you look only at the output devices, and their transfer function of voltage in to voltage out, an emitter follower (typical BJT output stage connection) will have better regulation than a source follower MOSFET output device. However, the input impedance of the MOSFET is orders of magnitude higher than the BJT. This higher input impedance makes it much easier to create a gain stage that has very high open loop gain. As an example, if the open loop output impedance of the BJT output stage is 1 Ohm, and the amp has a closed loop gain of 25dB at 100Hz, and the open loop gain of the amp is 65dB at 100Hz, then the output impedance will be reduced by a factor of 40dB. That 1 Ohm output impedance will now be .01 Ohm.If the output impedance of the MOSFET output stage is 5 Ohm, the closed loop gain is 25dB, and the open loop gain is 85dB at 100Hz, then the 5 Ohm output impedance is reduced by a factor of 60dB, or .005 Ohm.
