Complex Emitter follower

[Jeff R]

"The emitter resistors are a necessity - the base resistors are not. However, I think the value of the emitter resistors could be reduced to maybe .1 Ohms (per Self)."

You're still being silly. Even if you reduce the emitter resistors to 0R1 from 0R33 there will still be 600mV drop compared to a small fraction of that in the base resistor. Your sense of perspective is skewed.

I accept that I may appear "silly" for my concern about the 10 Ohm base resistors. 16 mV is pretty small compared to the base-emitter voltage drop, as you say. However, my main point was why use a component if there is not a clear and real need for it? Others have noted that different types of resistors impact the sound of an amplifier. If that is true, then we have to look critically at each resistor we use and, if we cannot justify it being there, then why use it? Trying to justify omitting them due to concern about a 16 mV drop may be a silly reason, but the justification that the resistors can impact the sound negatively is not.

As for the emitter resistors, at a 3 Amp loading condition, .33 Ohm resistors will drop nearly one volt. If we can reduce this down to .1 Ohm, the drop is reduced by about 1/3rd. These voltages are certainly significant compared to the Vbe drop. We cannot eliminate the resistors as they are needed for stability and for current sharing reasons.

 

[peranders]

I accept that I may appear "silly" for my concern about the 10 Ohm base resistors. 16 mV is pretty small compared to the base-emitter voltage drop, as you say. However, my main point was why use a component if there is not a clear and real need for it?...

41 through R44 are in series with the bases of the output transistors to suppress parasitic oscillations that could occur in the output stage. These are bursts of oscillations on peaks of the audio signal

Text taken from Prof Leach.

If you don't get any oscillations (can be very nasty) just simply omit the resistors! Under some circumstances you can get oscillation from a single transistor. MOSFET's are very sensitive for that, BJT less.

 

[Jeff R]

I agree, Peranders, and I have given that same advice in other threads.

Regarding Leach, I believe he also says that he never had any problem with such oscillations on any Leach Amp, but there was one case with the Super Amp that oscillated. The resistors cured that problem and he decided to implement the change in all of his amplifier designs as insurance. My Version 3 Leach Amp design did not include the base resistors, so he only added them with Version 4.

I appreciate the discussions.

 

[jan.didden]

[snip] - Distortion: As mentioned above, the feedback will try to channel the excess current to the other output device, but this will unavoidably lead to higher THD, as the feedback cannot compensate 100%, especially at higher freqs where this happens most, as the loop gain is lower and consequently feedback is weaker there.

Jan Didden

PS I just learned that this is called "dynamic crossover distortion" as opposed to the "static xover dist" resulting from the two output devices not exactly matching each others' gain curve at xover. For what it's worth.

Jan Didden

 

[mikek]

PS I just learned that this is called "dynamic crossover distortion" as opposed to the "static xover dist" resulting from the two output devices not exactly matching each others' gain curve at xover. For what it's worth.

Jan Didden

...i reckon i'll stick with cross-conduction....