Since the signal is between base and emitter then what is the practical influence of that BC junction capacitance non-linearity that needs fixing?
But ok, since this discussion is straying away from the main topic then from my side we can close it here - it is unlikely that I will become an expert on that level 😉
This is getting beyond my expertise, but non-linearity can cause unexpected gain resulting in oscillation. (I am an electrical engineer, but my field is computer architecture. I can tell that quite a few members here specialized in signals or devices).madis64 said:
Ed
Oh, just imagine, emitter really tracks base signal, no BE capacitance recharge.
But BC capacitance are at full rail potential, exhibit full output signal swing and need to be recharged.
You can get a few ideas here https://www.pcbway.com/project/shar...audio_power_amplifier_a_rebirth_4a7881e9.html
I put some notes together on EF instability based on Cordell's book and my own experience. Its not perfect, but it does provide some general guidelines about how to make an EF3 bulletproof.
Here is the oscillation WITHOUT EF3 instability precautions :-
I used 10-15 nH PCB trace inductances in this model, whereas Cordell used 35nH in his which will in general result in lower oscillation frequencies. On small signal emitter followers, I've had problems at 180 MHz, and on a beta helper in a VAS circuit, 20-40 MHz. My first EF3 in 2006 oscillated at 4 MHz (used MJ21193/4 output devices) which was cured with base stoppers in the output devices and a 22 Ohm and 1nF driver base damping network. I've not had a EF3 problem since then, using the simple techniques 1-4 outlined in the 1st graphic above.
Here is the oscillation WITHOUT EF3 instability precautions :-
I used 10-15 nH PCB trace inductances in this model, whereas Cordell used 35nH in his which will in general result in lower oscillation frequencies. On small signal emitter followers, I've had problems at 180 MHz, and on a beta helper in a VAS circuit, 20-40 MHz. My first EF3 in 2006 oscillated at 4 MHz (used MJ21193/4 output devices) which was cured with base stoppers in the output devices and a 22 Ohm and 1nF driver base damping network. I've not had a EF3 problem since then, using the simple techniques 1-4 outlined in the 1st graphic above.
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It’s a “ground is not ground” problem. See the parasitic inductances in Bonsai’s post.
Thanks for doing that. I knew about base resistors and collector decoupling.
By now, I have seen on this board examples of everything that can go wrong in amplifiers. I look back on my design as having avoided all pitfalls, mostly by skill but sometimes by luck. 🙂
Ed
Yes.
If you can keep it to act as a capacitor in the whole its network. And namely this remark accurately provides us to the solution of placing it straight to the leads of the BC junction.
That is likely good enough. Oscillations tend to be gross - the voltages appear wrong and the amplifier is getting hot! It may even be jamming radio receivers.
Ed
Ed
Depends if it is digital or analog. 20 MHz analog scope will show a 100 MHz signal. Digital will not.
Then it would be easiest to judge their presence by power consumption (rail current draw)?