Choices of bias network

[Leolabs]

What is the significant differences among these bias network:
1)single transistor+resistors,very very popular;
2)single transistor+resistors+diodes,something like the Leach amp;
3)two transistors+resistors,usually NPN + PNP,claimed that produce lower AC resistance.

????????

[unclejed613]

i've seen the 2 transistor one, not sure if there is any advantage to it, "speed" is not an issue for a bias network.

[AndrewT]

Hi,
I have seen the multi transistor type and they are often promoted by designers that put low distortion above all else.

I think the main route for audio signals from the VAS to load is the capacitor between the collectors (if active load) rather than through the Vbe multiplier. I believe a good cap here is better value than extra transistors.

[Piercarlo]

Originally posted by AndrewT
Hi,
I have seen the multi transistor type and they are often promoted by designers that put low distortion above all else.

I think the main route for audio signals from the VAS to load is the capacitor between the collectors (if active load) rather than through the Vbe multiplier. I believe a good cap here is better value than extra transistors.

Two transistors properly connected as in this circuit may assure a virtual "AC short circuit between the bases of devices to be driven. See the allegate for my idea of "properly connected".

Hi
Piercarlo

[lineup]

Originally posted by AndrewT

I think the main route for audio signals from the VAS to load is the capacitor between the collectors (if active load) rather than through the Vbe multiplier. I believe a good cap here is better value than extra transistors.

I also has done some tests - simulations,
if this capacitor across the Vbe multiplier transistor
can lower distortion.

The result that I have found so far:
In some circuits it will definitively lower distortion figures.
In other amp circuits they do almost no difference.


A funny thing about a capacitor before the output stage
I found the other day.
See my attachment!

First I tried with a cap across the Vbe transistor - the common way.
Then I tried with putting this capacitor between the BASE of npn/pnp output pair.
This is after the base stoppers resistors.
Voila!
This was the best solution in this circuit.
Lowered the distortion by considerable figures.

I guess it has to do with pulling out current from the bases.
Like is found at several other investigation, by other known amplifier designers.


lineup

[Jan Dupont]

Leolabs;

You forgot:
4) multiple diodes+resistor



lineup;

I also have good experience with this cap placement

[roender]

Originally posted by ACD
Leolabs;

You forgot:
4) multiple diodes+resistor

The best of all, especially when diodes are encapsulated in power BJT's like Onsemi NJL's series.
http://www.diyaudio.com/forums/attac...amp=1168269380

[teemuk]

http://www.essex.ac.uk/ese/research/...ed%20diode.pdf

[Jan Dupont]

roender;

Agree

[lineup]

Originally posted by ACD

lineup;
I also have good experience with this cap placement

thanks, good to know
I know you have good experience, ACD
as can be seen in your website: Audio Circuit Denmark

lineup - en del are blåe and gule, andre are vide and roede