The Importance of Being Biased

[Nelson Pass]

(With apologies to Oscar Wilde...)

There's always a lot of talk about output stage bias, but you
don't see too many graphic examples designed to illustrate the
value of bias current in lowering distortion.

Here's a curve of a complementary Mosfet follower operated
without feedback into 8 ohms and biased at a number of
idling current values from 62 mA to 2A. (The lowest curve is
2 amps).

Notice the heavy dependence of distortion on the bias current.

Notice also that there is roughly an inverse proportion between
bias and distortion.

We are running curves from Class AB through twice the Class A
value for the 20 watts.

0.062A corresponds to leaving Class A at 0.125 watts peak,

0.125A = .5W, 0.25A = 2W, 0.5A = 8W, 1A = 32W, 2A = 64W

I get a lot of people who worry about the exact point where an
amp leaves Class A, as if some sort of switch gets thrown and
the amp which was delicious at 5 watts is now disgusting at
6 watts.

As you can see, there is no such abrupt change as you move from
Class A to AB mode, and we see that higher bias improves the
linearity at all power levels, not just in the Class A region.

bon apetite

[gl]

Nelson,

Thank you for this set of examples. They do indeed graphically illustrate the point.

There has been considerable discussion recently about the relative merits of SE bias vs PP bias and the optimal ratio between the two if both are used simultaneously. I for one would appreciate some illumination on this.

One does wonder how one class A biasing electron can sound different from another. Perhaps I should ask, with apologies to George Orwell, if all electrons are equal how can some be more equal than others?

Thank you,
Graeme

[jacco vermeulen]

What a gay curves, Oscar would agree.
Why do the curves seem to move towards eachother at higher output levels, ye great warrior ?

[Zen Mod]

Originally posted by jacco vermeulen
What a gay curves, Oscar would agree.

ya think that Papa is off topic?




not in sexual meaning,except if you imply that he is transsexual, just because of lack of avatar ..........

[jacco vermeulen]

Gay means flamboyant, flamboiante in French sounds even nicer.

[Vix]

Hi there!

It's getting hot these days...bias is increasing....I need a cold drink...

Sorry for OT, but I am getting married on Sunday, July 1st, and may be offline for a few days...

I'll hopefully be back on July 16th...

Have a nice time!

(maybe I'll peek at this forum once or twice, but that's risky )

Regards,

Vix

[Zen Mod]

Originally posted by Vix
................

(maybe I'll peek at this forum once or twice, but that's risky )

.............

[macka]

Nelson,

Thankyou for the illustration.

What I find interesting is that increasing the bias in a pure SE design (no active CCS) the effect is quite audible as though there is a zone or optimum point (ref JHL 15 watt).

Of course this may vary depending on the devices and topology.

macka

[Vix]

Originally posted by Zen Mod

Thanks!

[GRollins]

Originally posted by gl


There has been considerable discussion recently about the relative merits of SE bias vs PP bias and the optimal ratio between the two if both are used simultaneously. I for one would appreciate some illumination on this.

One does wonder how one class A biasing electron can sound different from another. Perhaps I should ask, with apologies to George Orwell, if all electrons are equal how can some be more equal than others?

Thank you,
Graeme

Well, there's equal...and then there's equal...
No one electron can tell you much about the circuitry it has passed through. It takes a continual flow of them so that you incorporate the element of time, rather than attempt to draw conclusions from a single, discrete event. Once you have time, you can have cycles. Once you have cycles, you can begin to see patterns of deviation.
Regarding single-ended vs. push-pull: Although you would notice a decrease in overall distortion with increased bias, that particular story would be more appropriately told by a graph showing the relative levels of second harmonic to other harmonics.
And I would like to add my thanks to Nelson for initiating such an earnest discussion.

Grey