What exactly is "adaptive biasing" for amplifiers

[[email protected]]

Could some electronics guru shed some light
on this ? What exactly is "adaptive biasing"
in respect to power amplifiers ? How exactly
does the "adaptive" term come in, that is
adapt to changes in which parameter -- input
signal amplitude maybe ? Any hints, suggestions
would be helpful. Thanks in advance.

 

[Bill Sloman]

Could some electronics guru shed some light
on this ? What exactly is "adaptive biasing"
in respect to power amplifiers ? How exactly
does the "adaptive" term come in, that is
adapt to changes in which parameter -- input
signal amplitude maybe ? Any hints, suggestions
would be helpful. Thanks in advance.

Beats me. In audio amplifiers, the output transistors get hotter if
you play a lot of loud music, and if the class A/B biassing hasn't
been set up right, the bias current flowing through the output
transistors at zero signal can get quite a lot bigger than it is when
the transistors are cold.

 

[[email protected]]

Beats me. In audio amplifiers, the output transistors get hotter if

you play a lot of loud music, and if the class A/B biassing hasn't

been set up right, the bias current flowing through the output

transistors at zero signal can get quite a lot bigger than it is when

the transistors are cold.

It's just a general term to describe a variable power amplifier bias that is input signal level dependent. It can run the gamut from audio to RF, in applications where 1) relatively large signal dynamic range must be accommodated and 2) there is some necessity to conserve amplifier loading of the DCsupplies and/or 3) adaptive biasing is required to meet linearity performance.
No expert here, just what I surmise from a quick look at the Google search abstracts.

 

[miso]

In the audio range, adaptive biasing is such that the Iq is very low
unless you need the power. There are some tricks such as when a slew
situation is detected you juice the long tail pair to attempt to recover
faster.

Eric Vittoz from Swiss Federal has papers and a book or two of such tricks.

 

[Mark]

Here's what I did to conserve battery power in a hearing aid, yet get
bias up in a hurry, to provide low distortion at high amplitudes in a
class-A output stage...

   http://www.analog-innovations.com/SED/HearingAidSlidingClassA.pdf

                                        ...Jim Thompson

If one of the requirements is to conserve power, why would you use a
Class A output stage in the first place?

Mark

 

[Grant]

If one of the requirements is to conserve power, why would you use a
Class A output stage in the first place?

700uA standby current is hardly low power either, but how else do you make
an amplifier for 1V operation, if not class A?

Not a lot of wiggle room there.

Grant.

 

[miso]

I think when you reach the point where a hearing aid is required,
harmonic distortion isn't much of an issue due to reduced bandwidth. ;-)

 

[miso]

Fortunately I know nothing about hearing aids.

 

[Bill Sloman]

   With their size, they should be using picoprocessors. ;-)

Nanoprocessors would be quite small enough. The diameter of most atoms
is between 200 and 400 picometres, so building a picoprocessor would
be tricky.

 

[josephkk]

700uA standby current is hardly low power either, but how else do you make
an amplifier for 1V operation, if not class A?

Not a lot of wiggle room there.

Grant.

At the era of the design, probably not much choice; use class D today.

?-)