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.
-- Bill Sloman, Sydney
It sounds like a term invented by marketing types that is designed to get you so excited that you wet your pants and throw way more money at the manufacturer than you should.
Do you have any context? Like, brand and model? The term could cover anything from some really new and effective way of biasing amplifiers to your basic garden-variety negative feedback that's been around since the
1930's.-- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
It's just a general term to describe a variable power amplifier bias that i s input signal level dependent. It can run the gamut from audio to RF, in a pplications where 1) relatively large signal dynamic range must be accommod ated and 2) there is some necessity to conserve amplifier loading of the DC supplies and/or 3) adaptive biasing is required to meet linearity performa nce. No expert here, just what I surmise from a quick look at the Google search abstracts.
By "power amplifier", I'll assume that you mean RF power amplifier.
"Improving amplifier?s efficiency using a linearizer in conjunction with adaptive bias modulation."
"On Adaptive-Biasing of Linear Power Amplifiers in Devices with OFDM Transceivers"
The power amp in a cell phone running on low power supply voltages has problems staying linear, which is needed for todays high data rate OFDM system. Since the TX power level is dynamically controlled by the cell site's receiver BER (bit error rate), keeping the handset PA linear and simultaneously efficient is not a trivial exercise.
No. Feedback from the RF output and the cell phone APC (adaptive power control) section changes the power supply voltage and output stage bias point. The intent is to not run the amplifier continuously in maximally linear Class A, but rather to vary the operating point to a lower power level when a full power output swing is not required. This also dramatically improves the amplifier efficiency.
There's also adaptive biasing in hi-fi amplifiers, of which I know nothing.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
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.
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...
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
...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
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.
Low distortion, it's a hearing-aid.
I made a class-B version, push-pull, center-tapped transducer. Lots more power, but about the same Q-current.
Indeed. Working at 1V is "interesting" ;-) This is a many-years old design... now it would be interesting to do it in a BiCMOS process.
...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
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. ;-)
Not true. ...Jim Thompson
-- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at
He was talking about super cheap hearing aids for liberals, who never listen anyway. :)
That must be it. All the modern hearing aids I know of have fractional octave equalizers... and microprocessors :-( ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Fortunately I know nothing about hearing aids.
With their size, they should be using picoprocessors. ;-)
. ;-)
ever
Nanoprocessors would be quite small enough. The diameter of most atoms is between 200 and 400 picometres, so building a picoprocessor would be tricky.
-- Bill Sloman, Sydney
make
At the era of the design, probably not much choice; use class D today.
?-)
Some detail on the relative merits of different amplifier modes in hearing aids:
"Micropower Low-Voltage Digital Class D Amplifier for Hearing Aid Applications"
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
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