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3rd May 2004, 05:23 PM  #1 
diyAudio Member
Join Date: Apr 2004
Location: BC

Biasing for different classes
i was wondering if anyone could tell me the rules of thumb for biasing different classes of amplifiers ie A, AB and B...i would like to know how to work out output power (peak and RMS) and Current (peak and RMS) please

3rd May 2004, 07:30 PM  #2 
The one and only

OK, the basic formulas are W = V^2 / R and W = I^2 * R
and V = I * R where V = volts, R = ohms, I = amps and W = watts For a clean sine wave, the peak wattage is twice the average wattage. So 100 watts rms into 8 ohms is 40 volts peak and 5 amps peak, and 28.32 volts average and 3.54 amps average. Figure that your amplifier supply will require a few more volts than the peak output voltage. For an ordinary pushpull amplifier, the Class A bias is 1/2 that of the peak output current, which for the above example is 2.5 amps bias. For other types of amplifiers, the relation between bias current and Class A output varies according to the design, but realistically, it's always high. 
3rd May 2004, 08:56 PM  #3 
diyAudio Member
Join Date: Apr 2004
Location: BC

thank you nelson..i really appreciate the help

4th May 2004, 05:59 PM  #5 
The one and only

You need a little more than 30 volts, and a little less than
4 amps. V^2 = 2*120*4 = 960 V = 30.98 (PEAK) I^= 240 / 4 I = 7.75 (PEAK) I/2 = 3.87 (BIAS) 
4th May 2004, 07:03 PM  #6  
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Join Date: Dec 2003
Location: Near Seattle

Re: Biasing for different classes
Quote:
I'm using the definitions from the Sedra/Smith book: Class B is essentially a mirrored PNP/NPN EF with no base bias and direct driven from the VAS. Inherently it has no bias current which seperates it from class AB. Class AB is the same mirrored PNP/NPN with some bias circuitry. The Self book has some notes about how to determine the optimum bias for this topology but I can't remember off hand how it's done. Usually it's between 50ma  300ma with 300ma being fairly high. The leach amp uses about 100ma.  Danny 

4th May 2004, 08:15 PM  #7 
diyAudio Member
Join Date: Nov 2003
Location: Brighton UK

As ever confusion reigns.
Class aB is optimal minimum bias, and the majority of amplifiers. (note that this is what D.Self would like to call class B) Class AB is purposefully class A up to a few watts. (also according to D.Self) sreten. 
4th May 2004, 09:32 PM  #8  
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Join Date: Dec 2003
Location: Near Seattle

Quote:
The trouble between distinguishing between aB and AB is defining at what point the transition occurs. If you assume 3 watts, then in the same output topology is a difference class for 150ma bias vs 300ma bias (8ohm load). It becomes hard to determine from the topology what class the output is but determinable by observing the output. But then, on the other hand using my definitions, the topology is quite different for each class but hard to determine by observing the output. For example, it is hard to determine exactly at what point does the class B like output instead become class C? At output voltage levels, say about 5Vpp > 3 watts, conduction only occurs for 75% of the cycle. Clearly a class C response... I guess you just pick your own definitions from the way you like looking at it and stick to it...  Danny 

4th May 2004, 11:31 PM  #9  
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Join Date: Feb 2004
Location: Silicon Valley

Re: Re: Biasing for different classes
Quote:
Self recommends class B highly over class AB because it has the lowest measured distortion. He considers there is a precisely optimal class B bias for any output BJT stage, whether Emitter Follower EF or Complementary Feedback Pair CF, which yields minimum measured distortion. The optimal bias current is a function of the emitter resistors and output configuration, but for a typical EF output with 0.22 ohm emitter resistors, it is 107ma (presumably at 20C), while for the CF stage it is 11.5ma. [Yes, strictly speaking this is not class B since there is some simultaneous conduction of the outputs  but clearly, not much, and this is called class B by most authorities.] The optimal way to set bias is using a distortion analyzer for minimum distortion, regardless of the output configuration. Overbiasing will yield higher distortion, so it should be avoided. Self does not consider mosfets acceptable as output devices and makes no recommendations for bias current (his head is clearly in the sand on that topic). However, most designers bias at or above the knee in the transconductance curve of the mosfet, typically 100200 ma per device. There may be unique circuit characteristics which call for different bias currents, but those are reasonable starting points. 

5th May 2004, 01:09 PM  #10 
diyAudio Member
Join Date: Mar 2004
Location: Como

What is the difference (for audio quality) to use a pushpull amplifiers in classAB or in classA?
The crosover distorsion disappear and with hight polarization of output stage (the other stage are in class A) the armonic distorsion decresed? 
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