Biasing for different classes

[demons_wing]

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

[Nelson Pass]

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 push-pull 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.

[demons_wing]

thank you nelson..i really appreciate the help

[Danyele82]

A "normal" class AB output stage (like tihs ) with about 4A (@ +-30v) of bias current can "give" 120w in class A into 4 ohm?
And into 2 ohm 60w in class A and about 200w in class AB???

[Nelson Pass]

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)

[azira]

Originally posted by demons_wing
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

There is a lot of touchiness to the classes as I found out in a post I made a while back.

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

[sreten]

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.

[azira]

Originally posted by sreten
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.

I refernce Sedra Smith as an authority, sreten references Self for validity, equally qualified authors, equally qualified definitions...

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 (8-ohm 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

[slowhands]

Originally posted by azira


...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

Let's back up here. These output stage class definitions were made in the tube era and remain unchanged today. Class A means simply that current always flows through an output device all the time during the output waveform. Class B implies a push pull amp of some sort, and the output devices each are on for either the positive or negative output polarity only; thus with a sine wave without offset, each output device would be on 50% of the time. Class AB is a hybrid, with the devices simultaneously on for significantly greater than 50% of that same sine wave. These definitions are true regardless of the amplification device -- transistor, mosfet, tube or whatever.

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 100-200 ma per device.

There may be unique circuit characteristics which call for different bias currents, but those are reasonable starting points.

[Danyele82]

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