Leach Amp different emitor resistors

[Steven]

With reference to John Curl:
http://www.diyaudio.com/forums/showt...369#post244369

This is derived from an article published by HP in about 1971. This is the best area of operation for lowest distortion in the transition between class A and AB.

I just digged up the paper describing the 13-26mV voltage drop across the emitter resistors. It is an article in Hewlett Packard Journal, february 1971, Volume 22, Number 6, page 11-16, written by Barney Oliver (actually Dr. Bernard M. Oliver), at that moment HP's vice president for research and development. In this article he describes the two principle types of distortion in class B amplifiers, difference in current gain of the two transistors on one hand and crossover distortion on the other. It is derived that the output resistance of the amplifier is most constant in the crossover region, when g.R =1 with R is the emitter resistor and g is the transconductance at the operating point (in case of voltage drive). This is easy to see. If only one of the transistors is conducting (class B region) the output resistance would be R because 1/g is very small (large current), either from the NPN or from the PNP. With no drive (class A region), the output resistance would be (1/g+R)//(1/g+R)=R for 1/g=R, or g.R=1. The output resistance of NPN and PNP are equal and in parallel. But the output resistance is not constant, there is a bump in between these regions. This bump dissapears for a smaller bias current; when g.R=1/2 the output resistance falls motonically from 2R to R, if driven harder. So, the optimum R is between 1/2g and 1/g. This can be translated to a voltage across the emitter resistor in the range of kT/q ... kT/2q, which is 26mV ... 13mV at room temperature.
Mr. Oliver points out that this is a very small value compared to the Vbe change of 250mV if the temperature changes from 0 to 100 degr C, making thermal tracking of the biasing diodes a potential problem. At that moment he suggests to make R >> 1/g for thermal stability and rely on negative feedback to reduce the resulting distortion. I think this is a pity because of the negative aspects of negative feedback related to the increasing amount of higher order harmonics in the distortion compared to the lower order harmonics. Anyway, to avoid wasting too much power in the (large) emitter resitors he suggests to bypass these with diodes. He did the same for his "The Barney Oliver Audio Amplifier", where the emitter resistors are 8.2 Ohm (!), bypassed by a diode.

Around the same time others have made similar calculations or simulations, like Blomley in Wireless World (also february 1971! ) and later Self.

Steven

[dimitri]

Steven,
What a nice reference. Could you please mention exact paper title?

[mlloyd1]

author = "B. M. Oliver",
title = "Distortion in complementary-pair class-{B}
amplifiers",
journal = j-HEWLETT-PACKARD-J,
volume = "22",
number = "6",
pages = "11--16",
month = feb,
year = "1971",
CODEN = "HPJOAX",
ISSN = "0018-1153",
bibdate = "Tue Mar 25 14:12:15 MST 1997",
acknowledgement = ack-nhfb,
classcodes = "B1220 (Amplifiers)",
keywords = "amplifiers; complementary pair class B amplifiers;
crossover distortion; difference distortion; electric
distortion; feedback distortion suppression; negative;
transistor beta",
treatment = "P Practical; T Theoretical or Mathematical",
from http://www.math.utah.edu/pub/tex/bib...liver:1971:DCC

mlloyd1

[Borc]

Thanks Steven, that was what i need.

[john curl]

Thanks Steven, I lost the reference in the firestorm around here. That is the original article that I learned it from.

[john curl]

That Japanese patent should be voided. I learned about this in 1971 from Dr. RG Meyer at UC Berkeley.

[Luke]

Im planning a Leach with 12 transistors using +/- 80V rails.
I have massive sinks so I would like to bias into Class A, I can dissipate around 300W per channel.
Im a little confused because I have read that you dont want to go over 50mV across the emmitter resistor, and that you dont want to go as low as 0.2R in high bias for stability reasons, but I cant see how it could possible work.
Heres a couple of options, which is the best way to proceed?

v across resistor resistor bias x 6 total idle watts
0.1 0.33 .818181818 290.9090909
0.065 0.20 1.909090909 312


the spacing disapears for some reason, if I use:

0.20 ohm resistors I get 312 watts with a Vre of 0.65V
0.33 ohm resistors I get 290 watts with a vre 0f 0.1V

[Sebastiaan]

Dear,

This golden "25mV rule" Do you mean 25mV over each emitter resistor? or 25mV total over both NPN and PNP emitter resistors? (means 12,5mV over each emitter resistor duh )

In this case using 0.1R emitter resistors is in most situations a no go in a class B amplifier, since you need a lot of idle current to reach the 25mV value.

If memory serves me well, the Parasound JC1 has 0.1R emitter resistors.

With kind regards,
Bas

[AndrewT]

The 25mV rule applies to an optimally biased ClassAB complementary EF output stage.
It does not apply to CFP. It does not apply to Quasi. It does not apply to ClassA, It does not apply to single ended.

The 25mV rule is actually 26mV less a correction for the effective internal impedance of the output device.
As Re goes down so does the Vre. eg. Re=0r47 Vre~24 to 26mV, Re=0r1 Vre~18 to 20mV.

You cannot bias a +-80Vsupply amplifier into ClassA for conventional speakers.

Let's look at the numbers for an 8ohm speaker.
Vpk ~70V.
Ipk~ 70/8= 8.75A.
total Iq >= 8.75/2 ~4.4A.
Total power dissipated in the output devices is 80* 2 * 4.4 = 704W.
Dissipation per device is ~59W.
What size of heatsink would this need?
What device could stay within it's temperature de-rated SOAR while Pq is 59W?
What amp like this could survive a 4ohm speaker load?
Could any amp like this survive a 1r3 test load?

[satoru]

For who may be interested, the link for downloading a PDF copy of the the Oliver paper is as follows. Thanks Steven and mlloyd1, for posting the complete journal info!

http://www.hpl.hp.com/hpjournal/pdfs...Fs/1971-02.pdf

Regards,
Satoru