For all the years I have repaired solid state amplifiers, I have just set bias and left it alone. I have never given much thought to the quiescent bias conditions before, during or after Hot laps so to speak. So Now I am trying to understand what is acceptable Bias behavior.
new amplifier BJT outputs mounted on a very large heatsink and fan cooled. Bias set for 100ma per device (20mv across .2ohm emitter resistors) run the amp at 1/3rd power into a 4 ohm load for 15 minutes. the heatsinks get hot, but not so hot you can't hold your hand on them. turn off the input signal. the bias immediately drops down to about 160ma per device and slowly starts to drop as the heatsinks cool. eventually getting back down to 100ma after 10-15 minutes etc.
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Is That acceptable behavior? or should the bias immediately, or quickly, drop down the the 100ma bias setting? What is an acceptable/normal Bias hysteresis after a hot event? what should I be seeing?
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the amp seems to be stable otherwise. power on from cold takes about 10-15 minutes for the bias to come up to 100ma and stabilize. and is repeatable. will idle for hours on end just fine. and seems to be ok after a hot run, let it sit and it settles back down, idles ok, no signs of thermal runaway.
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I have just never considered what happens after running an amp hard, never thought that the bias would climb with heatsink temps etc. just has never been something I have thought through. I consulted all my books, Self, Cordell, Duncan, etc etc etc and I see mentions of the Thermal hysteresis, but nothing that talks about what normal is.
new amplifier BJT outputs mounted on a very large heatsink and fan cooled. Bias set for 100ma per device (20mv across .2ohm emitter resistors) run the amp at 1/3rd power into a 4 ohm load for 15 minutes. the heatsinks get hot, but not so hot you can't hold your hand on them. turn off the input signal. the bias immediately drops down to about 160ma per device and slowly starts to drop as the heatsinks cool. eventually getting back down to 100ma after 10-15 minutes etc.
.
Is That acceptable behavior? or should the bias immediately, or quickly, drop down the the 100ma bias setting? What is an acceptable/normal Bias hysteresis after a hot event? what should I be seeing?
.
the amp seems to be stable otherwise. power on from cold takes about 10-15 minutes for the bias to come up to 100ma and stabilize. and is repeatable. will idle for hours on end just fine. and seems to be ok after a hot run, let it sit and it settles back down, idles ok, no signs of thermal runaway.
.
I have just never considered what happens after running an amp hard, never thought that the bias would climb with heatsink temps etc. just has never been something I have thought through. I consulted all my books, Self, Cordell, Duncan, etc etc etc and I see mentions of the Thermal hysteresis, but nothing that talks about what normal is.
@jan.didden looked into that years ago and wrote an article in Linear Audio about it. I just avoid the whole issue by using a class-AB bias loop with transistor arrays.
Jan Didden, "Thermal transient variation of power amp quiescent current - Instrumentation and findings", Linear Audio vol. 9, pages 171...187, 1 April 2015
He measured variations of a few milliamps around a 60 mA average, but his article is about short bursts of audio, rather than 15 minutes at 1/3 power.
Jan Didden, "Thermal transient variation of power amp quiescent current - Instrumentation and findings", Linear Audio vol. 9, pages 171...187, 1 April 2015
He measured variations of a few milliamps around a 60 mA average, but his article is about short bursts of audio, rather than 15 minutes at 1/3 power.
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For a class AB amplifier with 0.2 ohm emitter resistors, the bias current going from 100mA at idle to 160mA at maximum dissipation is very good thermal design.
Bad design would be the bias current exceeding ~0.3A or falling below 100mA.
All of this comes from Vbe decreasing by 2mV/C. A small temperature difference can swamp the nominal 20mV drop.
The heatsinks have a thermal time constant of several minutes. The transistor dies have short time constants. For example, when my PC's CPU goes from 100% to idle, the CPU die temperature drops by half immediately, and then returns to idle temperature over the course of a minute or so.
Ed
Bad design would be the bias current exceeding ~0.3A or falling below 100mA.
All of this comes from Vbe decreasing by 2mV/C. A small temperature difference can swamp the nominal 20mV drop.
The heatsinks have a thermal time constant of several minutes. The transistor dies have short time constants. For example, when my PC's CPU goes from 100% to idle, the CPU die temperature drops by half immediately, and then returns to idle temperature over the course of a minute or so.
Ed
You need to do the analysis, but 200mA is probably >2x away from the point at which the amplifier blows up.
Ed