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Old 25th September 2003, 05:08 AM   #1
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Default Class AB biasing...

I've biased the amps I built by the first transistor. I used a 1M resistor on the center pin of a pot that went from 0v to 1.2v to bias the entire chain of transistors on each side. My past amps have used capacative coupling for the inputs. The amp I'm building will use no capacitors except for rail stiffening purposes. So in the transistor chain in this amp, biasing it will be very tricky since the current gain per side will be well over 16000x. I thought about using two pots, one to adjust and the other for fine adjust. Is there a better way? I haven't built it yet but I can tell it's going to be REALLY sensitive and prone to blowing up if I don't bias it perfectly.
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Old 25th September 2003, 06:18 AM   #2
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As temperature of power transistor rise, Vbe of them reduce by coefficient of -2mV/C. Less Vbe means more collector current at same Vbias. You need to make your biasing circuit temperature dependent (usually small transistor placed at the same heatsink as power transistors, and you also need Vbe multiplier), otherwise you can expect thermal runaway of output transistors or worse - second breakdown in them.
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Old 25th September 2003, 03:49 PM   #3
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So you're saying as the transistor gets hotter, the voltage between base and emitter gets smaller? Is this a constant across all silicon devices, diodes also?
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Old 25th September 2003, 04:36 PM   #4
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Yes, this is constant for BJT transistors and for diodes, for silicon the coefficient is -2.2mVC, it means that transistor heated from 30C to 50C will have Vbe voltage lower of 44mV (compared to Vbe at 30C). For germanium (not used now) this coefficient is different. For Field Effect Transistors (FETs) this is not true, they have different temperature behavior.
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Old 25th September 2003, 04:51 PM   #5
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You can see biasing technique in the following image - it is Douglas Self's amplifier. The biasing is performed by TR13 transistor:
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Old 26th September 2003, 08:38 PM   #6
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So if I'm biasing a NPN grounded transistor using the + rail and a resistor, is there any need to correct for temperature since there is already 20+ v across the resistor? A few mV over that range shouldn't make a difference. What I'm asking is, is it normal for amps to be biased using the first transistor in the chain?
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Old 1st October 2003, 08:41 AM   #7
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How about posting the circuit for us. It's difficult to picture what you're describing exactly. It sounds like you're attempting to bias the entire amps operating point from one pot. If so this is not a good idea.

The o/p stage generally requires a separate temp-compensated bias arrangement as explained by PMA. With certain types of MOSFETS this may be unnecessary, eg J50/K135 types and similar.
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Old 1st October 2003, 02:35 PM   #8
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Well I'm trying to bias it with two pots, one for the + side one for the - side. Do you suggest additional pots for a fine adjust?
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