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Old 10th April 2013, 02:47 PM   #1
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Default driver emitter resistors.

What is the criteria for selecting the emitter resistors for the drivers in a simple output stage such as the "basic power amp" in the beginning of Cordell's book?
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Old 11th April 2013, 01:55 PM   #2
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anyone? I just want to know what criteria I should use fore setting the quiescent current in the driver transistors.

Same for the VAS for that matter.
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Old 11th April 2013, 02:07 PM   #3
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If you are using a resistor in each emitter tied to the output then 1xVBE/Iq=R (VBE being the base emitter on voltage of the output transistor being driven and Iq the desired idle current of the driver transistor). If your using a single resistor between the driver emitters then it is 2xVBE/Iq=R . Iq for the VAS is determined by its load.
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Old 11th April 2013, 10:55 PM   #4
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Sorry, let me re-phrase. I know how to get the current I want. I want to know how to choose that current.
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Old 11th April 2013, 11:40 PM   #5
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Lots of packaged amps on here have 20 to 40 ma per TO3 output transistor as the quiescent current; there must be a reason. Using djoffe's circuit to increase the cold idle current to 20 ma on my ST120 made it sound better, IMHO.
One further consideration for multiple output transistor amps, is that the higher the value of the emitter resistor, the better the current sharing between mismatched parallel output transistors. Or to put it another way, the less time and the fewer temperatures you have to match Vbe on the various transistors installed in parallel. The highest I have seen is 0.5 ohm on Peavey amps where the factory states that transistors can be replaced one at a time with factory selected parts. The lowest value I have seen on here is 0.1 ohm on designs tested with simulations but not sold commercially.
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Last edited by indianajo; 11th April 2013 at 11:43 PM.
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Old 12th April 2013, 12:20 AM   #6
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You probably have checked a few amplifier schematics as they appear on the forum. Most "basic" and conventional BJT amps settle for 0R22 emitter resistors and many calculations you see assume this. There are some reasons like efficiency and device mismatch already mentioned, to go lower or higher in value but given the better matching possible and likely in purchasing modern output transistors from authorised sources, there is not much reason to change from 0R22 - a good compromise.
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Old 12th April 2013, 08:24 AM   #7
sajti is offline sajti  Hungary
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Quote:
Originally Posted by Dan Moos View Post
Sorry, let me re-phrase. I know how to get the current I want. I want to know how to choose that current.
It's not easy to decide. Mainly depending by the type of the driver.
The Toshiba and Sanken types such as 4793/1837 and 5171/1930 works properly with 5-6mA, but the Motorola/Onsemi types such as MJE150xx like the higher bias up to 50-100mA.

Sajti
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Old 12th April 2013, 09:02 AM   #8
DF96 is offline DF96  England
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The correct current depends on topology; low for complementary pair, high for Darlington. The current and 'emitter' resistor value are related, as the aim is to ensure that the effective transconductance of the output pair at quiescent current is roughly equal to the transconductance of one side when the other side is switched off.

Alternatively, follow the 'first watt' brigade and use a lot of current, thus enlarging any 'crossover' distortion but also moving it away from the crossover point.
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Old 12th April 2013, 09:18 AM   #9
sajti is offline sajti  Hungary
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Quote:
Originally Posted by DF96 View Post
The correct current depends on topology; low for complementary pair, high for Darlington. The current and 'emitter' resistor value are related, as the aim is to ensure that the effective transconductance of the output pair at quiescent current is roughly equal to the transconductance of one side when the other side is switched off.

Alternatively, follow the 'first watt' brigade and use a lot of current, thus enlarging any 'crossover' distortion but also moving it away from the crossover point.
Maybe my misunderstanding, but the thread starter asked about the driver bias, not the output.

Sajti
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Old 12th April 2013, 09:34 AM   #10
DF96 is offline DF96  England
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I assumed he meant the first BJT in the output stage pair. Other people's answers seem to assume this too.
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