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Biasing power transistors

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Old 04-06-2013, 07:05 PM
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i agree with shockwave,

the bias voltage you need to apply in order to obtain the correct idle current varies with the transistors you are biasing, not just different types eg 1446/2290/2879 but from transistor to transistor of the same type,
closely matched transistors will need close to the same bias voltage and current from the bias supply,
small changes in bias voltage can cause big changes in idle current,

for best linearity the bias circuit should be capable of keeping the bias voltage stable under different drive levels,
a bias supply that sags as you increase drive is a poor bias circuit,

the minimum current the bias supply needs to be able to provide without voltage sag is the maximum current draw for the transistor at full drive divided by the current gain of the transistor at full drive level,

lets say you are biasing a transistor that draws 10amps at full output and operates with a gain of 10,
10amp/10 = a bias supply that will supply a minimum of 1amp of stable bias voltage,
anything less will give you backswinging bias on peaks & degrade linearity,

when you get the voltage stable under load you should add thermal tracking so that the bias voltage adjusts downwards as the transistor temperature increases,
when you have the tracking curve right the idle current should stay fairly stable with changes in transistor temperature,

we find many crude/simple bias circuits in cb amps that do work to mask the switching distortion from the human ear but they are far from ideal.

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