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Biasing power transistors

Discussion in 'Amplifiers' started by 9Lives, Mar 27, 2013.

  1. 9Lives

    9Lives Active Member

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    Hi everyone. I'm wanting to do something, I'm having trouble simulating due to simulator imperfections. I want to design a discrete 2 transistor regulator like this:
    [​IMG]



    Problem is when I plug it into the simulator with a 50 ohm load the "regulating" part of the circuit doesn't show voltage spikes and dips.

    Can I use a circuit like this to supply current even though it's a voltage regulator? Or is there better designs for this?

    If I use a Darlington/heat sink transistor will it supply bias current to our favorite rf transistors?
     
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  2. 9Lives

    9Lives Active Member

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    Bump,

    No one can help me? When under load will this stabilize in class ab?
     
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  3. KAP68

    KAP68 Member

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    Just curious, what simulator are you using? I use National Instruments MultiSim.
     
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  4. 9Lives

    9Lives Active Member

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    I use pspice but I just found a bad ass one for android called everycircuit. There's a free trial but the whole version is worth it. You can change characteristics and values. And has 4 probes scope view
     
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  5. 9Lives

    9Lives Active Member

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    First attempt. I used "amplified diode current". It's where you use a resistor from v+ to the base of a Darlington and 2 series diodes cathode to gnd. Instead of grounding the emitter a capacitor is used. The emitter is then tied to the base of the rf transistors. I attempted to have it switching with the relay.

    I wouldn't say fail but, when the circuit was by itself and a 50 ohm load resistor (this maybe my problem) I had .6 v. Perfect!

    In circuit I only got .25. Im guessing this is bc of impedance and drive. Coupling capacitor on input of the amp??
     
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  6. High Voltage Mobile NJ

    High Voltage Mobile NJ BANNED

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    im not sure what your trying to build , but the voltage is the applied voltage always , there is no current flowing through it . current drain is in the bias of the transistors. use shockey diodes cause they do not have any voltage drop . is your input unregulated ? and your output is regulated ?
     
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  7. Shockwave

    Shockwave Sr. Member

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    The problem is the resistor value is too large and will not support the current to bias the transistor. What type of transistor and how many are you using? Most transistors require between 25 and 100 ma of bias current. You should not just adjust the bias voltage to .6 and assume the idling current is right.

    The value of the resistor in your bias circuit should be adjusted to provide the specified collector current with no drive. You'll have to install a current meter inline with the DC feed to the power amp to measure this. You may have to key the relay or apply bias manually since you do not want to apply RF drive to key the amp.

    Be careful when you adjust the bias. Too much and you can burn open the final transistors very quickly. Sometimes just .1 volts can make or break the alignment. That's why you must measure current and not voltage for this application.
     
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  8. High Voltage Mobile NJ

    High Voltage Mobile NJ BANNED

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    I have to pull a data sheet on those transisters and find out more about that circuit.
     
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  9. bob85

    bob85 Supporting Member

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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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