What does Unregulated AB bias mean?

[Onelasttime]

Ok I understand the different bias scheme's and most of the reason behind each one. I heard someone mention "Unregulated AB Bias" on a new 12 transistor amp they had built. The two guys where where either ignoring everyone or could not hear them. Go figure? I did try to break in and ask. I googled it when I got home and could not find anything on it. I tried the search function on this sight but had no luck.

So does unbiased AB just equal really poorly biased AB using cheap sandbar's or is it really B biasing with a bit of current always applied at low levels to nudge B closer to AB etc??? This was on SSB so I am just trying to figure out why anyone would do that on purpose and what the pro's and con's are. Can this be used only on push pull designs? What are the harmonics like on this sort of setup? I am guessing they are better than class c but still full of flywheel effect?

Correct me if things have changed since I had electronics in high school but I was taught that some tube amps could easily maintain -30db in class B gronded grid on SSB but most could not and that transistors amp should never be used in this way for anything other than CW and FM. Does it still hold true that class C and B are splater boxes waiting to happen and that neither should ever be used for SSB? Does this still hold true or was I just taught wrong. That or maybe I am just remembering things wrong I am that ham fisted DIY that knows enough to be dangerous but smart enough to let a pro do it when I get in over my head. LOL Praise God I have not been electrocuted since I was 8 or 9 and tried to fish some stuck toast out of the toaster with out unplugging it!! LOL I have had a healthy respect for electricity since that day! Well that is not entirely true a mean set of bad spark plug wires has zapped me a few times! LOL

Thanks for explaining it to me!

 

[543_Dallas]

Most CB amps that are biased are unregulated. Texas star being one if them. You've got the right idea with the big ceramic resistor because that's how it works. Bias current is set with the amplifier manually keyed and no drive. The hotter it gets and the harder its driven makes it drift towards class C.

The reason it's font this was is because it's a cheap and easy way to make an amplifier play well on SSB. It keeps the transistors from switching on and off between words and sounding like a class C amplifier on SSB.

 

[Captain Kilowatt]

Unregulated bias can also mean that the bias voltage is derived from an unregulated source. As the amp draws current this voltage supply swings up and down with current. This leads to a constantly changing bias voltage which causes the amp to operate somewhere between the different classes. It may be bad enouigh to swing all the way from class AB to class C. It is best to regulate the bias voltage before applying it too the amp.

 

[bob85]

For true AB biasing on bipolar amplifiers the bias supply must have a low source impedance able to deliver as much current as the devices been biased need at maximum drive level,

for instance if your transistor is operating @10db gain & full ouput pulls 10amp of collector current from your psu the bias supply should be able to provide 1amp or more without folding back the bias voltage,

the only way I know of doing that with good regulation is using an actively regulated bias supply,
just like the circuit in your linear type psu but setup for low voltage,

The other methods even when using a regulated supply for the bias divider & clamping diode vary the bias voltage with drive level even though the bias supply from the regulator is stable,

the worst of all are the simple unregulated supply feeding a divider like Texas star or xforce,
active regulated bias supplys allow a lower idle current while still remaining in class at all drive levels,

Bipolar output transistors and bias switching transistors conduct harder the hotter they get causing idle current to go up with increased temperature,

a good bias circuit not only keeps bias voltage stable under all drive levels it compensates for the increased conduction with temperature by reducing the bias voltage proportional to temperature keeping the output devices much closer to the correct idle current.