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Old 12th January 2012, 03:34 PM   #1
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Default bias stabilizer

often there is a capacitor across the bias transistor often called a bias stabilizer ....

i have seen some 0.1mfd film type caps and sometimes electrolytics even of 100mfd

anybody cares to explain some around this issue ???

biased regards
sakis
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Old 12th January 2012, 04:17 PM   #2
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Primarily it lowers the impedance around the bias transistor so that rail sag doesnt influence the bias current much. Higher value is better but no need to go overboard, 10 to 100 uf is perfect. Other way of better results is use of darlington, cfp or super beta transistor. These bring even more improvement but a added advantage of super beta is higher temperature sensitivity and faster response.

Another way to lower the impedance was brought to the forums attention by Keatoken, instead of a cap accross emitter collector use one accross base collector of the transistor (parrallel with top voltage divider resistor). In the case of impedance its the best performing.
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Old 12th January 2012, 05:37 PM   #3
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I think that the location and valu of this capacitor will also alter the amplifiers FFT spectrum.
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Old 12th January 2012, 07:29 PM   #4
Elvee is offline Elvee  Belgium
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One of the problem with this cap, particularly when it is large value, is its inverse peak detection effect: at higher power levels, the transistors bases steal more of the charging current, and as a result, the voltage across it decreases, because of the non-zero impedance of the bias network.
And the reason why the capacitor was installed in the first place happens to be this non-zero impedance. See?
That's one of the reasons to opt for a good automatic bias. Class i thread is one of the examples of what can be achieved using these techniques.
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Old 13th January 2012, 12:46 AM   #5
ontoaba is offline ontoaba  Indonesia
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It help prevent asymetrical load of OPS most in push pull VAS. This one has ~200mA biased, but without that caps, crossover distortion appeared in HF operation because the differences of irf540 with irf9540.
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