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Hello. I have a small RF amplifier that needs 9V.I have a 13.8V power supply. Should i use a 4.7V zener between to drop voltage at about 9.1V or just use a 9V regulator?Which one will be better in terms of lower ripple and noise?
For lower frequencies a regulator is better, but for high frequency noise rejection the regulator or Zener is not too good, those have some bypass capacitance from the input to output. Use a simple resistor for filtering with some capacitance.
If the 50/60 or 100/120Hz ripple is high, that doesn't matter too much at an RF amplifier, just use smaller coupling capacitors to filter out the lower frequencies and bigger supply filter capacitors.
Oh, and for an RF amplifier actually it is a bad idea to put 220Ohm resistor in series with the base of the transistor. The point of an RF transistor is the small values of the parasitic resistors and capacitors which can increase the thermal noise and decrease the bandwidth. Series resistors ruin the bandwidth and noise figure.
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