Question about Bias in RF
I want to bias my transistor by using an large inductor. At the same time, I want to shunt a capacitor at power supply. How should I choose the value of inductor and capacitor, at the mean time I can filter the noise?
I'm also confused about the noise. Is the noise come from power supply or from the input?
Thanks.
Noise comes from everywhere. Figuring the incoming
amplitude and path gain is key to RF design and the
figuring of noise factor etc. (as far as I can tell, by
watching). Inductor will clean HF noise but often it's
the close-in noise and near-DC noise that is the problem
(mix-tones from LF noise, are in-band while mix-tones
from HF noise may be out-of-band and more easily
trapped). Your supply caps should have a SRF that
is well above the operating frequency.
A transistor bias takes such small current level that you can expect to need a large expensive inductor. Therefore in this case you might prefer to choose a gyrator (active inductor) using an op amp and capacitor. Or just a capacitor because there are situations where a capacitor filter does a similar job as an inductor choke.
Also look into a 'capacitance multiplier' on the chance it could do what you want.
I assume you are looking to suppress the noise that comes on the DC bias lines.
You didn't mention if is about a low power, medium power, or high power circuit.
If is about a low power circuit is enough to use RC filter on the drain/collector bias and also an RC filter on the gate/base bias supply. This would give an efficient wideband noise filtering.
If is about a power amplifier things are a bit more complicated, because any resistors placed on the above lines will affect the amplifier linearity, so you have to use a combination of RLC filters.