Broad band biasing for power broadband amplifiers

I am working on a PA design that is supposed to work upto 3 GHz. It seems that some of the designs I have come across in my study use an Murata High frequency wirewound inductor in series with an RF bead for the bias. I understand that the purpose of an RF choke is to present a large impedance and choke out the resonance, while letting the dc go. However I dont understand why an RF bead would be used in series with the large inductor.

The value of the large inductor is 880 nH, with an SRF at 200 MHz, and the RF bead is a ferrite chip bead rated at 1000 ohms, and 200 mA.

I understand that the RF bead is usually added as a series element with a choke to make a broad band choke. How exactly does it accomplish that. Can someone please explain it from first principles. I would be really grateful.

Thanks,

DK

The bead in the bia circuit is for "power integrity" . It makes sure the bias voltage going into your transistor in "clean". Bypass capacitors to GND and gerrite beads in series pretty much serves the same purpose. A sereis bead (inductor) and bypass capacitor to GND are low pass filters.
THe challenge is to select these bead and bypass capacitor values to ensure you get rid of noise on the bias line (usually small around 10 KHz or less).
Essentially in nut shell you are low pass filtering your supply voltage to get rid of noise..

for SHF the bead its self presents a high enough impedance, how ever as the frequency falls, the beads impedance falls, so more inductance is required, which in this case is another choke wound on a ferrite core.
Frank

chuckey/ashok,

Thanks for replying. If broad banding is the requirement, then why not use a cascade of inductors from SRFs in the low frequencies to high frequencies. Why use an RF bead? Could it be that the RF bead is being used to deal with the self resonance of the 880 nH inductor. The 880 nH inductor will introduce resonance at frequencies that would be multiple of its SRF and they will affect the band of operation (in my case upto 3 GHz). So it may be that the bead is there to prevent these resonances from affecting the band of operation. Am I correct?

You are correct. The ferrite bead assures no narrow-band resonances occur due to a single inductor or series combination of multiple inductors in the frequency band, where the ferrite bead is effective.