Important parameters for parts?

[Dave Lapp]

I am looking to build the MRF101 amp, am sourcing parts, and I would like to understand the important parameters for some of the parts.

Specifically the 2W 100ohm resistors used in the feedback. These are thick film and obviously rated 2W but are there other parameters for these parts that matter? There seem to be a lot of different 100ohm 1W resistors and Bourne (for example) have several types for different target applications. It isn't obvious (to me) if the one spec'ed in the BOM has a characteristic which is critical or can I sub some other cheaper part? One example is that these are 1%. Is 5% OK?

Thanks,

Dave L.

[jcve...@gmail.com]

Hi Dave,

I have certainly used many different types of 100 ohm resistors in this circuit. Using multiple low-value resistors is important because resistors also have parasitic parallel capacitance so the impedance goes down as frequency goes up. This effect is lessened in lower value resistors. 

At 100W the drain has a 100V P-P sinewave which is 35.4V RMS. If we assume the RF voltage at the gate is 0 then the 400W resistor (100 X 4) should see a bit over 3W. I chose 2 watt resistors to keep the total available dissipation over 2X the total expected dissipation. You could easily use 1.5W resistors or even 1W if the duty cycle and ambient temp are low.

I would expect that 5% resistors would work OK too.

73,

Jim WA2EUJ

[Dave Lapp]

Thanks. That's what I thought but the manufacturers (e.g. Bourne) have some otherwise equivalent looking parts with very specific target applications. Clearly some applications do have some critical parameters. I just wanted to make sure this wasn't one of those applications.

Thanks,

Dave L.