23cm Amplifier SBB2088Z - More Navel Gazing

It occurred to me some time after I had prepared the PCB layout that I had left out the small input "stability" resistor in the transmission line version. Since the minimum resistance needed to ensure stability was 1 ohm I wasn't too concerned. Just putting 1 ohm into the actual circuit wouldn't change much would it?

Well, it turns out it did.I started from scratch and derived a circuit with a 1 ohm resistor and transmission line matching. I found that the capacitors adjacent to the 50 ohm ports were reduced by around 50%. And the transmission lines were around 20% different in length. Now 1 ohm is a really small amount of resistance and these are significant changes.

So I inserted a 1 ohm resistor into the original transmission line version and compared the two.

While there is a small variation in gain, all versions will be substantially the same in practice. Should I be concerned that 1 ohm made such a difference?

I put the difference circuits giving almost the same result as comforting in that the matching will be smooth and relatively insensitive. I look forward to receiving the boards to find out if this is indeed the case.

73's
Richard

23cm Amplifier Musings - SBB2089Z

While I got myself organized to order PCB's for testing the SXB4089 mentioned in the last few posts, I thought it would be worth looking at some of the other devices. The first of these I turned my attention to was the SBB2089Z, a 100mW device.

What I found, if you study the large image below, was that the bare device was acceptable for 6m through 70cm. However, at 23cm the output was not as good a match to 50 ohms as I had hoped (RL= 8dB ).

As expected, matching to 50 ohms at 23cm introduces loss of gain at other frequencies. There was also some slight chance of instability hence the series resistor in the second version to prevent these.

I'm going to get two boards fabricated for testing: one for 6-70cm using just the MMIC, and one for 23cm using transmission lines.


73's
Richard