Common base amplifier.

[dmowziz]

Hii...

This amplifier isn't working

Please what can be wrong?
I attached the schematic, and AC-Equivalent schematic as well

Expecting about 9dB gain but the prototype is not amplifying


Attached a pic of the board.

Thank you very much for any suggestion

[Herald1360]

What's the stray capacitance of L2 100mH(?!) like? Could it be shorting collector to ground at signal frequencies of interest?

[Radio Wrangler]

There are a few things there that make it a good candidate for oscillation.... both at the SRFs of those chokes and also at UHF with track inductances.

David

[dmowziz]

Originally Posted by Herald1360

What's the stray capacitance of L2 100mH(?!) like?

Hmmm... Thanks
I don't know.. Please how can I measure it?

and it is 2.2mH not 100mH. sorry

I will change to another choke soon and see
Thanks


@David... If it is oscillating at a higher frequency, will it affect amplification at desired frequency?


88 - 108 MHz Amplifier

[dmowziz]

Originally Posted by Herald1360

Could it be shorting collector to ground at signal frequencies of interest?

Thanks thanks.
That inductor was the problem. I used a toroid at hand, about 8.6 uH


Please how can I measure the stray capacitance you mentioned?

[Radio Wrangler]

The stray capacitance is distributed. From each part of each turn to each other part. It isn't directly measurable but has to be inferred from the self-resonant frequencies of the choke. There will be multiple resonances, so there are multiple valid values of stray capacitance. It quickly gets too complicated for everyday use, so while stray-C is spoken about, people use the lowest SRF, and are then careful to never try to use that inductor at anything approaching that frequency.

There are models for inductors for all the usual RF CAD packages, some are more comprehensive than others, but none are fully comprehensive. Inductor manufacturers supply data for some of these models, but again, it's not fully comprehensive and only models the part in its easy band, up to the first resonance.

Oh, and if it's oscillating at high frequency, then it can affect low frequency performance and even DC bias conditions. Oscillations run at high enough level for non-linearities to limit the amplitude. That non-linearity gives self rectification and that affects bias. The effect may be large or may be too small to notice. The oscillation can use up an appreciable portion of the device's power capability, voltage swing etc and the non-linearity gives intermodulation with the wanted signals. However, sometimes effects on low frequency signals and bias can't be relied upon to show that there is no oscillation. You can get instability triggered only when a signal drives the output a bit away from the quiescent condition, and the oscillation can be a short burst. Look with a spectrum analyser and you may see the spurious oscillation, but the power level looks low. This seems to counter the statement that oscillations drive themselves into non-linearity of some sort. Don't worry, the oscillation is large, but only for a short time and so the average power is what you measure. If you go looking for it, it's less obvious than it ought to be, but the effects are still there when you try to make a clean receiver or transmitter.

David