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Old 22nd Oct 2018, 8:47 pm   #21
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: 0.1-2000 MHz Wide Band Amplifier ~£4

It's going to be fairly noisy at the output anyway because of the 30dB gain but yes, it might not be obvious if it is hooting up at a GHz or more.

However, the board must be fairly stable most of the time or they wouldn't be selling them and it does look like someone has tried to add lots of vias but with this type of device I'm not sure if it's enough to preserve unconditional stability, especially with a 1.5mm thick PCB.

I recall about 25+ years ago Avantek brought out a series of low noise, high gain MMICs in SOT-86 and I think they were marketed as 'Magic Modamps' and they used an extra gain stage to produce high gain, high bandwidth performance like this. On paper they almost looked too good to be true as they claimed unconditional stability for these parts as well as low noise and high/flat gain over a huge bandwidth.

These MMICs used the familiar darlington stage but they also had an extra (high gain!) grounded emitter stage driving the darlington. I think the problem with these old MMICs was that both stages had to share the common ground pins of the SOT-86 package and this meant that the design was a bit marginal unless the device was grounded almost perfectly when mounted on a PCB. I suspect that the device on your PCB will be doing something similar internally. It's unlikely to contain two darlington stages in series as that would probably be even worse in terms of stability if both stages shared the common ground pins of the SOT-86 package.

The best thing to do is play with it and seen what it does and if it behaves itself. It's amazing how the sellers can buy the bits and the PCB and build it and then ship it to the UK for just £4!
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Regards, Jeremy G0HZU

Last edited by G0HZU_JMR; 22nd Oct 2018 at 8:57 pm.
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