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How to cascade BGA2869 amps

Question asked by Dave Rajnauth on Apr 20, 2017
Latest reply on Jun 20, 2017 by Heiko Leutbecher

I need help with BGA2869 MMIC amps.  I'm trying to cascade two amps for a 7 Mhz HF application to get more gain and running into a brick wall. Each amp works great on its own.  Tested with input of -107dbm, -73 dbm and -33 dbm from a 50R source. I get at least 28dB gain for each amp connected to 5V supply.  Each amp input/output is decoupled using 100nf caps.  The current draw to each amp is around 30mA.

 

I've created a PCB following power supply, ground plane and decoupling recommendations.  However, when I interconnect amps together (using  100nf coupling capacitor and a straight trace between input and output of respective amps) I run into problems.  On this PCB if I remove and bypass one amp and test the other I get the results noted above.

 

First I don't get the expected gain (i.e. 28db + 28 db). I measure less than 20db of gain for both amps.  Second, the current draw for the second amp drops to almost half of the spec (about 15mA) and the first amp is consuming about 30 mA.  Third, the noise floor comes up substantially more that for one single amp.  Would expect this if both amps were operating but based on current draw the second amp does not seem to be working well.  Again this ONLY happens when the amps are connected together. 

 

I have a 7805 regulator that is rated at 1 amp of current and i have various caps (100uf, 1uf, and .1uf) to control power supply noise from 7805 and power supply.  Also have a 470pf cap 7.4 mm from Vcc pin as recommended on data sheet.

 

Is there anything special that is needed to cascade these amps?

 

First time using MMIC amps and I'm lost.   Any help would be greatly appreciated.

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