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Re: Re: DEM Pre-Amp



That seems awfully odd, since Vdd for the GaAsFET comes from a 7805 voltage
regulator--allegedly 5V solid output for inputs 7.5-16V (right off the IC
spec sheet, hence the LNA spec).
The 5V is dropped thru a handful of resistors to 2V actually on the device
drain.  So that's why there's no NF vs. supply voltage; the supply voltage
is constant.

Since the problem is happening right when the 7805 is turning on (7.5V),
maybe the 7805 is going into oscillation; that could happen if the
capacitors across the 7805 input and output were defective/missing/poor
quality.

If the 7805 actually were oscillating I wouldn't have a good feeling about
the GaAsFET, whose Vdd,max is 5V.

It's unlikely the pot for the gate voltage was unadjusted; it would have to
be set for the specified drain current.

Anyway, some ideas...


At 10:06 PM 7/27/01 -0500, ken durham wrote:
>My .7db NF preamp arrived several days ago and just got put in the
>system with the converted Drake (spec 3 db NF.) It seems to have a
>problem or is causing a problem. It increases the S meter reading
>from S1.5 to about S3, but the s/n ratio seems worse. It seems to
>work only at about 7.5 V for a decent gain and NF.  S meter increases
>to  >S4 at this supply voltage and the NF improvement is obvious
>listening to the AO-40 beacon. My 12V supply isn't useable
>for this preamp. The spec sheet says that the supply range is
>7 to 16 V. The spec doesn't have a curve for NF or gain vs supply
>voltage.
>Does anyone know if this is typical of this preamp (model 13ULNA)?
>I sure hate to send it back since it took so long for them to ship
>and it's up on my tower now.  I'm tempted to just use a 7.5V supply
>and keep the preamp. The schematic shows a variable resistor from the
>IC that generates the internal negative voltage to ground and the wiper
>to the gate of the FET.  It may be that this pot just didn't get
>adjusted
>at the factory to a proper voltage for the gate. I don't want to take
>a chance on blowing the FET by trying to adjust  this for optimum
>gain and s/n ratio. Any suggestions?
>
>
>Ken   K5MBV
>
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>
------------
Scott Townley		
nx7u@arrl.net
Gilbert, AZ  DM43
     
HF/VHF/UHF/Satellite MOBILE station
http://www.primenet.com/~nx7u

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