# [VHFcontesting] PA Question

George Sintchak wa2vnv at optonline.net
Wed Oct 24 00:15:07 EDT 2012

```Where is the pad added - input or output?
You don't say what the gain of the amplifier is.

Case 1.  If you put the 10dB pad on the input of an amplifier, the Noise
Figure becomes 17 dB,  (10+7) and the overall gain is reduced by 10 dB.
If you put the pad on the output of the preamplifier, it also reduces the
gain of the preamp by 10 dB, and the Noise Figure will also be reduced
(somewhat) depending on the gain of the amplifier.

Case 2.  With the pad on the output, let's assume the amplifier gain is 20
dB.
The Ft = 7 + (10-1)/20 or 7+ 0.45 = 7.45 dB NF
The Friss equation is applied for (usually) amplifiers in series, showing
that the overall NF is dominated by the 1st stage NF (and gain) as long as
it has resonable (high) gain. You need to be more specific about what you

73's, George, WA2VNV
HF+50-1296, SOLP, FN30kv

----- Original Message -----
From: "John Santillo" <u1004467 at warwick.net>
To: "'VHF Contesting Reflector'" <vhfcontesting at contesting.com>
Sent: Tuesday, October 23, 2012 9:39 PM
Subject: [VHFcontesting] PA Question

> Hello,
>
>
>
> I have a PA Noise Figure Question.
>
>
>
> If a power amplifier has a Noise Figure of say 7dB and a series pad of
> 10dB
> is added does the Friss Equation hold true for calculating the increased
> Noise Figure?  For example, using the equation below I calculated a Noise
> Figure of just over 19dB with the pad inline.  If this is the case, can
> the
> carrier coming out of the PA be overwhelmed by the noise?
>
>
>
> Is the 7dB Noise Figure really the noise out of an amplifier with the
> input
> terminated above kTB?
>
>
>
>
>
> Ft = F1 + ( F2-1) / G1
>
>
>
>
>
> Thanks in advance,
>
>
>
> 73,
>
>
>
> John
>
> N2HMM
>
>
>
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> VHFcontesting mailing list
> VHFcontesting at contesting.com
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```