[VHFcontesting] PA Question

[George Sintchak]

John,

Yes, the NF will increase but only by a very small amount, as Tom has 
previously shown...
<<<Snip<<<
> If the amp is first (and having described it as a power amp, this might
> raise some eyebrows), then:
>
>    Ft = 5 + (10-1)/100 = 5 + .09, or just over 7 db.
>
Ft = 10 log 5.09 = 7.067 dB, so there is a small increase of 0.067 dB.
Not easy to measure the difference, you certainly will not hear the 
difference!
YMMV

What's driving this question for you?
73's, George

----- Original Message ----- 
From: "John Santillo" <u1004467 at warwick.net>
To: "'George Sintchak'" <wa2vnv at optonline.net>; "'VHF Contesting Reflector'" 
<vhfcontesting at contesting.com>
Sent: Wednesday, October 24, 2012 8:16 AM
Subject: RE: [VHFcontesting] PA Question


> George et al, let me rephrase.
>
> Let's say I have a radio with a spec'd output Noise Figure of 7dB.  Now I
> add a 10dB pad to the output of the radio.  Using the Friis equation does
> the Noise Figure increase?  I understand having to linearize the dB 
> values.
>
> TNX,
>
> N2HMM
>
>
>
> -----Original Message-----
> From: VHFcontesting [mailto:vhfcontesting-bounces at contesting.com] On 
> Behalf
> Of George Sintchak
> Sent: Wednesday, October 24, 2012 12:15 AM
> To: 'VHF Contesting Reflector'
> Subject: Re: [VHFcontesting] PA Question
>
> 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
> are asking.
>
> 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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>
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