[time-nuts] OP-Amps for 10MHz distribution...?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Feb 28 19:02:26 UTC 2012

A lower limit to the phase noise floor of an opamp can be estimated from 
its output noise and the output signal level.
Half of the output noise power contributes to PM, the other half to AM.

At low offsets modulation of internal capacitances by power supply noise 
and opamp noise modulates the phase shift produced by the opamp.
Such modulation occurs even when the amplifier isnt driven into saturation.
The same PM mechanisms occur in discrete amplifiers.
Keeping the dc gain low is helpful in reducing phase noise.
This doesnt mean AC coupling is a useful solution it isnt as a high dc 
gain from an input base of a BJT to its collector will produce 
significant modulation of its collector base capacitance and the phase 
shift of the amplifier. Shunting output load resistors with inductors 
can be useful in reducing such phase modulation, merely using a 
capacitor to couple the output to the next stage has no effect on close 
in phase noise produced by the amplified input noise (or power supply 
noise - e.g. via a bias divider).

Discrete JFETs tend to be somewhat noisier than BJTs for low impedance 
sources so using a JFET input device isnt usually as effective as using 
a BJT at least for 50 ohm sources.


Bill Fuqua wrote:
>   Discrete amplifiers are always less noisy than integrated amplifiers.
> If you want really low noise design a one with JFETs and Bipolar 
> transistors.
> I am trying to understand the contribution to phase noise by the opamps.
> Perhaps the "threshold" is shifting and amplifier is being driven to 
> saturation?
> I am new to this group but have had lots of RF experience and weak signal
> detection experience.
> 73
> Bill wa4lav
>> Date: Tue, 28 Feb 2012 07:34:09 -0500
>> From: Bob Camp <lists at rtty.us>
>> Hi
>> Very cool. How much power can you run through the device? Put another 
>> way, if you drive it with +13 dbm do all the numbers get 5 db better?
>> I doubt very many of us will be worrying about weather it's below 
>> -153 at 10 Hz or not?
>> Bob
>> On Feb 28, 2012, at 5:42 AM, Bruce Griffiths wrote:
>> > The attached plot indicates the phase noise performance obtainable 
>> with a wideband FET (OPA653) input opamp.
>> > With a 10MHz +9dBm input, the phase noise floor is around 
>> -163dBc/Hz at 1kHz offset and around -154dBc/Hz at 10Hz offset.
>> > A quieter test source would be useful particularly for offsets 
>> below 10Hz.
>> >
>> > Bruce
>> > <OPA653PN2.gif>_______________________________________________
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> William L. Fuqua III P.E.
> Sr. Electrical Engineer
> CP 177 Chemistry Physics Building
> Department of Physics and Astronomy
> University of Kentucky
> Lexington,KY 40506-0055
> Phone: 1-859-257-4155
> e-mail:  wlfuqu00 at uky.edu

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