[time-nuts] Distribution amp - Use a video amp unit ?

Joseph M Gwinn gwinn at raytheon.com
Tue Mar 27 23:42:08 UTC 2012


I have seen National LMH6702 current-feedback video amplifier chips in
non-inverting amplifier configuration used to implement a wideband 10 MHz
distribution amplifier.

Joe




From:	"Charles P. Steinmetz" <charles_steinmetz at lavabit.com>
To:	Discussion of precise time and frequency measurement
            <time-nuts at febo.com>
Date:	03/27/2012 07:28 PM
Subject:	Re: [time-nuts] Distribution amp - Use a video amp unit ?
Sent by:	time-nuts-bounces at febo.com



Bruce wrote:

>The above reverse isolation [~35 dB] is about 25dB lower than I would
expect.

D'oh!  Bruce is right -- I calculated the reverse isolation
incorrectly.  I had only been expecting 40 dB, so I didn't question
the result.  The breadboard actually measured nearly 63 dB.

>Stable operation at unity gain is necessary if a feedback capacitor is
used.

The Miller capacitance of the output transistors sees to that (with
an even greater phase margin when a faster transistor is used for Q1).

>An LM329 has similar noise without the dissipation of the internal
>heater in the LM399

I know.  I just particularly like the 399, and have a pile of
them.  I rationalize using it in this case by noting that the range
of frequencies where phase noise of the DA is important includes sub-
to low-Hz frequencies at which thermal effects could make the
unheated 329 significantly noisier (though if you keep drafts off
both of them, it might not be by a large amount).

Best regards,

Charles







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