[time-nuts] Looking for high reverse isolation amplifier

Richard (Rick) Karlquist richard at karlquist.com
Fri Jan 31 02:25:01 UTC 2014



On 1/30/2014 12:30 AM, John Miles wrote:
> Depending on how much forward gain you're after, I'd suggest looking at the
> LMH6702 current feedback opamp.  I keep a few of them around in Hammond
> boxes, powered by NiMH rechargeables.  Measured S12 is about 70 dB at 100
> MHz, and I'm sure it could do at least 40 dB at 200.   If I remember
> correctly the 3 dB point is about 400 MHz in a stage designed for +8 dB of
> gain, and unity gain is about 700 MHz.
>
> Residual PN at 80 MHz is around -135 dBc/Hz @ 1 Hz, flicker corner around 1
> kHz, floor around -165 dbc/Hz.  You can do better with discretes but you can
> also do a lot worse...
>
> -- john, KE5FX
> Miles Design LLC

Great suggestion!  TI has eval boards available for them so I can
get up to speed quickly.  I guess the idea is that I set it
up for a gain of 2, and put a 50 ohm resistor in series with the
output.  Now it is a unity gain buffer in a 50 ohm system.  A
signal trying to go through it backwards has a source impedance
of 100 ohms driving the output impedance of the amplifier, which
is spec'ed at 30 milliohms, at least at low frequencies.  That
works out to 70 dB, which is what you observed.  So
I can see how it could have good reverse isolation.

Do you run it in inverting or non-inverting configuration?

Rick N6RK



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