[time-nuts] NIST isolation amplifiers
bill at hsmicrowave.com
Tue Nov 25 15:16:16 EST 2014
Thanks a bunch for the comments and the article reprints. This is just what
I was looking for to get started on my distribution amplifier.
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Charles
Sent: Tuesday, November 25, 2014 11:35 AM
Subject: [time-nuts] NIST isolation amplifiers
A couple of people were asking about NIST isolation amplifiers recently.
I'm attaching circuit diagrams of the 5-10 MHz amp from
1997 and the 1-200 MHz amp from 1990. I think Bruce has the papers linked
at his ko4bb.com pages.
I built some of the 5-10 MHz amps with minor variations and they work very
well (I used a separate capacitance multiplier for the base divider string,
and changed the first 4.3k resistor to 6.65k to achieve symmetrical clipping
and a small increase in headroom). I used 2N3904s for the two lower
transistors and a 2N2219A for the top transistor, which dissipates over
I tried some fancy transistors with very low base spreading resistance,
which reduced the noise -- but the increased junction capacitance made the
AM to PM conversion worse, so the overall residual PM was worse. On the
other hand, GHz transistors had higher noise due to lower gain. So the
3904/2219A combination appears to be just about optimum. (Note that the 200
ohm resistor at the input contributes about half of the circuit's noise, and
you can't use the Norton trick because it would ruin the isolation.)
The transistor stack draws 32mA and the base divider stack draws ~1.5mA.
The amplifiers have an input impedance of 250 ohms, so paralleling the
inputs of 5 sections creates an overall 50 ohm input impedance. When a
circuit has reverse isolation of well over 150dB, as this one does, you need
to pay very careful attention to shielding.
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