I never had much luck with current feedback amplifiers such as the
LMH6702. Their input current noise (at the time) was too high for
my needs and their output peaks at higher frequencies if the
feedback resistors aren't optimal for the part.
I had the best results with voltage feedback op amps like the
MAX4104/MAX3404 when I needed gain on the input stage and the
LMH6609 when I needed a buffer.
My applications were broadband. If I remember correctly,
aggressive bandwidth limiting can cause phase shift problems due to
temperature changes unless one is careful in the design of the filter.
I've successfully put as many as four op amps in parallel in an
input stage to reduce phase noise.
Bob M (another bob)
On 10/24/2017 6:24 AM, Bob kb8tq wrote:
Hi
One would guess that they put them in parallel to get more drive. If that’s
correct,
details of the loading are going to get into the simulation pretty quickly.
In a lot of cases, these amplifiers were designed against a specific need. If
you have
a signal source that is in the -180 dbc / Hz range, they are unlikely do
perform well.
In many cases a floor in the -140 dbc / Hz range was considered “good enough”.
If you are simply driving common test gear, it probably *is* good enough. If the
application was video rather than a standard the specs could have been very
different.
In the case of an amp with a LMH6702, you are not going to get super close in
phase noise. The device is *very* noisy under 1 MHz. It also starts to increase
distortion
by 10 MHz so you will see up conversion. It probably did quite well against the
intended
design spec.
=====
If you need a system that will distribute one frequency today and a totally
different
frequency tomorrow, broadband makes sense. For the more common task of
something like “only 10 MHz”, it does not make much sense at all. Gain other
frequencies is just going to spread around noise from this or that source
of crud. Driving filters with op amps can be problematic. It often is easier to
go another
way.
Bob
On Oct 24, 2017, at 6:09 AM, Anders Wallin <[email protected]> wrote:
FWIW I recently took a peek inside a commercial distribution-amplifier and
it seems to use two LMH6702 op-amps in parallel.
There are two of these dual-LMH6702 stages with a 1:2 splitter after the
first, and then a 1:4 splitter after the second stage. 8 outputs in total,
with an additional op-amp driving each output.
A simulation that shows the difference in PN between a single LMH6702 and
the dual-op-amp idea would be nice.
For far-out (>100Hz from carrier?) PN only SNR might matter, so a SPICE
noise-simulation giving noise PSD at relevant (5-10MHz) frequencies might
give something?
Anders
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