Where to put the attenuation?

Project is a multi-input wide-band pre-amplifier with a plug-in card for
each of several signal sources. Most of the cards will differ only in the
bandpass filter at the input, perhaps 3 - 4 different types (at first).
Each card will contain an input buffer amplifier (possibly also implementing
a 1st order filter), an additional bandpass filter, and a gain stage [in no
special order and two or more of these could be combined into a single stage
if advantageous]. Output of each card goes to the backplane when enabled
via digital selection lines. Each card's output impedance is such that more
than one card can be enabled at a time and their outputs are summed.
Further down the line are additional signal conditioning cards -- master
gain, filtering, monitoring, etc -- and then a final distribution amplifier
(or 2-3 such).
While there are a variety of signal sources for this (sub)system, most will
remain connected for long terms, and while it's tempting, I don't want to
create a specific card for each source, but a very few generic cards which
differ mostly in the bandpass filtering. This means that each card has to
have a variable attenuation to equalize the input which will be adjusted
when an input source is changed. I've envisioned this being done via a
multi-turn pot mounted at the front edge of the card. There are several
places in the circuit I can see putting this attenuator:
1. As a voltage divider at the input connected directly to the input
capacitor and ground with the wiper going to a unity gain voltage follower.
2. As a variable resistance -- in series with a fixed resistor -- going
into the - input of an inverting op amp serving as the input buffer with a
fixed feedback resistor to provide variable gain
3. As a variable resistance -- in series with a fixed resistor -- in the
feedback path of an inverting op amp serving as the input buffer with a
fixed input resistor to provide variable gain
4. As a voltage divider providing the input and feedback resistance for
inverting op amp serving as the input buffer to provide variable gain.
5. As a voltage divider providing the input and feedback resistance for
a non-inverting op amp serving as the input buffer to provide variable gain.
6. As a voltage divider after a unity gain voltage follower at the input
feeding the next stage
[Phew -- have I missed any configurations?]
Anyway, there's obviously a large number of places it can be put. Assuming
that the range of adjustment is relatively small so that the equivalent
Thevenin resistance seen by the op amp to which it's connected doesn't
change too much, I can't see where any of these configurations has any great
advantage or disadvantage relative to any other -- but I'm sure I'm missing
something here. Please comment on these alternatives and tell me which
ones, if any, are significantly better or worse than the others.
TIA
Norm
Reply to
Norm Dresner
Loading thread data ...
I was bored to read all this.What you wanna do?It's much easier to buy a factory-made amplifier (as I am buying factory made switches, circuit-breakers, receptacles and household appliances).
-- Dimitris Tzortzakakis,Iraklion Crete,Greece major in electrical engineering freelance electrician dimtzort AT otenet DOT gr ? "Norm Dresner" ???a?e st? µ???µa news:gHu2d.600009$ snipped-for-privacy@bgtnsc04-news.ops.worldnet.att.net...
implementing
Reply to
Tzortzakakis Dimitrios
+: Full adjustment range down to 0. Scale of pot gives linear reading. Input range exeeds opamp input range. -: Input impedance is lowered by paralleling the pot, but is constant. In the middle position of the pot the noise can substantially increase if the signal source is of lower impedance than the pot.
+: Input range can be greater than opamp range -: input impedance changes and shifts the input highpass cutoff frequency. Noise gets increased. scale non linear
+: input range higher than opamp, constant input impedance, linear scale, gain and attenuation possible -: much more noise, stray capacitance can limit bandwidth of opamp, stability issues with some opamps
see 2, but more pronounced
+: very high input impedance (you still need a resistor for the bias), lowest noise -: input range limited to opamp, only for gain >=1, non linear scale
as 5. but also for gain [Phew -- have I missed any configurations?]
Since you use a couple capacitor, 2. and 4. are out, best would be 1, 5, 6 depending on the required gain range. If you use electronic pots, 6 will also protect from overvoltage on the pot, you can use a low impedance pot even with very high input impedance, and have a linear scale. So a 10 turn knob with direct readout can calibrate the gain.
Reply to
Ban
give it more thought [or wait for more responses].
Norm
Reply to
Norm Dresner

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.