Norm Dresner said:
uses
Just another thought: The MAT-04 is four NPN transistors in a single
package. Does anyone know about the matching for an MPQ2222? It's got to
be cheaper and more readily available. Not that I'd seriously do this
much work to replace a ~$5-10 IC, but there's the curiosity in my
engineering side that's pushing thru.
I don't think that you'll see the desired matching from devices like the
MPQ2222. There is no specficiation for matching, and the component
might even be built from multiple chips. Even if it is built from a single
chip, the matching could be +-25mv or worse, which is of little use for
you.
This 'Gain Control' amplifier thing is ugly, no matter how it is done. It
is fortunate that matching and component characteristics nowadays can
be impressive.
When looking at old tube designs (or even old FET designs), they tend to
be quite ugly, and amazing that they had performed as well as they did.
Early on, I had even seen the use of the dynamic resistance of diodes (and
using ad-hoc techinques to remove the thump.)
If absolute precision isn't needed, and super fast attack/release times
aren't needed, and some drift is allowed, then the Lamp(LED) along
with resistive photocell scheme can be quite nice.
The technology today can be very repeatable and very precise. Such
approaches (like the That stuff, or some of the SSM stuff from AD) do
as well as someone can build.
Except for input AGC (before the 18-20bit A/D converter), I prefer
using DSP techniques for gain control. For before the A/D converter, then
all I'd tend to use would be a peak limiter which would avoid harsh
distortion
on peaks. With the large bit depth, then the recording can be done with
plenty of headroom.
Nowadays, my approach would be (in order of preference, but also with
prejudice): (For audio)
1) DSP!!!
2) Gilbert or That gain control element.
2A) Switched resistor scheme??? ( I don't know how well it might work.)
3) Photo resistor
4) FET
5) Dynamic resistance or simple variable gm method.
The photo resistor can be done with very low distortion, but might
have more limited lifetime and more drift. The FET is mostly just
useful for peak limiting, but matching across channels across a wide
gain control range isn't very good.
A WORST approach is to use the dynamic reisstance of a diode or simple
gm change for a transistor. The linearity will tend to suck badly (without
some compensation.) By the time that one would apply excellent distortion
compensation, then the Gilbert or That gain control cell would be best.
John