[sdiy] phantom power: how to avoid electrolytics?

[René Schmitz]

Hi Jürgen, Martin and List,

jhaible wrote:
> Doesn't this National Semiconductor low noise pair have something
> like 100 transistor pairs inside? (forgot the part number. LM194 ?)

I'm not sure its really 100 pairs, or just one pair of quite some size, 
but you can think of it as lots of transistors in parallel anyway. 
(MAT02, LM194, 394..) At least I know the die of the MAT02, which has 4 
transistors which are connected criss crossed. (Gradient cancellation.)

> In theory, the signal adds arithmetically (correlated), and the
> noise adds geometrically (not correlated; power adds instead of voltage
> or current.) So connecting a lot of transistors in parallel will
> increase SNR. If all transistors are equally noisy, N transitors will
> produce N times the signal, and SQRT(N) times the noise.
> (Is this formula right?)
> So your SNR would be increased by 20log(N/SQRT(N)) dB =
> 20 log SQRT(N) dB.
> Two pairs will give 3dB (the same amount you loose if you're
> using a pair instead of a single transistor, btw!).
> 100 pairs will give 20dB.
> More realistic for diy:
> 10 pairs will give 10dB over 1 pair.
> (Think of it: 20 transistors connected as 10 pairs will only give 7dB
> improovement over a single transistor stage!)

I think one would be paralleling a lot of cheap transistors before one 
would get into the regions of one LM194 or MAT02 alone.

Just for the record: a 1:10 transformer would yield a 20dB improvment in 
SNR.... Typical are 26dB or 1:20, that would be an awful lot of 
transistors. Ok, its actually a bit unfair and idealized, because you 
would have to take the effects of the different impedance into account. 
(Since noise figure depends on the impedance as well. )

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159