# S/N Ratio dependent on impedance match?

Discussion in 'Electronic Basics' started by Bill Bowden, Jan 27, 2013.

1. ### Bill BowdenGuest

Why is S/N ratio dependent on impedance match?

It would seem a ferrite loopstick antenna would deliver twice the
voltage if connected to a high impedance rather than matching it to
it's characteristic impedance. But I have heard this is not a good
idea since the S/N ratio would degrade. Any truth to this idea?

-Bill

2. ### Phil AllisonGuest

"Bill Bowden"
** No.

FET ( source follower) pre amps for ferrite antennas usually have no gate
resistor - cos it only reduces the Q and the signal level.

..... Phil

Absolutely..

Jamie

4. ### Bill BowdenGuest

So, why does the s/n ratio double if you go through a transformer?
Doesn't the transformer double everything going into it?

5. ### Bill BowdenGuest

Yes, thanks Phil. I think you are saying that jacking up the signal
with a transformer is a good idea until it degrades the Q due to non-
ideal conditions of the transformer? I guess any reduction in S/N
ratio would result from a lowered Q and wider bandwidth? So, if you
can maintain the Q, there will be no change in S/N ratio? Is that a
correct statement?

-Bill

6. ### Phil AllisonGuest

"Tim Wescott"

** Equal source/load impedance matching is virtually never used in audio -
with the exception of long cable runs where the characteristic impedance of
the cable may be matched with a resistive load at the receiving end to
neutralise the undesired effects of cable inductance and/or capacitance.

With low noise sources (ie mics and other passive transducers) the practice
is to make the load 5 to 10 times the source impedance. This is most easily
done with FET and tube inputs and also BJT stages where local or loop
feedback makes the actual load impedance quite high.

But as Tim said, it is highly desirable to match the "noise impendence" of
the amplifier to the source impedance.

FETs and tubes have optimum noise impedances in the megohms range while BJT
stages can be tailored to give much lower values - down to a few ohms for
MC pickups and ribbon mics.

** Not so quickly - a ferrite loop antenna is VERY inefficient so the
noise generated by the loop can dominate over atmospheric sources. A low
noise FET makes an excellent ferrite loop pre-amp as it adds only about 2dB
to the theoretical noise for source impedances in the 10k to 100 kohms
range.

.... Phil