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Philips Semiconductors
Using the NE/SA5234 amplifier
signal-to-noise ratio at the output of this stage is determined by first
multiplying the gain times the signal which gives 1V
resultant noise of 400mV
. The signal-to-noise ratio is calculated
RMS
as
S N
20log
(1.0 4x10
10
This is quite adequate for good quality audio applications.
Next assume that the bandwidth is cut to 3.0kHz with an input of
1mV
. The RMS noise is modified by the ratio of the root of the
RMS
noise channel bandwidths.
3
3x10
EN
1.6 V
3
20x10
Amplified Noise = 160 V
RMS
ST1700
VI. MULTIPLE STAGE CONSIDERATIONS
Since multiple noise generators are non-coherent, their total effect is
the root-of-the-sum-of-the-squares of the various noise generators
at a given amplifier input.
This makes orders-of-magnitude lower noise sources less important
than the higher magnitude source. Therefore, when considering the
combined signal-to-noise of multiple stages of gain, the first stage in
a chain dominates making its design parameters the most critical.
For this reason it is good practice to make the preamp stage gain as
high as practical to boost signal levels to the second stage allowing
at least an order-of-magnitude above the second-stage noise. For
1991 Oct
with a
RMS
4
(EQ. 5.)
)
68dB
(EQ. 6.)
RMS
10k
1 F
2.2 F
600
1 F
10k
3
2
1 F
600
1k
40dB CIRCUIT
Figure 9. NE5234 THD Test Circuits
S N
20 log
10
A 56dB S/N will provide superior voice channel communications .
1.6 V
e
n
+
1mV
RMS
–
x100
+
V
CC
UNITY GAIN
100k
V CC
2
3
+
1
2
–
R
= 600
L
+
V
CC
10k
V CC
2
4.7 F
+
1
–
10k
100pF
instance, a signal input which exceeds the input noise of the
following stage by a factor of 10:1 will only be degraded by 0.5% or
-46dB, neglecting the first-stage noise. If we use the preceding
example with a first-stage output signal of 100mV
S/N, and an output noise of 0.16mV. Following this with a 10kHz
band limited gain-of-10 second-stage, with a 100k noise source at
the non-inverting input, the combined S/N is calculated as follows:
(assume a 100 source resistance from amplifier #1)
The Second stage output noise is:
6
Application note
AN1651
3
100x10
4
1.6x10
56dB
100k
e
n
+
–
R
= 100
S
1mV
RMS
+1.6mV
x10
Figure 8.
ST1700
DISTORTION
ANALYZER
and a 56dB
RMS
(EQ. 7.)
SIGNAL
NOISE
RMS
SL00636
SL00637
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