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Philips Semiconductors Application note
AN1651Using the NE/SA5234 amplifier
1991 Oct
6
signal-to-noise ratio at the output of this stage is determined by first
multiplying the gain times the signal which gives 1V
RMS
with a
resultant noise of 400mV
RMS
. The signal-to-noise ratio is calculated
as
Sń N 20log
10
(1.0ń 4x10
* 4
) + 68dB
(EQ. 5.)
This is quite adequate for good quality audio applications.
Next assume that the bandwidth is cut to 3.0kHz with an input of
1mV
RMS
. The RMS noise is modified by the ratio of the root of the
noise channel bandwidths.
ƪ
3x10
3
Ǹ
20x10
3
Ǹ
ƫ @ EN + 1.6mV
RMS
(EQ. 6.)
Amplified Noise = 160µV
RMS
S
ń
N 20 log
10
ƪ
100x10
*
3
1.6x10
*
4
ƫ
(EQ. 7.)
+ 56dB
A 56dB S/N will provide superior voice channel communications .
+1.6mV
RMS
NOISE
+
–
+
–
1mV
RMS
e
n
x100
R
S
= 100Ω
1.6µV
e
n
x10
100kΩ
1mV
RMS
SIGNAL
SL00636
Figure 8.
UNITY GAIN
+
–
10k
100k
3
2
1
600Ω
2.2µF
1µF
R
L
= 600Ω
V
CC
+
V
CC
2
+
–
3
2
1
10k 10k
+
1k
100pF10k
4.7µF
1µF
1µF
600Ω
V
CC
2
40dB CIRCUIT
ST1700
DISTORTION
ANALYZER
V
CC
ST1700
SL00637
Figure 9. NE5234 THD Test Circuits
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
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
RMS
and a 56dB
S/N, and an output noise of 0.16mV. Following this with a 10kHz
band limited gain-of-10 second-stage, with a 100kΩ noise source at
the non-inverting input, the combined S/N is calculated as follows:
(assume a 100Ω source resistance from amplifier #1)
The Second stage output noise is: