REV.
SSM2019
–6–
V+
V–
OUT
R
G
+IN
–IN
R
G1
R
G2
SSM2019
REFERENCE
G =
V
OUT
(+IN) – (–
IN)
=
10k⍀
R
G
+ 1
Figure 1. Basic Circuit Connections
GAIN
The SSM2019 only requires a single external resistor to set the
voltage gain. The voltage gain, G, is:
G
k
R
G
=+
10
1
W
and the external gain resistor, R
G
, is:
R
k
G
G
=
10
1
W
–
For convenience, Table I lists various values of R
G
for common
gain levels.
Table I. Values of R
G
for Various Gain Levels
R
G
(⍀)A
V
dB
NC 1 0
4.7 k 3.2 10
1.1 k 10 20
330 31.3 30
100 100 40
32 314 50
10 1000 60
The voltage gain can range from 1 to 3500. A gain set resistor is
not required for unity gain applications. Metal film or wire-wound
resistors are recommended for best results.
The total gain accuracy of the SSM2019 is determined by the
tolerance of the external gain set resistor, R
G
, combined with the
gain equation accuracy of the SSM2019. Total gain drift combines
the mismatch of the external gain set resistor drift with that of
the internal resistors (20 ppm/∞C typ).
Bandwidth of the SSM2019 is relatively independent of gain,
as shown in Figure 2. For a voltage gain of 1000, the SSM2019
has a small-signal bandwidth of 200 kHz. At unity gain, the
bandwidth of the SSM2019 exceeds 4 MHz.
1k 10M
VOLTAGE GAIN – dB
60
10k 100k 1M
40
20
0
V
S
= ⴞ15V
T
A
= 25ⴗC
Figure 2. Bandwidth for Various Values of Gain
NOISE PERFORMANCE
The SSM2019 is a very low noise audio preamplifier exhibiting
a typical voltage noise density of only 1 nV/÷
Hz
at 1 kHz. The
exceptionally low noise characteristics of the SSM2019 are in
part achieved by operating the input transistors at high collector
currents since the voltage noise is inversely proportional to the
square root of the collector current. Current noise, however, is
directly proportional to the square root of the collector current.
As a result, the outstanding voltage noise performance of the
SSM2019 is obtained at the expense of current noise performance.
At low preamplifier gains, the effect of the SSM2019 voltage
and current noise is insignificant.
The total noise of an audio preamplifier channel can be calculated by:
EeiRe
nnnSt
=+ +
2
2
2
()
where:
E
n
= total input referred noise
e
n
= amplifier voltage noise
i
n
= amplifier current noise
R
S
= source resistance
e
t
= source resistance thermal noise
For a microphone preamplifier, using a typical microphone
impedance of 150 W, the total input referred noise is:
EnVHzpAHz nVHz
nV Hz kHz
n
=+¥+ =()(/ )(./)
./@
12150 1 6
193 1
2
22
W
where:
e
n
= 1 nV/÷Hz @ 1 kHz, SSM2019 e
n
i
n
= 2 pA/÷Hz @ 1 kHz, SSM2019 i
n
R
S
= 150 W, microphone source impedance
e
t
= 1.6 nV/÷Hz @ 1 kHz, microphone thermal noise
This total noise is extremely low and makes the SSM2019
virtually transparent to the user.
