ssm2275s Analog Devices, Inc., ssm2275s Datasheet - Page 9

• Manufacturer Part Number: ssm2275s
• Description: Rail-to-rail Output Audio Amplifiers
• Manufacturer: Analog Devices, Inc.

REV. A
THEORY OF OPERATION
The SSM2275 and SSM2475 are low noise and low distortion
rail-to-rail output amplifiers that are excellent for audio applica-
tions. Based on the OP275 audiophile amplifier, the SSM2275/
SSM2475 offers many similar performance characteristics with
the advantage of a rail-to-rail output from a single supply
source. Its low input voltage noise figure of 7 nV/√Hz allows the
device to be used in applications requiring high gain, such as
microphone preamplifiers. Its 11 V/ s slew rate also allows the
SSM2275/SSM2475 to produce wide output voltage swings
while maintaining low distortion. In addition, its low harmonic
distortion figure of 0.0006% makes the SSM2275 and
SSM2475 ideal for high quality audio applications.
Figure 27 shows the simplified schematic for a single amplifier.
The amplifier contains a Butler Amplifier at the input. This
front-end design uses both bipolar and MOSFET transistors in
the differential input stage. The bipolar devices, Q1 and Q2,
improve the offset voltage and achieve the low noise perfor-
mance, while the MOS devices, M1 and M2, are used to obtain
higher slew rates. The bipolar differential pair is biased with a
proportional-to-absolute-temperature (PTAT) bias source, IB1,
while the MOS differential pair is biased with a non-PTAT
source, IB2. This results in the amplifier having a constant gain-
bandwidth product and a constant slew rate over temperature.
The amplifier also contains a rail-to-rail output stage that can
sink or source up to 50 mA of current. As with any rail-to-rail
output amplifier the gain of the output stage, and consequently
the open loop gain of the amplifier, is proportional to the load
resistance. With a load resistance of 50 k , the dc gain of the
amplifier is over 110 dB. At load currents less than 1 mA, the
output of the amplifier can swing to within 30 mV of either sup-
ply rail. As load current increases, the maximum voltage swing
of the output will decrease. This is due to the collector to emit-
ter saturation voltage of the output transistors increasing with an
increasing collector current.
Input Overvoltage Protection
The maximum input differential voltage that can be applied to
the SSM2275/SSM2475 is 7 V. A pair of internal back-to-back
Zener diodes are connected across the input terminals. This
prevents emitter-base junction breakdown from occurring to the
input transistors, Q1 and Q2, when very large differential volt-
ages are applied. If the device’s differential voltage could exceed
IN–
Q1
M1
IB1
IB2
M2
Q2
Figure 27. Simplified Schematic
IN+
–9–
with both inputs. The minimum value of the resistor can be
determined by:
There are also ESD protection diodes that are connected from
each input to each power supply rail. These diodes are normally
reversed biased, but will turn on if either input voltage exceeds
either supply rail by more than 0.6 V. Again, should this condi-
tion occur the input current should be limited to less than
In practice, R
reduce offset voltages caused by input bias current. This is
shown in Figure 28.
Output Voltage Phase Reversal
The SSM2275/SSM2475 was designed to have a wide common-
mode range and is immune to output voltage phase reversal with
an input voltage within the supply voltages of the device. How-
ever, if either of the device’s inputs exceeds 0.6 V above the posi-
tive voltage supply, the output could exhibit phase reversal.
This is due to the input transistor’s B–C junction becoming for-
ward biased, causing the polarity of the input terminals of the
device to switch.
Figure 28. Using Resistors for Input Overcurrent Protection
7 V, then the input current should be limited to less than
5 mA. This can be easily done by placing a resistor in series
5 mA. The minimum resistor value should then be:
IN
should be placed in series with both inputs to
R
R
IN
R
R
IN
IN
IN
V
SSM2275/SSM2475
V
DIFF MAX
CFI
IN MAX
5
0 01
mA
V+
V–
V
V
OUT
EE
CC
7
(2)
(1)