SSM2275/SSM2475

REV. 0

–9–

input transistors, Q1 and Q2, when very large differential volt-

ages are applied. If the device’s differential voltage could exceed

±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

with both inputs. The minimum value of the resistor can be

determined by:

R

V

IN

DIFF MAX

=

−

,

.

7

001

(1)

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

±5 mA. The minimum resistor value should then be:

R

V

mA

IN

IN MAX

=

,

5

(2)

reduce offset voltages caused by input bias current. This is

shown in Figure 28.

+

–

V+

V–

Figure 28. Using Resistors for Input Overcurrent Protection

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-

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/

µs 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

Ω, 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

IN–

IN+

Q1

Q2

M2

M1

IB2

IB1

CF1

OUT

Figure 27. Simplified Schematic

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