The MUSES02 is a dual opamp optimized for high-end audio applications. It was developed by the Japanese company New Japan Radio. It is now part of Nisshinbo Micro Devices, a joint venture between New Japan Radio and Ricoh Electronic Devices. Digi-Key offers the MUSES02 at a unit price of 56€.
The operating voltage range extends from ±3,5V to ±16V. The typical current consumption is 8mA. The datasheet prominently highlights the noise voltage density of 4,5nV/√Hz and the voltage gain of 110 dB. The maximum voltage rise time is typically 5 V/µs. The bandwidth is specified at 5,8MHz. The manufacturer guarantees isolation with an attenuation of 150dB between the two integrated opamps. Changes in common-mode voltage and supply voltage are typically attenuated by 110dB.
The two opamps are located on two separate dies. This explains the low crosstalk from one side to the other.
The die measures 2,5mm x 1,2mm. The process does not appear to be particularly advanced. The structures are relatively large, and there are no special elements. Large structures are definitely an advantage in the field of precision circuits. For example, larger elements inherently generate less noise.
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The design appears to date from 2008. The letter B could stand for a second revision, but it could also have a different meaning. The symbol, which resembles the number 7, is often found on components from Nisshinbo Micro Devices. Its meaning remains unclear.
If you take a closer look at the circuit, you’ll notice that a great deal of effort went into the power supply. Both the negative and positive potentials are routed via two bondwires each to different bondpads. In addition, the potentials have been distributed in a star configuration from these bondpads so that the individual circuit blocks interfere with each other as little as possible.
The integrated circuit is not overly complex, but it does have a few unique features. Two PNP transistors, embedded in a classic differential amplifier, serve as the input stage. The traces from the bondpads to the input transistors are designed to be of equal length and thus have equal resistance. The transistor pairs Q1/Q2 and Q4/Q5 have been placed roughly in the center and in a crisscross pattern. Both measures reduce the influence of thermal gradients on the input stage.
Transistor Q6 acts as a buffer stage that reduces the unbalanced load on the differential amplifier caused by the subsequent amplifier stage. Transistor Q7 provides the voltage gain of the opamp. The voltage amplifier stage and the output stage use the second negative potential V2-, so that their dynamic currents do not affect the potential V1-. The voltage amplifier is powered by a constant current supplied via transistor Q11. Since the operating current is constant, the potential V1+ can be used for this purpose.
The Capacitor C2 reduces the effect of the base-collector capacitance of transistor Q7. This so-called Miller capacitance is voltage-dependent and can distort the signal accordingly. (I know most of you know the topic but sometimes there are newbis in the room.) C2 also limits the frequency response of the opamp. The RC network R7/C3 provides additional local negative feedback. Transistor Q8 protects the output stage from overload. If Q7 is driven to the point where its collector potential drops significantly, Q8 turns on and draws base current from Q6, which ultimately reduces the drive level of the chain all the way to Q9.
The Q12/R8/Q13 block generates a specific voltage drop that ensures both output stage transistors always allow a certain amount of quiescent current to flow in the crossover region. This reduces crossover distortion at the signal’s zero-crossing point. The output stage is configured in a complementary configuration. The two paths are combined via two low-impedance resistors.
The control of the two current sources, Q3 and Q11, is unusual. It is not achieved via a common bias potential. Each current source has its own current mirror, which gets its reference current from its own current sink. Clearly, great care was taken to isolate the current sources from one another as much as possible. In contrast, the control of current sources Q16 and Q17 is designed to be relatively simple. The reference voltage on which the currents are based is generated by the two base-emitter paths of transistors Q18 and Q19. Their operating current is set solely via resistor R11. Capacitor C4 acts as a stabilizer against high-frequency disturbances in the power supply. It is located on the resistor loops and is thus, strictly speaking, distributed across the resistor. The significant influence of temperature on the voltage drop across transistors Q18 and Q19—and thus on the operating currents—remain uncompensated.
The MUSES02 contains several variable and optional elements. The operating current of the differential amplifier can be adjusted via the three elements of resistor R1. The capacitance of C1 can also be adjusted via the metal layer. It limits the frequency response of the differential amplifier. Similarly, the frequency response of the opamp can be influenced via the metal surface of capacitors C2 and C3. A second, unused resistor is connected in parallel with R8. Both strips have a widening in the center that could be used as a contact area. This can be used to vary the quiescent current of the output stage. To the right of transistor Q13 are two unused transistors. Perhaps these transistors were intended to allow for a different configuration of the quiescent current setting. The reference currents of the current sources can also be adjusted extensively. Resistor R11 has two optional contacts. Above transistor Q18 is another transistor that could be used to increase the reference voltage by one additional base-emitter voltage drop. Less obvious are the extended contacts on resistors R9 and R10. These can also be used to slightly vary the currents in the differential amplifier and in the voltage amplification stage.
The substrate of the MUSES02 is p doped. It is accordingly connected to the negative supply potential (black). The two elongated contacts have been deliberately placed next to the output stage and next to the voltage amplifier. The transistors and resistors are located in n-type wells and are thus isolated from one another. The differential section of the differential amplifier has an additional, wider frame (yellow). Additional p-doping has apparently been introduced there. A similar frame is found in the bias section. There, the frame is additionally connected to the less-loaded negative supply potential (blue). It remains unclear why the frame of the differential amplifier was not connected in the same way. Nevertheless, the frame has a balancing effect on the local substrate potential.
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