Working Principle and Application of Operational Amplifier TS321 and Voltage Comparator TS391

Abstract: The development of portable devices requires electronic components to develop in the direction of miniaturization and low power consumption. As ultra-small package single operational amplifier TS321 and single voltage comparator TS391, they meet this requirement very well in all aspects. This paper mainly introduces the working principle and some basic applications of these two devices. They have low power consumption and small size, which can extend the battery life, save circuit board area and reduce product cost.
Keywords: ultra-small; operational amplifier; voltage comparator; TS321; TS391

Since Robert J. (Bob) Widlar designed the first recognized monolithic integrated operational amplifier μA702 in 1963, the development of operational amplifiers has gone through the process from general-purpose operational amplifiers to high-precision special-purpose operational amplifiers with low offset, low noise and high gain. During this period, 324, as a general-purpose operational amplifier, has been widely used for a long time due to its extremely high ease of use. However, since 324 integrates four identical operational amplifiers in a single chip, it causes cost waste in some applications where only one operational amplifier is used, and also consumes more power consumption and circuit board area. As a single op amp version of 324, TS321 provides the same parameters and performance in a smaller package size, and has broad prospects in application with lower power consumption. As a single comparator version of the quad voltage comparator 339, TS391 has great advantages in miniaturization like TS321.

1 Working principle of TS321 and TS391
TS321 uses bipolar PNP transistors as the input stage, and uses two-stage amplification to effectively improve the gain of the circuit. T321 has an extremely wide power supply voltage range. When powered by a single power supply, the voltage range is from 3V to 30V, and when powered by positive and negative power supplies, the power supply range is from ±1.5V to ±15V. It has a lower power supply current (typical value is 500μA) and lower input bias current, and can still work stably under high capacitive loads.
TS391 also uses bipolar PNP transistors as input stage, which can accept a wide range of power supply voltages, including single power supply or positive and negative power supply, extremely low power supply current (0.2mA), low output saturation current (250mV), a large differential input voltage range, which can be equal to the power supply voltage value at most, and the output level can be flexibly and conveniently selected.
Both TS321 and TS391 use SOT23-5L ultra-small package, as shown in Figure 1.

TS391 is similar to an operational amplifier with unadjustable gain. The comparator has two input terminals and one output terminal. One is called the non-inverting input terminal (+), and the other is called the inverting input terminal (-). When used to compare two voltages, a fixed voltage is added to any input terminal as a reference voltage (also called a threshold level, which can be selected at any point in the TS391 input common mode range), and a signal voltage to be compared is added to the other terminal. When the voltage at the "+" terminal is higher than that at the "-" terminal, the output tube is cut off, which is equivalent to an open circuit at the output terminal. When the voltage at the "-" terminal is higher than that at the "+" terminal, the output tube is saturated. It is equivalent to connecting the output terminal to a low potential. When the voltage difference between the two input terminals exceeds a certain value, it can ensure that the output can be reliably converted from one state to another. Therefore, it is ideal to use TS391 in occasions such as weak signal detection. The output terminal of TS391 is equivalent to a transistor without a collector resistor. When in use, a pull-up resistor is generally required from the output terminal to the positive power supply. Selecting pull-up resistors of different resistance values ​​will affect the value of the high potential of the output terminal. Because when the output transistor is cut off, its collector voltage basically depends on the value of the pull-up resistor and the load.
2 Application of TS391
2.1 TS391 as a zero-crossing comparator
Figure 2 shows a zero-crossing comparator with a limiting circuit, and DZ is a limiting voltage regulator. The signal is input from the inverting input of the op amp, and the reference voltage is zero, which is input from the non-inverting end. When Ui>0, the output U0=-(UZ+UD), when Ui<0, U0=+(UZ+UD). Its voltage transfer characteristics are shown in Figure 3.

The zero-crossing comparator has a simple structure and high sensitivity, but poor anti-interference ability.
2.2 TS391 as a hysteresis comparator
Figure 4 shows a zero-crossing comparator with hysteresis characteristics. A resistor voltage divider positive feedback branch is drawn from the output end to the non-inverting input end. If u0 changes state, the ∑ point also changes potential, causing the zero-crossing point to leave its original position. When u0 is positive (denoted as U+), then when ui>U∑, u0 changes from positive to negative (denoted as U-), and U∑ becomes -U∑. Therefore, only when ui drops below -U∑ can u0 rise again to U+, so the hysteresis characteristics shown in Figure 5 appear. The difference between -U∑ and U∑ is called hysteresis. Changing the value of R2 can change the size of the hysteresis.

3 Conclusion
As single operational amplifiers and single voltage comparators in ultra-small packages, TS321 and TS391 have low power consumption and small size, which can extend battery life, save circuit board area, and reduce the cost of end products. These devices are suitable for portable applications, such as personal handheld devices such as walkmans and mobile phones, so their application prospects are extremely broad.