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Using Precision Matched-Pairs, Duals, and Quads
5.1
Precision BJT matched-pairs As we learned from the previous chapter, some of the most accurate BJT-based current sources that the designer can create at the circuit board level use precision matched-pairs and quads. These are available from Analog Devices, National Semiconductor, and second-source specialists Linear Integrated Systems. Precision matched monolithic BJT pairs and quads represent the highest quality and precision available in a silicon transistor. All three manufacturers make NPN matched-pairs. Analog Devices and Linear Integrated Systems also offer PNP matched-pairs. Additionally, Analog Devices makes a quad NPN device. Overall, Linear Integrated Systems provides a wider range of different package options than any other manufacturer. Some of the company's matched-pairs are super-gain transistors (not Darlingtons), with hFES upward of 1000. Many of the BJT circuits shown in Chapter 4 can be optimized by using any of these high-performance matched-pairs and quads. What makes precision matched-pairs so special? Unlike dual transistors or multiple transistors that are packaged as an array, matched-pairs have specifications far exceeding those of high-performance discretes or a typical multitransistor array. Precision matched-pairs rely on a specially designed paralleled transistor structure (not unlike today's power MOSFET structure, which also uses many paralleled cells in order to create one single FET device with very low on-resistance). With the precision matched-pair/quad BJT, a similar technique dramatically reduces the base-emitter bulk resistance to levels typically less than an ohm (this resistance is often more than 50 Q in a regular discrete transistor). They also receive special processing, in order to achieve these exotic characteristics. This includes a multiple passivation process, which stabilizes the important parameters over a wide temperature range, reduces leakage currents, and enhances the isolation between individual transistors. These parameters are also stabilized over a range of collector currents, which typically span from one microamp to several milliamps. Additionally, each transistor is individually tested against its published data sheet specifications over the full operating temperature range, and those characteristics, such as noise levels, gain matching, and offset voltage drift, all come close to matching the specs for the ideal transistor. To illustrate this concept further, the best NPN matched-pairs have VBES that are matched to better than 50 pV (referred to as AVBE or Vos ) and high hFES that are 125
Precision BJT matched-pairs As we learned from the previous chapter, some of the most accurate BJT-based current sources that the designer can create at the circuit board level use precision matched-pairs and quads. These are available from Analog Devices, National Semiconductor, and second-source specialists Linear Integrated Systems. Precision matched monolithic BJT pairs and quads represent the highest quality and precision available in a silicon transistor. All three manufacturers make NPN matched-pairs. Analog Devices and Linear Integrated Systems also offer PNP matched-pairs. Additionally, Analog Devices makes a quad NPN device. Overall, Linear Integrated Systems provides a wider range of different package options than any other manufacturer. Some of the company's matched-pairs are super-gain transistors (not Darlingtons), with hFES upward of 1000. Many of the BJT circuits shown in Chapter 4 can be optimized by using any of these high-performance matched-pairs and quads. What makes precision matched-pairs so special? Unlike dual transistors or multiple transistors that are packaged as an array, matched-pairs have specifications far exceeding those of high-performance discretes or a typical multitransistor array. Precision matched-pairs rely on a specially designed paralleled transistor structure (not unlike today's power MOSFET structure, which also uses many paralleled cells in order to create one single FET device with very low on-resistance). With the precision matched-pair/quad BJT, a similar technique dramatically reduces the base-emitter bulk resistance to levels typically less than an ohm (this resistance is often more than 50 Q in a regular discrete transistor). They also receive special processing, in order to achieve these exotic characteristics. This includes a multiple passivation process, which stabilizes the important parameters over a wide temperature range, reduces leakage currents, and enhances the isolation between individual transistors. These parameters are also stabilized over a range of collector currents, which typically span from one microamp to several milliamps. Additionally, each transistor is individually tested against its published data sheet specifications over the full operating temperature range, and those characteristics, such as noise levels, gain matching, and offset voltage drift, all come close to matching the specs for the ideal transistor. To illustrate this concept further, the best NPN matched-pairs have VBES that are matched to better than 50 pV (referred to as AVBE or Vos ) and high hFES that are 125