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The information presented in this cross reference is based on TOSHIBA's selection criteria and should be treated as a suggestion only. Please carefully review the latest versions of all relevant information on the TOSHIBA products, including without limitation data sheets and validate all operating parameters of the TOSHIBA products to ensure that the suggested TOSHIBA products are truly compatible with your design and application.
Please note that this cross reference is based on TOSHIBA's estimate of compatibility with other manufacturers' products, based on other manufacturers' published data, at the time the data was collected.
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There is ±30% variation in the value of the series bias resistor (R1) from the typical value as shown in the datasheet. The value of the base-emitter resistor (R2) is not specified separately; instead, the resistor ratio (R1/R2) is specified. There is ±10% variation in R1/R2 from the typical value.
The values of bias resistors are dependent on temperature, as shown in Figure 2. The resistance decreases at a rate of roughly 0.2%/°C.
About resistor R1:
R1 converts the voltage applied to the B terminal of a BRT into current. A bipolar transistor is a current-driven device. When a bipolar transistor is voltage-driven, the rate of change of the collector current with respect to voltage becomes large, making it difficult to control the collector current. R1 in a BRT makes it relatively easier to control the collector current. When a BRT is on, the internal transistor operates in the saturation region where hFE (=IC/Ib) is in the range of 10 to 20, depending on the input voltage. Therefore, a relatively large current (IB) on the order of a few milliamperes flows through R1. Since the allowable power dissipation of R1 is 1/8 W, the maximum input voltage (VI) of BRTs with a high R1 value is determined by the value of R1.
(See the FAQ entry “What is the maximum voltage that can be applied to the base of a bias resistor built-in transistor (BRT)?”.)
About the resistor ratio (R1/R2)
The input voltage (ON) specification (VI(ON)) is dependent on R1/R2. Since the base current (IB) does not flow immediately before the transistor turns on, the voltage applied to the B terminal (VI) is divided by R1 and R2. Let the turn-on threshold voltage of the internal transistor be Vbe. Then,
Vbe ＝ R2 / (R1+R2) * VI(ON)
The Vbe of BRTs is not affected by the values of R1 and R2 if they contain the same transistor.
VI(ON)＝Vbe＊(R1+R2) / R2 ＝ Vbe＊( 1 + R1 / R2 )
Hence, VI(ON) is dependent on the resistor ratio (R1/R2).