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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.

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_{1}) from the typical value as shown in the datasheet. The value of the base-emitter resistor (R_{2}) is not specified separately; instead, the resistor ratio (R_{1}/R_{2}) is specified. There is ±10% variation in R_{1}/R_{2} 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 R_{1}:

R_{1} 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. R_{1} 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 h_{FE} (=I_{C}/I_{b}) is in the range of 10 to 20, depending on the input voltage. Therefore, a relatively large current (I_{B}) on the order of a few milliamperes flows through R_{1}. Since the allowable power dissipation of R_{1} is 1/8 W, the maximum input voltage (V_{I}) of BRTs with a high R_{1} value is determined by the value of R_{1}.

(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 (R_{1}/R_{2})

The input voltage (ON) specification (V_{I(ON)}) is dependent on R_{1}/R_{2}. Since the base current (I_{B}) does not flow immediately before the transistor turns on, the voltage applied to the B terminal (V_{I}) is divided by R_{1} and R_{2}. Let the turn-on threshold voltage of the internal transistor be V_{be}. Then,

V_{be} ＝ R_{2} / (R_{1}+R_{2}) * V_{I(ON)
}The V_{be} of BRTs is not affected by the values of R_{1} and R_{2} if they contain the same transistor.

V_{I(ON)}＝V_{be}＊(R_{1}+R_{2}) / R2 ＝ V_{be}＊( 1 + R_{1} / R_{2} )

Hence, V_{I(ON)} is dependent on the resistor ratio (R_{1}/R_{2}).

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