Digital Transistor Selection
Internal Resistor Selection Tool
Enter conditions such as voltage, current and resistance values, then click on the Calculate button.
The result of the Ic/IB calculation will be displayed, along with items marked as Recommended, Confirmation required, or Review required.
1. Enter the conditions
2.IC/IB Calculation Results
IC/IB≦20：Recommended
20＜IC/IB≦80: The transistor will turn ON, but the output voltage will be high, so it is important to verify the characteristics
(Please contact us if you have any questions)
IC/IB>80：Conditions need to be reviewed
IC/IB＜0：Recommended
IC/IB≧0：Conditions need to be reviewed
■How to Use the Tool
・Digital Transistors (NPN)
・Digital Transistors (PNP)
We will explain how to select the resistors integrated into a digital transistor.
Digital transistors are offered in products with both R1 and R2 built in, models with only R1, and types with only R2. The roles of R1 and R2 remain the same across all variants.
R1: How to select the resistance value
The R1 resistor is used to control the bipolar transistor via voltage.
In the case of a MOSFET, it turns ON when a voltage greater than the threshold voltage (Vth) is applied.
For a bipolar transistor, it turns ON when the Base-Emitter voltage VBE exceeds the forward voltage VF.
At this point, the Base current IB rises sharply. When VBE is lower than VF, almost no current flows, but when VBE is higher than VF, a large IB flows.
Relationship VBE and IB
What’s more, due to variations in VF, controlling ON/OFF with the Base voltage can be challenging.
Incorporating resistor R1 makes it possible to suppress input current variations due to input voltage fluctuations.
Relationship VIN and IIN
The input current of the bipolar transistor is determined by the input voltage and resistance value of R1, while the output voltage is governed by the ratio of the output current IC to the Base current IB (IC/IB).
As such, the resistance of R1 is an important parameter.
When the input voltage is low or output current is large, selecting a smaller resistance value makes it easier to switch the transistor ON/OFF.
R2: How to select the resistance value
For products equipped with R1 and R2, R2 controls the OFF region.
If VBE does not exceed VF, current flows through R2.
For products equipped with only R1, all of the current flowing through R1 becomes IB, but in models equipped with R2 as well, a current IR2 flows through R2, and until R2 x IR2 exceeds VF, an OFF region can be established for a certain range of input voltages.
The key point when selecting R2 is to choose a larger resistance value when the input ON voltage is low and a smaller resistance value when the input OFF voltage is high.
Also, under the ON condition, since a portion of the current flowing through R1 also flows through R2, selecting a larger resistance value for R2 makes it easier to switch ON/OFF when the output current is high.
Selection Method
① To saturate the transistor, it is recommended to ensure IC/IB≦20.
② Input resistance: R1should be within ±30% and for E-B resistance：R2 should be R2/R1=±20%
③ VBE is between 0.55V and 0.75V *This is a typical value and not guaranteed
The following relationship holds for digital transistors.
■ Relationship formula for the DC current gain of a digital transistor
GI：DC current gain of the digital transistor
GI=IO/Iin
hFE=IC/IB
IO=IC , Iin=IB +IR2, IB=IC/hFE , IR2=VBE/R2
The voltage relationship is given by: VIN=VR1+VBE
■ Collector current relationship
∴ IC= hFE×((Vin-VBE)/R1 )- (VBE/R2)) ・・・①
*hFE mentioned here is the value when VCE=5V、IC=1mA, and is not in the saturation state.
When using the transistor as a switch, we recommended operating it in the saturation region. For the following calculation, assume IC/IB=20/1.
∴ IC= 20×((Vin-VBE)/R1 )- (VBE/R2 ))・・・②
Replace hFE in equation ① with 20/1.
Furthermore, to account for variations,
substitute the worst-case values into equation ②: R1 max. +30%, R2 min. -20%, and VBEat max. 0.75V. Select resistances R1 and R2 for the digital transistor using the following equation so that IC of the digital transistor is greater than the maximum output current Iomax of the set.
∴ Iomax≦20((Vin-0.75)/(1.3×R1)-0.75/(1.04×R2))
Terminology of Digital Transistors
- VI(on) Min: Input Voltage (Input ON Voltage)
Refers to the minimum input voltage required to allow the specified output current to flow when a forward voltage VO is applied between the OUT terminal and the GND terminal, in other words, it is the minimum input voltage value in the region where the digital transistor is ON.
Therefore, to switch from ON to OFF, the input voltage must be reduced below this minimum value, so the acceptable value for a good product will be lower than this.
For example, a VI(on) min. of 3.0V indicates that at least 3.0V input is needed to turn the transistor ON. - VI(off) Max: Input Voltage (Input OFF Voltage)
Refers to the maximum input voltage between the IN and GND terminals when the specified supply voltage VCC and output current IO are applied between the OUT terminal and the GND terminal, in other words, the maximum input voltage value in the region where the digital transistor remains OFF.
To switch from OFF to ON, the input voltage must be increased above this maximum value, so the acceptable value for a good product will be higher than this.
For instance, a VI(off) max. of 0.5V means that input signals up to 0.5V are considered OFF.
Considering variations, the actual value will always be above 0.5V, typically around 1.5V in performance graphs. - VO(on): Output Voltage
The output terminal voltage under any input conditions that do not exceed the absolute maximum ratings. In a grounded amplifier circuit, when sufficient input current is supplied, the output voltage decreases and the IN-OUT junction become forward biased. When measuring at the specified VO and IO, II is divided by an integer (usually 10 to 20). - II(max): Input Current
The maximum allowable input current that can continuously flow through the IN terminal when a forward voltage VI is applied between the IN and GND terminals. - GI：DC Current Gain
The ratio IO/II at the specified VO and IO. - R1: Input Resistance
The resistor integrated between the IN terminal and transistor Base. The allowable range of R1 is ±30%. It also varies with temperature. - R2/R1: Resistance Ratio
The ratio of the transistor’s Base-Emitter resistance to the integrated input resistance.
