What is a pre-biased transistor?
Pre-biased transistors are transistors manufactured with integral biasing resistors. The result is a cost-effective design where the integrated components save Printed Circuit Board (PCB) cost and lowers the line count for the Bill of Materials (BOM). A typical device is the Diodes Incorporated DDTD113ZC-7-F NPN transistor in a SOT-23-3 package as shown in Figure 1. This pre-biased transistor includes a 1 kΩ series current-limiting resistor as well as a 10 kΩ turn-off resistor in parallel with the base-emitter junction.
Figure 1: Schematic representation of the featured pre-biased transistor with series resistor R1 and base to emitter resistor R2.
How is a pre-biased transistor used in a circuit?
The pre-biased transistor is designed for direct interface with digital logic such as a microcontroller thereby eliminating the need for supporting resistors. The result is a compact PCB design and reduced parts count as three traditional components are integrated into a single package.
What is the purpose of each resistor in the pre-biased transistor?
The typical pre-biased transistor contains two internal resistors. They may be classified by function as:
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Series current limiting resistor: Like the series resistor in an LED circuit, the series resistor limits the base current.
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Turn-off (shunt) resistor: The resistor in parallel with the transistor’s base to emitter junction aids in transistor shutdown. It provides a leakage path to hold the base at zero volts when the input series resistor is disconnected such as when the microcontroller I/O is set for high impedance at startup or when operating in Ultra-Low Power (ULP) Mode. This turn-off resistor also aids in the reduction of I_{CE} leakage current.
A variety of pre-biased NPN and PNP transistors are available. The designer may choose devices with the best series and turn-off resistors to match their application.
Prerequisite transistor theory for forced beta operation in digital (on-off) circuits
A transistor is characterized by its DC current gain (beta) described as the ratio of collector current to base current. This is an important metric for analog design for a transistor operating in its linear region.
Linear operation should be avoided in digital (on-off) design. Instead, we will focus on forced beta. With this design technique, we purposely overdrive the transistor to ensure full saturation – purposely avoiding the linear region. As a first estimation, we assume a forced beta of 10 with the caveat that the forced beta is significantly lower than the linear beta.
See this prerequisite article for additional information about forced beta.
Circuit analysis for a pre-biased transistor circuit.
The circuit analysis for the featured Diodes Incorporated DDTD113ZC-7-F is included as Figure 2. The transistor features an R1 value of 1 kΩ and an R2 value of 10 kΩ. It also features a transistor with a high gain.
In this application, the transistor is driven by a 3.3 VDC logic signal:
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The R1 series resistor limits the input current to 2.7 mA. This is relatively low drive level easily provided by most microcontrollers. However, this may still be too high as explored in the next Tech Tip.
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The turn-off resistor consumes a small portion of the input current. This may or may not be important to the circuit calculation depending on the R1 to R2 ratio for the chosen pre-biased transistor.
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The base current is calculated as the input current minus the turn-off resistor current.
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Assuming a forced beta condition, the maximum collector current is calculated as ten times the base current. Note that this is a conservative estimate used to ensure that the transistor enters saturation. You may be able to improve the situation through testing of the corner cases of your design.
Figure 2: Circuit analysis for the featured Diodes Incorporated DDTD113ZC-7-F driven by 3.3 VDC logic.
Tech Tip: The typical microcontroller has two current specification included a per pin specification and a total summation of all currents. An example is the Renesas Electronics Corporation R7FA4M1AB3CFM#AA0 as featured on the latest Arduino UNO R4 board. Most ports can sink and source 4 mA with a few 20 mA capable pins, yet the total of all output pin is limited to 60 mA. Consequently, careful circuit analysis is required to stay within device limitations.
Note that the featured Diodes Incorporated DDTD113ZC-7-F pre-biased transistor has chosen for its low R1 resistor value. A transistor with a higher input resistance may be a better match for a low-current microcontroller.
Be sure to evaluate on a case by case (load by load) basis.
How fast is a pre-biased transistor?
A transistor’s turn-on and turn-off speed is determined by factors such as the load characteristics, the Miller capacitance, and depth of saturation. Here are a few design considerations directly related to this engineering brief:
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A transistor driven deep into saturation will be slow to turn off. By definition, forced beta bias places the transistor deep into saturation to avoid the linear region. This is important consideration, as the transistor will not turn off until the carriers have been cleared from its silicon structure.
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The base current is a function of the microcontroller’s I/O voltage as well as the R1 / R2 ratio for the pre-biased transistor. The depth of saturation is also a function of the load. For example, the featured circuit is faster when the load is close to the 26 mA limit, as opposed to being lightly loaded with 5 mA. Stated another way, carefully choose the pre-biased transistor to match the load to avoid over-saturation.
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A direct connection to the microcontroller precludes application of a negative pulse to increase turn off speed. Also, without an exposed base connection there is no method to implement speed-up circuits such as a Baker clamp.
Taken together, these design limitations suggest that the pre-biased transistor operates at moderate speeds. However, there are many different loads and level-shifting application that would benefit from the pre-biased transistor. In all cases the designer should select the pre-biased transistor to best match the microcontroller with the load.
Tech Tip: Remember that the matching process was traditionally accomplished with proper resistor selection. Now, the designer must select the appropriate pre-biased transistor with internal resistors appropriate to the task at hand. For example, an R1 of 4.7 kΩ or even 10 kΩ may be a better suited to your design.
Parting thoughts
The pre-biased transistor is a convenient way to reduce PCB size and reduce the total number of parts used in your design. Be sure to match microcontroller to load by selecting a pre-biased transistor with appropriate internal resistors.
We would love to hear from you if have successfully use pre-biased transistors in your design.
Best wishes,
APDahlen
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About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.