Key Parameters for Transistor Selection
Ajit Nawale
Senior Design Engineer - TATA Autocomp Systems | Ex. OLA Electric 🎯1:1 Guidance for Aspiring PCB Designers | Helping Job Seeker |
Selecting a transistor for a specific application involves various parameters and characteristics. Here are some key parameters to select Transistor along with an example.
Transistor Selection Parameters:
Transistor Type: Decide whether you need a bipolar junction transistor (BJT) or a field-effect transistor (FET). BJTs are current-controlled devices, while FETs are voltage-controlled. Further decides wheteher it is NPN or PNP Transistor
a) NPN Transistor : NPN transistors are switched on by applying a positive voltage to the base terminal.
b) PNP Transistor : PNP transistors are switched on by applying a Low voltage to the base terminal.
Maximum Collector (or Drain) Current (Ic or Id): Determine the maximum current your application requires. Select a transistor with a current rating above this value.
Maximum Collector (or Drain) Voltage (Vce or Vds): Choose a transistor with a voltage rating higher than the maximum voltage in your circuit.
Gain (hFE or β): For BJTs, the gain determines the current amplification. For FETs, it's the transconductance (gm). Select a transistor with suitable gain for your amplification needs.
Switching Speed: Consider the switching speed required for your application. High-frequency applications may need transistors with fast switching times.
On-Resistance (for FETs): For FETs, particularly power FETs, low on-resistance (Rds(on)) is critical for minimizing power loss and improving efficiency.
Voltage Compatibility (for FETs): Consider the gate-source voltage (Vgs) requirements for FETs. Some FETs require a specific gate voltage for proper operation.
Temperature Ratings: Check the maximum operating temperature (Tj or Tc) and ensure it can withstand the expected temperature in your application.
Noise Considerations: For low-noise applications, consider the noise figure of the transistor.
Transistor Package: Choose a package type that fits your PCB and heat dissipation requirements.
Example : Turn an LED on and off using a microcontroller's digital output pin (3.3V or 5V logic) as a control signal. e.g. Transistor BC547
Collector-Base Voltage (V_CB): The maximum voltage that can be applied between the collector (C) and the base (B) without causing the transistor to break down. Typically, this value is around 50V for the BC547.
Collector-Emitter Voltage (V_CE): The maximum voltage that can be applied between the collector (C) and the emitter (E) without causing the transistor to break down. It's typically around 45V for the BC547.
Emitter-Base Voltage (V_BE): The voltage required to forward-bias the base-emitter junction and turn on the transistor. Typically, this value is around 6V for the BC547.
Collector Current (I_C): The maximum continuous current that can flow from the collector to the emitter. The BC547 is available in different current ratings, such as BC547 (100mA) and BC547B (200mA).
Base Current (I_B): The current required to forward-bias the base-emitter junction and control the collector current. It depends on the specific transistor's manufacturer and packaging but is typically in the range of 5μA to 100μA for the BC547.
DC Current Gain (hFE or β): This parameter represents the current gain of the transistor and varies with the collector current. For the BC547, it typically falls in the range of 110 to 800.
Power Dissipation (P_D): The maximum power that the transistor can safely dissipate as heat. It depends on factors like the package type and thermal resistance. In a TO-92 package, it's typically around 500mW for the BC547.
Transition Frequency (fT): This parameter represents the frequency at which the transistor's current gain begins to roll off. For the BC547, it's typically in the range of 100MHz to 300MHz.
Package Type: The BC547 comes in various packages, with TO-92 being a common through-hole package.
Operation:
When the microcontroller outputs a HIGH signal (3.3V or 5V logic) on the base of the NPN transistor, it saturates the transistor. This allows current to flow from the collector to the emitter, effectively turning the LED on.
When the microcontroller outputs a LOW signal (0V) on the base of the NPN transistor, the transistor is cut off, and no current flows through the LED. The LED turns off.
Note : Always refer exact components datasheets while components selection.
References :
https://www.sparkfun.com/datasheets/Components/BC546.pdf
https://www.electronics-tutorials.ws/transistor/tran_4.html
https://byjus.com/jee/transistor-as-a-switch/
https://www.tutorialspoint.com/transistor-as-a-switch