The Difference Between BJT and FET: Which One to Choose?
When it comes to choosing the right type of transistor for your electronic circuit, it’s important to understand the differences between bipolar junction transistors (BJT) and field-effect transistors (FET). Both transistors are critical semiconductor devices that amplify or switch current in a circuit. Here, we’ll explore the main differences between BJT and FET to understand their strengths and weaknesses.
BJT Overview
BJTs were invented in the late 1940s and have become a staple in electronic circuits. They are known as “current-controlled” devices because their output current is proportional to the input current at the base terminal. There are two types of BJTs: NPN and PNP. The NPN type has a heavily doped N-type emitter and collector, while a lightly-doped P-type base region separates the emitter and collector. The PNP type is the opposite, with a heavily doped P-type emitter and collector separated by a lightly doped N-type base region.
FET Overview
FETs were invented in the 1950s and quickly gained popularity due to their high input impedance and low power consumption. They are known as “voltage-controlled” devices because their output current is proportional to the input voltage at the gate terminal. FETs have three types: JFET, MOSFET, and MESFET. JFETs have a single PN junction and are good for low to medium frequency applications. MOSFETs have a metal gate and an insulating oxide layer, making them ideal for high-frequency and high-power applications. MESFETs have a metal gate and a semiconductor substrate, making them good for microwave frequencies.
Key Differences
The main difference between BJT and FET is their input impedance. BJT has a relatively low input impedance and requires a significant amount of input current to drive the output. FET, on the other hand, has a very high input impedance and requires only a small amount of input voltage to operate. This means that FET can be used in circuits where the input signal is weak, and BJT is better suited for circuits where the input signal is strong.
Another difference is the way they amplify signals. BJT amplifies current, while FET amplifies voltage. BJT has a higher current gain than FET – meaning that it can amplify a larger current than FET. Because FETs are voltage-controlled, they have a much higher frequency response than BJTs, making them ideal for high-frequency applications. BJT is better suited for low-frequency applications because of its limited frequency response.
Conclusion
In conclusion, both BJT and FET have their strengths and weaknesses. BJT is better suited for low-frequency applications that require high current gain, while FET is better suited for high-frequency applications that require high input impedance and voltage gain. Ultimately, the choice between BJT and FET will depend on the specific needs of your circuit. By understanding the differences between the two, you can make an informed decision and ensure the success of your electronic project.
Table difference between bjt and fet
| BJT | FET |
|---|---|
| BJT stands for Bipolar Junction Transistor | FET stands for Field Effect Transistor |
| They have a current-controlled operation | They have a voltage-controlled operation |
| They function as current amplifiers | They function as voltage amplifiers |
| They have high input impedance and low output impedance | They have high input impedance and high output impedance |
| They have a high gain at low frequencies | They have a high gain at high frequencies |
| They are less expensive compared to FETs | They are more expensive compared to BJTs |
| They are more suitable for low noise amplification | They are more suitable for high-frequency applications |