Diode are important basic component used in electronics circuit. They find application in power supply circuit, modulator with diode in AM transmitter, digital electronics and other functional circuits. But what if you don't have a diode at hand but you have plenty of transistors. So here is how you use a transistor as a diode. It's very simple, connect the transistor collector and base together. The junction forms the anode part and the emitter terminal becomes the cathode of the diode as illustrated in the picture below.
BJT Transistor as a Diode: Key Characteristics
When you connect the collector and base of a BJT transistor together, it forms a base-collector junction, effectively turning the BJT into a diode. This results in specific diode-like behavior, which includes the following:
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Forward Voltage Drop: The base-collector junction of an NPN transistor, like the 2N3904, typically exhibits a voltage drop of 0.6V to 0.7V, very similar to silicon diodes such as the 1N4148 (a small signal diode) and the 1N4007 (a general-purpose rectifier diode).
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Reverse Leakage Current: When reverse biased, the base-collector junction of a BJT exhibits a small reverse leakage current. This current is typically low but might be slightly higher than what you find in dedicated diodes like the 1N4148.
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Capacitance: The base-collector junction in a BJT has higher junction capacitance than most diodes. This could affect high-frequency applications where a diode with lower capacitance, like the 1N4148, would perform better.
BJTs are widely used in amplitude modulation circuits and other signal processing tasks where the diode-like behavior of their base-collector junction comes in handy. For example, when building a simple amplitude modulation (AM) circuit, you can take advantage of this BJT diode functionality for modulation purposes. Learn more about building such circuits by visiting our guide on simple amplitude modulation (AM) circuit where a regulator diode is used. While the following circuit diagram shows the same AM modulator version but replacing diode with BJT 2N3904 transistor.
How Does the BJT Base-Collector Junction Compare to Standard Diodes?
While the base-collector junction of a BJT behaves like a diode, it’s essential to compare its performance to that of standard diodes, especially in terms of forward voltage drop, reverse leakage current, and capacitance.
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1N4007: The 1N4007 is a silicon rectifier diode used primarily for power rectification in circuits. It has a higher forward voltage drop (0.7V to 1.1V), which is typical for rectifier diodes, and is often used in higher current applications. On the other hand, the BJT diode behaves similarly but is generally used in low-current circuits.
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1N4148: The 1N4148 is a small signal diode designed for high-speed switching. Its forward voltage drop is around 0.7V, and it exhibits a very low reverse leakage current. In contrast, the BJT base-collector junction also has a forward voltage of 0.6V to 0.7V, but with slightly higher reverse leakage compared to the 1N4148.
Practical Applications of BJT Diodes
A BJT diode can be used in a variety of applications where diode-like behavior is required but with the transistor's additional amplification. Some practical applications include:
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Modulation Circuits: Using the BJT base-collector junction for modulation is common in AM radio transmitters. The BJT diode can modulate a signal for AM transmission. Learn more about single diode modulator circuits in our article on how does a single diode modulator circuit work?.
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Signal Processing: The BJT diode can be employed for signal clipping, rectification, and even in diode-based mixers for RF circuits.
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Power Regulation: While BJTs are not typically used as voltage regulators, their base-collector junction can act as a clipping diode in voltage regulation circuits. If you're interested in learning how to build simple voltage regulators, check out our guide on simple voltage regulator with a transistor.
Voltage Regulation Using Diodes and BJTs
For voltage regulation, diodes are often used to set a reference voltage. For example, a Zener diode can regulate a voltage by allowing current to flow in reverse once a specified voltage is reached. BJTs, too, can be used in voltage regulation circuits, though they typically rely on their transistor properties for amplification. Learn more about Zener diode voltage regulators by reading our detailed post on Zener diode voltage regulators.
Conclusion
In summary, BJT transistors exhibit diode-like behavior when their collector and base are connected. While this setup behaves similarly to traditional diodes in terms of forward voltage drop and reverse leakage current, it’s essential to consider the capacitance and speed of BJTs when choosing between using them and dedicated diodes like the 1N4148 or 1N4007.
BJTs can be used in low-current signal processing applications, where the transistor's amplification properties provide additional functionality. Whether you’re designing an AM modulator, building a simple voltage regulator, or exploring Zener diodes for regulation, understanding how BJTs work as diodes opens up many possibilities in circuit design.
For more insights into building AM circuits, voltage regulators, and modulators, be sure to check out the following blog posts:
