Last Updated on October 22, 2023 by Kevin Chen
Photo of transistor source Freepik
Current sensing is a major feature in modern electronics applications. It is a unit that not only determines the presence of electric current in a device but is also able to quantify it. It can tell whether the presence of current is high or low. In other words, it is able to display a measurable demonstration or range of the current.
When it comes to electric current, the sensing is usually done by the current sensor. This is an electronic device or component that not only recognizes but also determines whether the current in a conductor or system is high or low. It is also able to detect the smallest presence of current.
The general working principle of a current sensor entails the ability to not only detect the current flowing through a wire or a system but also to convert that current into an output voltage. In all scenarios, this voltage will be directly proportional to the amount of current flow. Other than the current-voltage relationship, the sensor also works on the general properties of electric magnetism. This is derived from the fact that electric current tends to generate some magnetic field near the conductor.
A typical current sensor comprises different electronic components which work together to deliver the sensing capabilities. A transistor is among the key electronic components that work as current sensors. It is able to utilize its internal components, principles, and working mechanism to ensure that it delivers the correct data that is related to the electric current sensing.
Transistors working as current sensors
The technology behind the current sensing and monitoring has undergone a major transformation thanks to the use of integrated circuits. These semiconductor chips have different electronic components including transistors which are widely known for their amplification function. Building modern current sensors with these discrete electronic components has become a go-to solution in the modern world of electronics.
Transistors are designed to be part of the electric circuit and this is where they derive their current sensing capability.
The working mechanism of transistors as current sensors
Different operation modes allow transistors to work as current sensors in electric circuits. The three modes are the saturation mode, the cut-off mode, the active mode, and the reverse-active mode.
In the saturation mode, a transistor will act as a short circuit between the collector and the emitter. Both the terminals are forward-biased and there will be a free flow of current from the collector to the emitter.
At the cut-off circuit, the transistor operates as an open circuit. There is a limited flow of electric current from the collector terminal to the emitter. In the active mode, the flow of current between the collector and emitter will depend on the current at the base terminal. This current is sometimes referred to as the control current. The reverse-active mode has the same working principle as the active mode only that here electric current flows in the opposite or reverse mode.
Check out the image below for a further explanation
Image source Physics and Radio Electronics
Transistor configurations for sensors
When used for current sensing applications, transistors are configured in two main ways. These are the transistor-base current configuration and the emitter current configuration. As we have stated, the base current in most applications is always considered as the control current. It will determine the amount of output current in the emitter and the collector terminals of a transistor.
So, how does the base current affect the current sensing capabilities of a transistor? When the transistor in in an active region, the base current will control the collector current. A resistor that is connected to the base terminal regulates the flow of current. In most cases, several resistors are connected in series. This implies that the current at the base terminal will also depend on the resistance value of the resistors.
By using the base-current sensing configuration, it is clear that there is a direct relationship between the base current (IB) and the collector current (IC). This direct connection is what gives the transistor the current-sensing capability. Of course, we have to keep in mind the inherent current gain will also affect the final value of the collector current.
Another common current-sensing configuration of the transistor is the base-emitter configuration, which is also known as the common emitter (CE) configuration. Here, the emitter works as an output terminal for the transistor.
In this configuration, the emitter junction is forward-biased meaning that there will be a smooth flow of current from the base terminal to the emitter terminal. On the other hand, the collector current is reverse-biased. However, the base will still act as a current controller thanks to the resistors that are connected in series.
Amplification in the current sensing
Regardless of the transistor configuration, a constant factor in the whole operation of the transistors as sensors is the amplification. Transistors are able to amplify the level of electric current. Using the internal current gain the output current will be higher than the input current.
Amplification is quite important especially when you want to know the exact current measurements that have been sensed by the transistor.
Which type of transistor is used for current sensing
Other than the transistor configurations, the type also matters when it comes to analyzing the application of transistors as current sensors. Both the bipolar junction transistors (BJTs) and Field Effect Transistors (FET) can be used as current sensors. In the BJT, the base will act as the current controller while in the FET, the gate will act as a current controller. Otherwise, the rest of the working mechanism remains the same.
There is no doubt that current sensing is a critical function in modern electronics. There are many ways in which transistors have proven to be reliable electronic components when it comes to current-sensing applications. The fact that it is an active electronic component means that it is part of eh circuit and hence will be directly connected to the electric current. So, if you are planning to build a current sensor, consider including
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