In the previous post we learned BJT biasing, in this article we will learn what is transistor or BJT saturation and how to determine the value quickly through formulas and practical evaluations.
What is Transistor Saturation
The term saturation refers to any system where the specification levels have attained the maximum value.
We can take the example of a fully wet sponge, which may be in its saturated state when there's no space in it to hold any further liquid.
A transistor may be said to be operating within its saturating area, when the current parameter reaches the maximum specified value.
Adjusting the configuration may result in quickly changing the saturation level of the transistor.
Having said this, the maximum saturation level will be always as per the maximum collector current of the device as outlined in the datasheet of the device.
In transistors configurations it is normally ensured that the device does not reach its saturation point, since in this situation the base collector ceases to be in the reverse biased mode, causing distortions in the output signals.
We can see an operating point within the saturation region in the figure 4.8a. Observe that it is that specific region where the joint of the characteristic curves with the collector-to-emitter voltage is lower than VCEsat or at the same level. Also, the collector current is comparably high on the characteristic curves.
How to Calculate Transistor Saturation Level
By comparing and averaging the characteristic curves of Fig 4.8a and 4.8b, we are able to possibly achieve a quick method of determining the saturation level.
In Fig 4.8b we see the current level is relatively higher while the voltage level is at 0V. If we apply Ohm's law here, we are able to calculate the resistance between the collector and emitter pins of the BJT in the following manner:
A practical design implementation for the above formula can be seen in the fig 4.9 below:
This implies that whenever it is required to quickly evaluate the approximate saturation collector current for a given BJT in a circuit, you may simply assume an equivalent short circuit value across the collector emitter of the device and then apply it in the formula for getting the approximate collector saturation current. Put simply, assign VCE = 0V and then you can calculate VCEsat easily.
In circuits with fixed-bias configuration, as indicated in Fig 4.10 a short circuit could be applied, which may result in a voltage across RC equal to the voltage Vcc.
The saturation current developing in the above condition could be interpreted wit the following expression:
Solving a practical example to find the saturation current of a BJT:
If we compare the above result with the result that we acquired at the end of this post, we find that the result ICQ = 2.35mA is by far lower than the above 5.45mA which suggests that normally BJTs are never operated in the saturation level in circuits, rather at much lower values.