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Transistor Saturated: What It Is and How to Identify One

A Transistor Saturated

A Transistor Saturated

Source: Wikiwand

Transistor Saturated? What does it mean? Well, this term can only make sense if you’re a designer or engineer that’s quite familiar with transistor switches.

If not, we’ll break it down.

When you’re dealing with low DC devices, it’s normal to switch them Off or On. And you can achieve this by using transistor switches. But the transistor has to be in a saturated state to turn the DC device on or off.

Later in this article, we’ll discuss more on this topic, show you the operation modes, calculation, and more.

So, let’s get started!

What Is Transistor Saturation?

BD135 Transistor Saturation

BD135 Transistor Saturation

Source: Wikimedia Commons

Saturation happens when a system reaches its threshold or maximum value. So, a transistor operates within a saturated area when the current attains the highest specified value.

For instance, when you pour liquid into a glass till it reaches the brim—it’s in a saturated state. And it’s because the mirror can’t handle more drink. Also, when you modify the configuration of a transistor, it quickly changes its saturation level.

But, it’s crucial to note that when you configure transistors, the device doesn’t reach its saturation point. And it’s because the base-collector doesn’t remain in the reverse-biased mode. As a result, there will be distortions in the output signals.

What Are the Operation Modes?

Transistors operate in four different modes because they are non-linear devices. And the modes show the current flowing through them (i.e., from an NPN’s collector to emitter).

NPN Transistor

NPN Transistor

Further, if you want to know a transistor’s mode, you have to pay attention to the relationship and voltages of the three pins.

So, VBC is the voltage that moves from the base to the collector, and the VBE refers to the current moving from the floor to the emitter. That said, the operation modes include:

Saturation Mode

When a transistor is in saturation mode, it’s “On.” Plus, it behaves like a short circuit between the collector and emitter.

NPN Emitter

NPN Emitter

Source: Wikimedia Commons

Also, this mode makes the transistor’s diodes become forward biased. And forward biased is when the VBE and VBC are more than zero. Additionally, it means that VB is higher than VC and VE.

In other words, for a transistor to enter saturation, the VBE has to be higher than the threshold voltage. You can represent voltage drop with a few abbreviations like Vd, Vth, etc., and the value differs between transistors and even temperature.

So, at room temperature, we can estimate that many transistors have a voltage drop of about 0.6V.

Further, it’s vital to note that you may not have excellent conduction between the collector and emitter. As a result, you’ll notice a small voltage drop at the nodes.

Manufacturers often represent this voltage in transistor datasheets as VCE(sat) (CE saturation voltage). And you can define VCE(Sat) as the voltage from the collector to an emitter that the transistors need for saturation.

The value of the VCE(Sat) ranges from 0.05 – 0.2V. And the deal shows that VC must be a bit higher than VE for the transistor to enter saturation mode. Plus, VC and VE must be less than VB.

Reverse-Active

The reverse-active mode happens when a transistor amplifies and conducts but the current moves in the opposite direction (from emitter to collector).

Transistor Amplifier

Transistor Amplifier

Source: Wikimedia Commons

So, for a transistor to be inactive reverse mode, the voltage at the emitter should be more than the base. And this voltage must be greater than the collector. In other words, VC<VB<VE.

Also, it isn’t easy to see manufacturers design the active reverse mode for an application. And it’s because this model doesn’t drive a transistor.

Active

The transistor’s VBC and VBE must be harmful and higher than zero in this mode, respectively. Also, it means that the base voltage must be higher than the emitter but lower than the collector.

So, the collector has to be higher than the emitter, i.e., VC>VB>VE. Interestingly, this model is the transistor’s most potent mode because it changes the device to an amplifier.

Hence, the current that moves into the base pin increases. As a result, the wind that moves into the collector exits the emitter.

Ic = bIB

Where:

Ic = collector current

b = amplification factor

IB = base current

Cut-Off 

This mode occurs when the transistor is off—which is the opposite of saturation. So, in this mode, the transistor resembles an open circuit because it’s void of collector and emitter current.

How do you make a transistor enter this mode? You can do this by ensuring that the emitter and collector voltages are more significant than the base voltage. In other words, the values of VBE and VBC have to be negative.

You can represent the cut-off mode like:

VC > VB

VE >VB

It’s vital to note that we referenced NPN mode transistors throughout the article. So, for the PNP transistor, you’ll have the opposite characteristic of the NPN. For instance, in the saturation mode of PNP transistors, current moves from emitter to collector.

Also, you can reference the table below for a better understanding:

NPN MODEVOLTAGE RELATIONSPNP MODE
ReverseVE > VB > VCActive
Cut-offVE > VB < VCSaturation
SaturationVE < VB > VCCut-off
ActiveVE < VB < VCReverse

How to Calculate Transistor Saturation

It’s easy to calculate the transistor saturation when there’s a curve you can study. So, if your curve shows that the voltage level is at 0V while the current is relatively higher—use Ohm’s law.

That way, you’ll be able to determine the resistance between the pins (collector and emitter) of the transistor like this:

RCE =  VCE           0 V

       ——   =   ——   = 0 W

             IC             IC(Sat)

What if you need to determine the approximate saturation collector current for a transistor in a circuit? You can get that by assuming a corresponding short circuit value across the device’s CE (collector-emitter). Then put it in the formula above. You can put VCE as 0V and calculate for VCE(Sat).

Also, if the circuit has a fixed-bias configuration, you can apply for a short course. Consequently, RC (voltage across) will be equal to VCC. And you can express the condition like below.

  • IC(Sat)  =  VCC/RC 

How Do You Know if a Transistor Is Saturated?

Operating a transistor at saturation isn’t easy, but it’s possible. Also, it’s crucial to set your operation within the active region if you want to operate your transistor-like amplifier. Here are proven methods of knowing a saturated transistor:

1. By carrying out an actual measurement

2. Doing simulation—a better method than the previous one

3. Computation—an old method that’s cheap and without limitation. One of the ways you can use this method is by assuming that the circuit is saturated. With that, solve for the maximum gain of the course. Then, relate it to the minimum current progress of the device.

Wrapping Up

In reality, there are different ways you can identify transistor saturated. After all, that’s the only way a transistor will work as a switch to regulate a low DC voltage.

Also, it comes with four modes of operation, and the conditions differ for NPN and PNP transistors. Do you have questions or concerns about saturated transistors? Please feel free to contact us.

Hommer Zhao
Hi, I am Hommer, the founder of WellPCB. So far, we have more than 4,000 customers worldwide. If you have any questions, you can feel free to contact me. I really appreciate any help you can provide.

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