How is the saturation of a BJT determined?

How is the saturation of a BJT determined?

Saturation (for a BJT) is defined in several ways, but generally it relates to the collector-emitter voltage V CE. Here is an LTSpice simulation of a 2N4401 transistor driving a 160 ohm load with a 5V supply, which corresponds to about 30 mA collector current with the transistor turned ON.

What are the characteristic curves of a BJT?

In the linear (or active) region the curves would ideally be horizontal straight lines, indicating that the collector behaves as a constant current source independent of the collector voltage, as illustrated in the hybrid-π model (i. C = βi. B).

How does current flow in a BJT relay?

When the controlling device’s output is high, current flows through RB into Q1’s base, biasing Q1 into conduction. Current flows from V1, through the load, into Q1’s collector, out Q1’s emitter, and back to 0V. The load is energised: an LED lights up, or a relay activates and closes its contacts.

How to calculate the saturation of a transistor?

Divide the maximum collector current (I C) by the minimum current gain (h FE) at that collector current. For example, say I C (max) = 30 mA and h FE (min) = 83 at I C = 30 mA. The result is that I B (nominal) is about 360 µA. Multiply I B (nominal) by a “saturation factor” – a number between 2 and 10 – to ensure that the transistor saturates.

What do you need to know about transistor saturation?

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. The term saturation refers to any system where the specification levels have attained the maximum value.

When do you use a bipolar junction transistor?

When a bipolar junction transistor (BJT) is used to switch a load (e.g. a relay, an LED, a buzzer, a small motor, etc) ON and OFF, it is most often operated as a “saturated switch”. This article explains saturation in BJTs – why it is used, and how to calculate the base resistor to ensure saturation.

Is it possible to create a non saturating BJT switch?

This could damage the BJT. The snubbing diode effectively short-circuits the winding when it reverses voltage polarity, preventing the large spike. The remainder of the time the diode is reverse-biased and effectively out of the circuit. It is also possible to create a switch or driver that is non-saturating.

How does a BJT switch work with a driver?

With the driver, the logic circuit will only need to supply base current, not LED current. Here is how it works: If the logic input voltage is zero, there will be no base current. This means that there will be no collector current and therefore the LED will be off. At this point the BJT is in cutoff.

How does a saturated NPN transistor work?

Since the collector of a NPN will act like a current sink and in saturation the external circuit isn’t giving it as much current as it could pass, the collector voltage will go as low as it can. A saturated transistor typically has around 200mV C-E, but that also can vary a lot by the design of the transistor and…

Which is the most important characteristic of a BJT?

In order to distinguish these regimes we have to look at the i-v characteristics of the device. The most important characteristic of the BJT is the plot of the collector current, IC, versus the collector-emitter voltage, VCE, for various values of the base current, IBas shown on the circuit of Figure 6.

What are the parameters of a BJT amplifier?

For the design of the amplifier, the 3 parameter values required are VceSAT, ro and β. Derived from the transistor characteristics curve shown in BJT Figure 2 above, one can set an approximate Q-point (VCE and IC) in the active region and measure ro and β. We will solve for Vce and estimate IC.

When does a transistor go into saturation what happens?

A transistor goes into saturation when both the base-emitter and base-collector junctions are forward biased, basically. So if the collector voltage drops below the base voltage, and the emitter voltage is below the base voltage, then the transistor is in saturation.