What should the base resistor be for a bipolar transistor?
For hard saturation, engineers usually choose a value of 10. Remember that a bipolar transistor is a current amplifier, because a small amount of current “Ib” through the base controls a larger amount of current “Ic” flowing through its collector.
How are the characteristics of an NPN transistor calculated?
The actual position of the load line on the characteristics curves can be calculated as follows: Then, the collector or output characteristics curves for Common Emitter NPN Transistors can be used to predict the Collector current, Ic, when given Vce and the Base current, Ib.
How to calculate base resistance of a transistor?
Therefore, with simple transposition of the above formula, the following formula provides the base resistance: Rb = (Vi – Vbe) / Ib. To guarantee that the transistor operates in the saturation region, we multiply the base current by a factor of three.
What is the DC current gain of a base resistor?
This resistor determines the amount of saturation current I b (sat) flowing into the base junction, and that controls the amount of saturation current I c (sat) flowing through the collector and emitter junctions. For hard saturation, engineers usually use a DC current gain h FE value of 10.
How is the current flow in a collector related to the base current?
How large this current flow is depends upon a gain factor known as “h FE “, also sometimes called the DC current gain, and beta. Hence, the current flowing through the collector is proportional to the base current multiplied by gain, as shown by the formula below.
Why is the base of a transistor called a resistor?
As you can see, the current through these components is limited by the 1 k? “base” resistor, so called because it limits the amount of current that flows through the base of the transistor. Aside: The “larger” current through the LEDs isn’t actually all that much larger. Why?
What should the base resistor be for a BJT?
There’s a rule of thumb that you make the base resistor such that the base current is a good multiple (5, 10?) of the minimum base current needed to put the BJT into saturation – you can get this from the data sheets. You need to know V B E ( s a t) – but that’s in the data sheets too.
Why does a NPN transistor need a positive voltage?
An NPN transistor requires a positive voltage at the base junction to switch ON and control a load (RL) such as a low-voltage relay with a known resistance value. In these types of switching applications, we require it to behave as a switch and conduct fully in the saturation region.
What kind of resistor do I need for a switching calculator?
For switching applications, a ¼-watt resistor with 5-% tolerance usually works fine. In order to use this calculator, you will need to know the input switching voltage (Vi), supply voltage Vcc, and the load current iL. Use the Standard Resistor Values chart to find the nearest standard resistor value.