Initially when no current is flowing through base, the resistance across CE is very high that no current flows through it. This current is negligible since they are in pA or nA.For better understanding on current control, a transistor can be considered as a variable resistor across collector(C) and emitter(E) whose resistance varies based on the current through the base(B). Theoretically the switch is completely open but practically a leakage current flow can be observed. Thus below is the circuit with 12V to base same as that to emitter with respect to ground during which the switch is OFF state. Calculating the base current and the corresponding resistor to be used.įor complete saturation the base current is approximated to 2.5mA (Not too high or too low). In order to drive the transistor into saturation condition sufficient base current has to be drawn out such that the transistor is completely ON. In this case it will be 200mA (Parallel LEDs or loads) and resistor = 60 Ohms.Ģ. Find the collector current wiz the current consumed by your load. The corresponding saturation voltages and base currents are also available.ġ. From the datasheet the maximum continuous collector current is -600mA and corresponding gain(hFE or β) is given in datasheet as test condition. The first important thing to bear in mind to use a current limiting resistor at base. To explain with a PSPICE model, PN2907A transistor has been selected. The application of an PNP transistor is to work as a high side switch. Cut-off region,IC= 0 – Switch operation (Completely OFF) Saturation region, IC= Saturation current – Switch operation (Completely ON)ģ. Active region, IC=β×IB– Amplifier operationĢ. Operating region versus Mode of operation:ġ. The PNP transistor can act as a switch and an amplifier. Besides, the current from emitter to collector starts to flow, provided the voltage VCE is applied at collector terminal. Hence, the electrons flow towards the positive terminal and the base current flows (IB) is opposite to the electron flow. When the potential further decreases below 0.7V, the barrier voltage is reached and the diffusion occurs. When a negative potential VBE is applied across Base-Emitter junction decreasing from 0V, the electrons and holes start to accumulate at the depletion region. They are collector-base junction(CB) and base-emitter junction. The emitter region is heavily doped when compared with collector region.
The construction of pnp transistor is that the collector and emitter regions are doped with p-type material and the base region is doped with small layer of n-type material. Mostly the common type of transistor used is silicon because it is the most abundant element on the earth after oxygen. The barrier potential for a silicon transistor is 0.7V at 25☌ and 0.3V at 25☌ for a germanium transistor. The proportional constant is the Gain (β).Īs discussed above, the transistor is a current controlled device which has two depletion layers with specific barrier potential required to diffuse the depletion layer.
When a current flows through the diode D2 from emitter to base, the diode D1 senses the current and a proportional current will be allowed to flow in the reverse direction from emitter terminal to collector terminal provided ground potential is applied at the collector terminal. The diode D1 has a reverse conducting property based on the forward conduction of diode D2. The most important application of PNP transistor is high side switching and Class B combined amplifier. An unbiased transistor or a transistor without potential applied at the terminals is similar to two diodes connected back-to-back as shown in figure below. As the power rating of the transistor increases necessary heat sink need to be attached to the body of transistor. Mostly all PNP transistors are of above pin configuration. Let us consider the two examples of PNP transistors – 2N3906 and PN2907A, shown in the images above.īased on the fabrication process the pin configuration may change and these details are available in corresponding datasheet of the transistor. In this tutorial we will talk about the PNP transistors. There are two types of BJT - NPN and PNP transistors. Identifying the terminals of a transistor requires the pin diagram of a particular BJT part. BJT is a three terminal device with Collector (C), Base (B) and Emitter (E). “Two polarities”is abbreviated as bipolar, hence the name Bipolar junction transistor. The first bipolar junction transistor was invented in 1947 at Bell laboratories.