When the transistor is to be used as a switch, it is used either in cutoff (switch is off, or an open circuit) or saturation (switch is on, or very close to a short circuit). This is also called the linear region.Ĭutoff (v BE <0.6 volts). This is also called the "on" state.Īctive region (v CE >0.2 volts, v BE volts). We can now define the three regions of operation for the transistor: If the voltage at the base is too low to turn the base-emitter diode on, then there is no current. This model begins to fail when the collector voltage is approximately equal to the emitter voltage (actually v C-v E=v CE At this point the device no longer continues to amplify the signal. However you should be aware of shortcomings of this model: Thus the transistor can be thought of as a current amplifier device - the current at the output (collector or emitter) is b times large than the current at the input (base). The emitter current is the sum of the base and collector currents, but since the collector current is so much larger than the base current it is common practice to equate the collector and emitter currents. The quantity b (usually called h FE in transistor spec sheets) is a characteristic of the individual transistor and is typically in the range from 100-500 for the types of transistors we will be using. The current into the collector is b times larger than the base current. If the base is at higher ( volt) potential than the emitter then a current i B will flow into the base. The models we are going to use are shown below.Ĭonsider first the NPN transistor shown at the left. As with the diode there are many different models for the transistor. They have three terminals the Base, the Collector and the Emitter, labeled B, C and E in the drawing. On an NPN transistor the arrow is Not Pointed i N.Īs you can see the two transistors look very similar. The transistors we will be using are called bipolar transistor (in contrast with field effect transistors, or FETs, that you will see later in the semester) and come in two flavors NPN and PNP. Transistors are a bit more complicated than diodes in that they are three terminal devices. Note: this section is taken from Prof Cheever's website. The parts taken from the lab website have grey backgrounds. you can find the full lab description at Prof Erik Cheever's website. In order to make reading this lab easier, the lab procedure and out work are integrated. Finally we looked at a pumped op-amp and made slight modifications to improve performance. We also looked at voltage regulators and improved regulators from our previous labs using transistors. We looked at the common emitter amplifier and starting from basic versions, we built more efficient circuits. This link is the main feature of transistor action.In this lab, we primarily focused on the operation of bipolar junction transistors. Therefore if a transistor has a Beta value of 50, then for every 50 electrons flowing between the emitter-collector terminals one electron will flow from the base terminal.īy combining the expressions for both Alpha, α and Beta, β the current gain of the transistor can be given as:Īs seen from the equations above, electron mobility between the Collector and Emitter circuits is the only link between these two circuits. Beta values normally range between 20 and 200 for most general purpose transistors. NPN transistors are good amplifying devices when the Beta value is large. The current gain of the transistor from the Collector terminal to the Base terminal is signified by Beta, ( β ). The current gain of the transistor from the Collector terminal to the Emitter terminal, Ic/Ie, is a function of the electrons diffusing across the junction. The ratio of the collector current to the emitter current is called Alpha (α). Since the physical construction of the transistor determines the electrical relationship between these three currents, (Ib), (Ic) and (Ie), any small change in the base current ( Ib ), will result in a much larger change in the collector current ( Ic ). Note: “Ic” is the current flowing into the collector terminal, “Ib” is the current flowing into the base terminal and “Ie” is the current flowing out of the emitter terminal. The current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as A nother way to display a NPN Transistor is shown in Figure 2 below. The Base terminal is always positive with respect to the Emitter. The voltage between the Base and Emitter ( V BE ), is positive at the Base and negative at the Emitter. \): NPN Transistor schematic.įor a bipolar NPN transistor to conduct the Collector is always more positive with respect to both the Base and the Emitter.
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