A transistor is one of electronic component and semiconductor device that can basically amplify the current. The transistor is like the relay and it has a switching operation. There are three different terminals, such as base, collector and emitter. When the current flow from base to collector with enough voltage at around 0.6v for its operation, the gate will open between collector and emitter and the higher current runs with the lower voltage. In addition, there are two different types of the transistor which are called NPN and PNP type.
Meter check of a transistor
Bipolar transistors are constructed of a three-layer semiconductor “sandwich,” either PNP or NPN. As such, transistors register as two diodes connected back-to-back when tested with a multimeter’s “diode check” function as illustrated in Figure 9.1. Low voltage readings on the base with the black negative (-) leads correspond to an N-type base in a PNP transistor. On the symbol, the N-type material corresponds to the “non-pointing” end of the base-emitter junction, the base. The P-type emitter corresponds to “pointing” end of the base emitter junction the emitter.
BIPOLAR JUNCTION TRANSISTORS
Here I’m assuming the use of a multimeter has a diode test function to check the PN junctions.
If your meter has a designated “diode check” function, and the meter will display the actual forward voltage of the PN junction and not just whether or not it conducts current.
Meter readings will be exactly opposite, of course, for an NPN transistor, with both PN junctions facing the other way.
Low voltage readings with the red (+) lead on the base is the “opposite” condition for the NPN transistor. If a multimeter with a “diode check” function is used in this test, it will be found that the emitter-base junction possesses a slightly HIGHER forward voltage drop than the collector base junction. This forward voltage difference is due to the disparity in doping concentration between the emitter and collector regions of the transistor; the emitter is a much more heavily doped piece of semiconductor material than the collector, causing its junction with the base to produce a higher forward voltage drop.
Knowing this, it becomes possible to determine which terminal is which on an unmarked transistor.
This is important because transistor packaging, unfortunately, is not standardised. All bipolar transistors have three terminals, of course, but the positions of the three terminals on the actual physical package are not arranged in any universal, standardised order.
Suppose a technician finds a bipolar transistor and proceeds to measure voltage drop with a multimeter set in the “diode check” mode. Measuring between pairs of terminals and recording the values displayed by the meter, the technician obtains the data in Figure 9.2.
-meter readings between terminals.
The only combinations of test points giving conducting meter readings are terminals 1 and 3 red test lead on 1 and black test lead on 3), and terminals 2 and 3 (red test lead on 2 and black test lead on 3). These two readings must indicate forward biasing of the emitter-to-base junction (0.655 volts) and the collector-to-base junction (0.621 volts).
Now we look for the one terminal common to both sets of conductive readings. It must be the base connection of the transistor, because the base is the only layer of the three-layer device common to both sets of PN junctions (emitter-base and collector-base). In this example, that terminal is number 3, being common to both the 1-3 and the 2-3 test point combinations.
METER CHECK OF A TRANSISTOR
Those sets of meter readings, the black (-) meter test lead was touching terminal 3, which tells us that the base of this transistor is made of N-type semiconductor material (black = negative). Thus, the transistor is a PNP with base on terminal 3, emitter on terminal 1 and collector on terminal 2 as described in Figure 9.3.
Please note that the base terminal in this example is not the middle lead of the transistor, as one might expect from the three-layer “sandwich” model of a bipolar transistor. This is quite often the case, and tends to confuse new students of electronics. The only way to be sure which lead is which is by a meter check, or by referencing the manufacturer’s “data sheet” documentation on that particular part number of transistor.
Knowing that a bipolar transistor behaves as two back-to-back diodes when tested with a
Diode test function is helpful for identifying an unknown transistor purely by meter readings.
It is also helpful for a quick functional check of the transistor. If the technician were to measure Using the Diode test function in any more than two or any less than two of the six test lead combinations, he or she would immediately know that the transistor was defective (or else that it wasn’t a bipolar transistor but rather something else – a distinct possibility if no part numbers can be referenced for sure identification!). However, the “two diode” model of the transistor fails to explain how or why it acts as an amplifying device.
Transistor Symbol and semiconductor construction shown below.Diode test function is helpful for identifying an unknown transistor purely by meter readings.
It is also helpful for a quick functional check of the transistor. If the technician were to measure Using the Diode test function in any more than two or any less than two of the six test lead combinations, he or she would immediately know that the transistor was defective (or else that it wasn’t a bipolar transistor but rather something else – a distinct possibility if no part numbers can be referenced for sure identification!). However, the “two diode” model of the transistor fails to explain how or why it acts as an amplifying device.
Fig 10
Identify the legs of your transistor with a multimeter. For identifying and testing purposes, refer to the representation shown above. Diode test (V) meter readings
Transistor number VBE VEB VBC VCB VCE VEC
NPN 0.708v 0v 0.706v 0v 0v 0v
PNP 0v 0.705v 0v 0.703v 0v 0v
The emitter terminal has slightly higher voltage.PNP 0v 0.705v 0v 0.703v 0v 0v
EXPERIMENT No. 7
Transistor as a switch
Components: 1 x Small Signal NPN transistor, 2 resistors.
Exercise: Connect the circuit as shown in Fig 12 and switch on the power supply.
Transistor as a switch
Components: 1 x Small Signal NPN transistor, 2 resistors.
Exercise: Connect the circuit as shown in Fig 12 and switch on the power supply.
Connect the multimeter between base and emitter.
Note the voltage reading and explain what this reading is indicating.
Note the voltage reading and explain what this reading is indicating.
0.8v The current flows from Base to Emitter with transistor's operation at around 0.6v and the lower current goes through Base to Emitter.
Connect the multimeter between collector and emitter.
Connect the multimeter between collector and emitter.
Note the voltage reading and explain what this reading is indicating.
0.06v The gate is opened between collector and emitter so the higher current flows from collector to emitter with lower voltage drop of 0.06v.
In the plot given below what are the regions indicated by the arrows A & B?
A(saturated) : The supply voltage will appear across the load collector current will flow between collector and emitter.
B(cut-off) : Base current is too low or zero. So collector current will not flow from collector to emitter.
The transistor operates correctly in the active region with an amplifier and a switching function. As a result, higher current flows between collector and emitter.
reference by http://moodle.unitec.ac.nz/file.php/1309/Theory/Lesson_4_BJT/Lesson_4_BJT.pdf
What is the power dissipated by the transistor at Vce of 3 volts?
P=0.013 X 3 =0.039W
What is the Beta of this transistor at Vce 2,3 & 4 volts?
EXPERIMENT No. 8
Summary: Vary the base resistor and measure changes in voltage and current for Vce, Vbe, Ic, and Ib. Then plot a load line.
Set up the following circuit on a bread board. Use a 470R for Rc and a BC547 NPN transistor.
Rb 47K Vbe 0.7v Vce 0.09v Ib 0.11mA Ic 6.17mA
Rb 220K Vbe 0.68v Vce 0.16v Ib 0.03mA Ic 6.02mA
Rb 270K Vbe 0.68v Vce 0.20v Ib 15.9μA Ic 5.95mA
Rb 330K Vbe 0.68v Vce 0.32v Ib 13.1μA Ic 5.71mA
Rb 1M Vbe 0.65v Vce 2.16v Ib 4.2μA Ic 2.02mA
Your voltage drop measurements across Vce should vary from below 0.3 v (showing the transistor is in the saturated switch region) to above 2.0 v (showing the transistor is in the active amplifier region) If this is not the case, you may have to try a smaller or bigger resistor at Rb. Talk to your teacher to get a different size resistor, and redo your measurements. Discuss what happened for Vce during this experiment. What change took place, and what caused the change?
When I changed five different resistors(Rb) from lower values to higher values, the Vce(voltage between collector and emitter) values also changed. When the Rb increased, the Vce also goes up.
Discuss what happened for Vbe during this experiment. What change took place if any, and what caused the change?
The Vbe dropped slightly when the resistor values are increased. However, the Vbe has constant values from base to emitter for transistor's operation at around 0.6v.
Discuss what happened for Ib during this experiment. What change took place, and what caused the change?
When I changed five different resistors(Rb) from lower values to higher values, the Ib values also changed. When the Rb increased, the Ib decreased. The current Ib is lower than Ic values. The small current runs through the base for an operating transistor.
Discuss what happened for Ic during this experiment. What change took place, and what caused the change?
When I changed five different resistors(Rb) from lower values to higher values, the Ic values also changed. When the Rb increased, the Ic dropped. The current Ic is higher than Ib values. This means that the gate between collector and emitter is opened so the higher current flows from collector to emitter.
Plot the points for Ic and Vce on the graph below to create a load line. Plan the values for so you use up the graph space. Use Ic as your vertical value, and Vce as your horizontal value.
Using Vbe on the Vce scale, plot the values of Ib so the finished graph looks similar to fig 13.
Vce is significantly large at 2.16v when the Rb is 1MΩ from the load line graph and figure. Ib will be nearly zero and no collector current will flow, this means that the transistor is cut-off. The other end of the load line Ib increases with maximum current at 0.11mA, then Vce will be nearly zero or close to it, this state is called transistor's saturation. The transistor is working in saturation with the highest current of base bias. In both cut-off and saturation, minimum power is consumed in the transistor. The transistor can be operated correctly in the active area with different values of Ib and transistor's amplification characteristic.
In addition, β(gain) value should be smaller for the chosen transistor. Because if the β is too high the heat will produce in a transistor and it will be damaged.
Hi,
ReplyDeleteI was using a portion of your post to study, and I wanted to correct you on Figure 10. The diodes in the diagram should be revered to accurately describe the pnp and npn junctions.
Source : http://www.circuitstoday.com/pn-junction-diode-characteristics