FACULTY OF ELECTRICAL AND ELECTRONIC ENGINEERING
LABORATORY INSTRUCTION SHEET
Name
1. Nur Nadiah binti Mazlan (CE200002)
2. Muhammad Nabil Farhan bin Shariffudin (CE200116)
Lecturer’s Name Dr. Mohamad Nazib bin Adon Course Code BEE 20801
Course Name
ELECTRONIC ENGINEERING LABORATORY I
Experiment No. 3
Experiment Title BJT TRANSISTOR (DC)
(DC) Date
1.0 OBJECTIVES
i) To understand the operation of bipolar junction transistor (BJT);
ii) To observe the effect of IB and RC on the quiescent point of BJT biasing circuit;
iii) To determine the forward current gain, .
2.0 THEORY
A. INTRODUCTION
An NPN BJT structure, illustrated in Figure 2.1 has of three terminals, Base (B), Collector (C) and Emitter (E).
Figure 2.1. Structure and symbol for NPN BJT
From its structure, BJT has two PN junctions, Base-Emitter (BE) and Collector- Base (CB). Depending on the bias condition of each junction, different modes of BJT operation are obtained as summarized in Table 1.
Mode BE Junction CB Junction Application
Cut-off Reverse-bias Reverse-bias Used in switching application Saturation Forward-bias Forward-bias
Active Forward-bias Reverse-bias Used if the transistor is to operate as an amplifier
Table 1. Summary of operation mode of BJT
When BE junction is forward biased, a diode-type action takes place at that junction.
Hence, when the applied voltage, VBE 0.7 V, current will flow. Figure 2.2 is used to demonstrate the relation of current and voltage of each terminal. The current
relationship is given by equation 1 while voltage relationship is given by equation 2.
(DC) Date
Figure 2.2. Current and Voltage of NPN BJT
As a current controlled device, its output (collector) current,
by a constant known as forward current gain (or ℎfe ), indicated in equation 3. Different transistor has different value of β and its value is also temperature dependent. For further detail, please refer to the datasheet.
Ic = βIB (3)
Another parameter of interest in the active region is , the or which is defined by: = IC / IE. (4)
To make a transistor acts as a linear amplifier, it must be operating in the active mode (see Table 1). To achieve this, the transistor must be properly biased with a DC voltage. The transistor voltage and current levels that are set by the biasing circuit are called for example . As is temperature dependent, hence, it is
important to design a stable biasing circuit, .
B. LOAD LINE AND MODES OF OPERATION
The load line can help us to visualize the characteristics of a transistor circuit. For common emitter circuit (for examples, the circuits shown in Figure 3.1 and 5.1), we can use a graphical technique for both BE and CE portions of the circuit. The graphical technique for BE portion is illustrated in Figure 2.3. It shows the piecewise linear characteristics for BE junction and the input load line. The input load line is obtained from Kirchhoff’s voltage law equation around the BE loop.
The load line and the quiescent base current change when either VBB or RB
change (or both).
For CE portion, the load line is found by writing the Kirchhoff’s voltage law equation around the CE loop. This load line is drawn on the collector characteristic curve (or also known as common-emitter transistor characteristics) as depicted in Figure 2.3. From there, we could see if the selected Q-point is the right one or otherwise.
(DC) Date
Maximum amplification could be achieved for a particular amplifier circuit if the Q-point is located in the middle as illustrated in Figure 2.4.
Figure 2.3. piecewise linear i-v characteristics and the input load line
Figure 2.4. Common-emitter transistor characteristics and the
(Figure 2.3 and Figure 2.4 are taken from: Microelectronics Circuit Analysis and Design by Donald A. Neamen, McGraw-Hill, 2007)
(DC) Date
3.0 PRELAB TASK
(Prelab must be done and shown to the lab instructor when needed before lab session start)
(i) Use the circuit shown in Figure 3.1 to obtain the collector characteristic of BJT transistor 2N3904. You may use any simulation tool (e.g. PSpice) to plot the
similar to the one depicted in Figure 5.1. Print out the programme code (or schematic circuit with the probes) used to obtain this collector characteristic curve.
Figure 3.1. Circuit for generating collector characteristic curves
4.0 EQUIPMENT LIST Transistor: 2N3904 Resistors and capacitors DC power supply Multimeter (with hfe
measurement) Ammeters (mA and μA) Breadboard and jumper wires
For an online experiment:
Simulation software (For example, Proteus, Multisim and PSpice)
Breadboard Image (to ensure that you know how construct the circuit as it is performed in the laboratory)
(DC) Date
5.0 PROCEDURE
For online experiment,
(i) Draw the circuit components and jumper wires on the breadboard image based on Figure 5.1. Also, draw the probes and connection to ammeter etc. in this breadboard.
This image must be attached in your report.
(ii) In the simulation software, simulate the circuit and place the ammeter at the right locations to take the necessary readings.
Figure 5.1
DETERMINING THE REGION OF OPERATION
(a) Construct the circuit shown in Figure 5.1 with RB = 150 k , RC = 5 k and VCC = 5 V.
(b) With the input signal, Vin = 0.00 V, measure the output signal Vout and record it in Table 2. Repeat this process to complete Table 2.
(DC) Date
(c) Plot the voltage transfer characteristic curve (Vout versus Vin). Please indicate the operating region of BJT on this voltage transfer characteristic curve.
(DC) Date
1. What can you say about the graph of voltage transfer characteristic curve?
Please relate it with the region of operation of the BJT transistor.
2. Based on what you learned, how would you explain the relation of the base current, IB and the collector current, IC as well as the base-emitter voltage, VBE in each operating region?
WHAT CAN YOU CONCLUDED FROM THIS EXPERIMENT?
5.1. Also, draw the probes and connection to ammeter etc. in this breadboard.
ii. In the simulation software, simulate the circuit and place the ammeter at the right locations to take the necessary readings.
a) Construct the circuit shown in Figure 5.1 with RB = 150k, Rc = 5k and Vcc = 5V.
b) With the input signal, Vin = 0.00V, measure the output signal Vout and record it in Table 2. Repeat this process to complete Table 2.
Vin (V) 6 0.5 0.7 0.8 1 1.5 2 3 4
Vout (V) 4.97 5.00 5.00 5.00 5.00 5.00 4.99 4.99 4.98 Vbe (Mv) 711.98 495.34 593.7 611.36 631.82 656.96 670.68 687.29 698.0 Ib (uA) 35.3 0.03 0.71 1.25 2.45 5.62 8.86 15.4 22.0 Ic (mA) 5.85 0.00 0.07 0.13 0.288 0.75 1.27 2.38 3.53
Vin (V) 5 0
Vout (V) 4.98 5 Vbe (Mv) 705.7 0 Ib (uA) 28.9 0 Ic (mA) 4.69 0
1. What can you say about the graph of voltage transfer characteristic curve? Please relate it with the region of operation of the BJT transistor.
The voltage transfer value influences the operation of the BJT transistor. The higher the Vin, the lower the value of Vout. Referring to the graph above, we could see that as The greater the value of Vin, the lower the curves get showing that the value of Vout decreasing.
2. Based on what you learned, how would you explain the relation of the base current, IB
and the collector current, IC as well as the base-emitter voltage, VBE in each operating region.
According to what I've learnt, the base current (IB) in tiny signal transistors is generally 1% of the emitter or collector current. As a result, the collector receives 99 percent of the emitter current. The direct current that travels through the collector of a transistor is known as collector current (IC). As for base-emitter voltage VBE, one of the limits on transistor operation is that this voltage (commonly referred to as the diode drop) remains constant at around 0.6 volts.
Current amplification can be achieved by a tiny change in VBE resulting in a huge change in collector current. Referring to Table 2, as Vin increases, all three IB, IC and VBE slowly increases as well.
able to observe the effect of IB and RC on the quiescent point of BJT biasing circuit. Last but not least, we successfully determine the forward current gain. There were many findings in this lab, as in order to make this experiment a success, we stumbled upon several obstacles that led us to new knowledge.
While the group's skills and expertise are nowhere near those of a specialist, designing the concept would mark the beginning of each member's career as a professional engineer.
