ONLINE LAB - 6

Name of the experiment: High frequency characteristics of BJT

Objective

The objective of this experiment is to study the behavior of the BJT I-V characteristics at high frequency from a remote place (online).

Concept of Online Education

Online Education (OE) refers to a mode of education and a system where the interacting learner and the teacher are separated by space where the interaction can be done through high speed internet. It is an alternative method of instructional process to the traditional or conventional method. It enables a large segment of the learners with necessary aptitude to learn more knowledge and professional competence. Since OE is a form if instruction, which is capable of catering for large number of students, it is impossible to deliver the instruction and teaching without the help of a PC and an  internet connection. The easily available internet connectivity is helping to distribute the content and teach the same to the distance learners. This is an effective media and has been extensively used for educational purposes to spread literacy or to give formal and non-formal education all over the world. In the present decade, the online media is dominating in distance education in the developed and developing countries.

In India, continuous efforts are being made to improve the quality and quantity of distance education and several educational commissions have examined and made recommendations for bringing about the required innovations to meet the needs of the distance education system. Moreover, several research studies have indicated that the effective use of new instructional strategies through communication and information technologies, which provide individualized instructions like Learning Module, Programmed Learning Material (PLM) and Computer Based Instructional (CBI) materials. These methods together with its allied communication technologies reach a large number of learners and also help in improving the quality of teaching-learning process in distance education. Now, with the introduction of digital technologies like telecommunication, Interactive television (I-TV) and Virtual conferences (Video conferencing, teleconferencing, audio conferencing and computer conferencing), the virtual learning was established in distance education for teaching, learning and evaluation. In that regard, recently developed online experimentation is playing an important role for online education in India.

Concept of Online Labs

Advancing technology has opened many doors in education. The next step in this direction is interactivity at teaching. Student is able to, not only to see what is involved, but he or she is able to learn from hands on experience. Using computers can be a very effective way of accomplishing this. Students are more motivated and can learn more effectively if they have the opportunity to conduct experiments. Experiments allow a student to compare reality with simulations, collaborate with each other, and give them opportunity to follow their curiosity. Experiments allow a student to compare reality with simulations, collaborate with each other, and give them opportunity to follow their curiosity. Unfortunately, many engineering courses do not include lab component because of significant expense and space considerations. In response to this, I-Lab created remote web accessible laboratories are providing a new framework of science and engineering courses. Remote laboratories allow for much more efficient use of laboratory equipment and give students the opportunity to conduct experiments from the comfort of his home, with an Internet accessible browser. These online Internet accessible labs are important in several learning situations. The first of these is the distance learning scenario. In this situation, learners execute a laboratory oriented course or exercise from their homes or places of employment. Individual learners are remote from each other so that collaboration is distributed. There are currently an increasingly large number of efforts to provide the online analog of the university classroom in various parts of the world. However, there are comparatively few efforts to provide the online analog of the university laboratory, as lectures are much simpler to implement in the Internet environment. However, laboratory learning is a key part of a well designed curriculum. As the number of distance learners and distance learning programs increase, the demand for online laboratory access will also increase. This could for example, also make them available to other national community colleges or partnering Universities and colleges all around the world. So, laboratory based learning experiences that traditionally have been possible only at universities with abundant funds for research are now accessible to many. Third scenario of application is integration of reality into live lectures and seminars. In this situation, teachers present to classroom audience a live (but remote) experiment or demonstration controlled by the instructor. In this scenario, the lab is brought online to the classroom. Economic, space, and cost issues are extremely important and must be considered in setting-up any distance as well as conventional learning environment. Online Laboratories hold promise of being up to three orders of magnitude cheaper to setup than conventional laboratories, requiring less space to run the experiments and being accessible to much larger audience and utilized round the clock.

Typical online Internet accessible laboratory consists of:

v      Lab device, instrument or pilot plant equipment for tele-presence showing the lab to remote users

v      Teleconferencing equipment or at least built-in chatting capabilities for collaboration among students and instructor

v      Control software allowing users to perform experiments, program lab devices and/or run pilot plant.

 Introduction to High frequency characteristics of BJT

The BJT, was the first three terminal device in solid state electronics and continues to be a device of choice for many digital and microwave applications. The BJT essentially consists of a back-to-back p-n diode. The emitter doping is much higher than the base doping to ensure that the current gain is high for the device. That is, a small base current change produces a large collector current charge. The basic static characteristics of a BJT show how voltage and current flow in a BJT occurs. Also, the biasing of the device can be learnt through various modes of operation like forward active, cutoff, saturation and reverse active. The Ebers-Moll model is used to get a clear picture of how the device is designed physically. This model is not used for device and circuit design.

High frequency behavior of BJT

In order to understand the full operation of the high frequency operation of a BJT, we need to go beyond the Ebers-Moll model, which neglected a number of important issues. An important application of bipolar transistor is the amplification of high frequency signals. For understanding the transistor performance in such applications, a small signal equivalent circuit is needed. The hybrid-pi model is one such description. The terminals E, C, B are referred to as extrinsic terminals of the BJT, where as E’, B’, C’ are known as intrinsic terminals.

                                                        Figure 1:- Cross-section of NPN transistor with parameters used in equivalent circuit.

A simplified version of the equivalent circuit model results if only the diffusion capacitance and resistance of the forward biased EB junction is included. The simple model is shown in figure.

                                                   Figure 2:- Hybrid pi-model for BJT

We shall briefly discuss the elements of the equivalent circuit.

·         r_e: this is the resistance associated with the emitter and depends upon the doping of the emitter and the mobility of the carriers.

·         C_π: since the EB junction is forward biased, the diffusion capacitance associated with the injection of minority electrons into the base can be quite important. By use of a narrow base, this capacitance is reduced.

·         r_π:  this Is the resistance associated with the forward biased EB junction and is dependent on the value of the injected emitter current.

·         C_je: this is the junction capacitance of the forward biased EB junction.

·         r_b: this is the base resistance and depends upon the base doping. To reduce r_b, the base doping must be high, but this adversely affects the emitter efficiency.

·         g_mV_b’e’: this represents the current source for the transistor.

·         R_o: this is the output resistance and is due to the finite slope of the I_c-V_ce curves. It should be as high as possible to ensure that the device current is controlled only by the input signal.

·         C_s: this represents the capacitance between the doped collector and substrate.

·         r_µ: this is the resistance of the reverse biased BC junction and should be very large.

·         C_µ: this is the junction capacitance associated with CB junction.

·         r_c: this is the resistance associated with the collector. Usually, the collector is doped lightly near the base and heavily further down to ensure a low value of r_c.

An important parameter representing frequency response is the cutoff frequency f_T related to the total delay time τ_ec representing the propagation of carriers from the emitter to the collector.

To achieve cutoff frequency, one needs narrow emitter stripes, large emitter current (to reduce r’_e), very thin base regions, and low parasitic capacitances. Of course, one needs to use material with superior transport properties. The value of f_T initially increases with increase in I_c due to decrease in τ_e, but it starts to decrease, which causes the base region to extend out into the collector so that τ_t starts to increase.

Several approximations can be made, which would make the hybrid-pi model much simpler to use. First, we identify that the CB resistance r_µ  is huge, and at a first order, it can be neglected. Next, the values of the capacitors used are of the order of pF and hence they would become important only when the operating frequency is of the order of MHz or more. For lower frequencies, the capacitors can be treated as open circuits. This approximation is called midband approximation. Finally, within the midband range, the resistances also can be neglected. Based on all these assumptions, the simplified hybrid pi model is shown in figure, which is also known as T-model of the transistor.

                                          Figure 3:- Simplified hybrid-pi model

                                     Fig: Output Characteristics of an n+pn BJT

The highest frequency at which the device can deliver a power gain of unity into a properly matched load. This frequency is called the maximum frequency of oscillation, f_max, and is obviously the upper limit beyond which the device is useless as an amplifier.  

Software(s) and Hardware(s) required

a.       PC with internet connectivity (preferably high-speed)

b.      Teamviewer software for remote user (free downloadable)

Procedure

Step 1 – Read the manual of your experiment from our Homepage link

Step 2 –  Register and Login, remember your “User ID” and “Password”. (Registration is required only for one time)

Step 3 – Read the procedure for doing the experiment and Click on the button to proceed

Step 4 -  Now, “log in” to connect the remote Hardware Setup (at Indian Institute of Technology, Kharagpur, India)

Step 5 - Click on the experiment of your choice (from left column)

Step 6 – Now answer the preliminary quizzes to be eligible for doing the experiment

Step 7 - click on “Book the experiment” to book the particular slot for your experiment (time and date)

Step 8 – Click on “Run Experiment” during your time and slot

 Step 9 – Give the “input parameters” as per requirement of the experiment and click on “enter”

 Step 10 – The output curve will appear on your screen gradually and automatically. The  numerical output data will be shown simultaneously.

 Step 11 – Take the graph and data for showing “Result” using other software

Step 12 - Analyze the data and graph.

 Step 13 - To Log out, click on the    and close the window.

Step 14 - Now type www.vit.ac.in/onlinelab to go the home page again.

Step 15 - You are ready for the next experiment

Results

The figure below shows the output characteristics of an n+pn BJT  device.

Conclusion

A look at various parameters that are in the hybrid pi model of the BJT shows how to extract the high frequency characteristics of the BJT. The simplified T-model circuit of the BJT can also be used in constructing high frequency amplifiers.

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