Faculty of Engineering & Technology

FACULTY OF ENGINEERING & TECHNOLOGY

LAB SHEET

COMMUNICATIONS ELECTRONICS

EEE3096

TRIMESTER 3 (2013/2014)

Note : Students are advised to read through this lab sheet before doing experiment.

Lab CE1 & CE2: Communication electronics system

design

Objectives

To sharpen students’ design skills, which after completing the two lab phases, students will be able to design, construct and perform analysis of the basic RF Class-A amplifier and phase locked- loop based on given specifications.

To expose students to the electronic circuit test and measurement equipments and test station used in analogue circuit industries. During the course of the lab experiments, students will be going through RF amplifier and phase locked-loop design steps, from theoretical calculation to practical circuit construction and actual designed circuit performance analysis.

Upon completion of the lab experiments, student should be able to:

• Apply the principle and design of RF amplifier and phase locked-loop circuitry for communication electronics system design.
• Design and construct a practical communication electronics system.
• Analyze the circuit performance on designed electronic systems using electronic circuit test station.

Programme Outcomes (% of contribution)

• Ability to acquire and apply fundamental principles of science and engineering.

• Capability to communicate effectively.

• Acquisition of technical competence in specialized areas of engineeringdiscipline.

• Ability to identify, formulate and model problems and find engineering solutionsbased on a system approach.

• Ability to utilize systematic approach in design and operational performance evaluation.

• Ability to work independently as well as with others in a team.

LAB Sheet CE1:

Design of RF Class-A Tuned Amplifier

1.0 Objective

There are many types of RF amplifier configuration using bipolar junction transistor. The Class-A amplifier is the most general. This experiment is designed to familiarize the student to the practical aspects of the design of a Class-A Tuned Amplifier. Issues such as the choice of components, the biasing requirement and prediction of the circuit performance will be illustrated.

2.0 Equipment Required

1. Function generator.

2. 100MHz bandwidth dual channel oscilloscope.

3. 5V power supplies.

4. A piece of breadboard and some single core wires.

5. A digital multi-meter.

2.1 Components Required (Quantities shown within parenthesis)

1. Resistors – 10K (1), 4.7K (1), 1K (1), 150 (1) and 470 (1).

2. Capacitors – 0.1uF (3), 1uF (1) and 100pF(1).

3. NPN Transistor – 2N3904.

4. Inductor – 4.7uH.

3.0 Introduction

An amplifier is a circuit that provides an output signal that is an enlarged replica of the input. They faithfully reproduce input at a higher power level. An amplifier is classified according to how it is biased. A Class-A transistor amplifier is biased so that the transistor conducts continuously. The transistor of the Class-A amplifier is biased in the active region during the whole duration of operation. The operation of a transistor in active region is almost linear, thus the Class-A amplifier is a linear amplifier. Figure 1A and 1B shows two types of Class-A transistor amplifier circuits – the tuned and un-tuned amplifier.

Figure. 1A Un-tuned Class A amplifier Figure. 1B Tuned Class A amplifier

3.1 Difference between Tuned and Un-tuned Class-A Amplifier

An un-tuned amplifier has a wider bandwidth than a tuned amplifier. This means an un-tuned amplifier will enlarge an input signal for a wider range of frequencies. Thisis illustrated in Figure 2 for the comparison of the amplifier voltage gain.

Figure 2 – Typical frequency response of tuned and un-tuned amplifier.

To implement a tuned amplifier, a resonance network such as the parallel RLC tank network is used. Such a network has high impedance at the vicinity of the resonance frequency. The voltage gain of an amplifier is dependent on its load impedance. If the parallel RLC tank network is used as the load impedance, the gain of the amplifier will be very high when the input signal frequency is near the RLC network resonance frequency fo. At other frequencies, the RLC tank network has low impedance, thus forcing the amplifier to have low voltage gain.

Figure 3 – Parallel RLC tank network.

The bandwidth of a tuned amplifier using the parallel RLC tank network invariably depends on the bandwidth of the network. The bandwidth of the parallel RLC tank network in turn depends on its parallel resistance R. The higher the value of R the more narrow the bandwith (as illustrated in figure 4). This means the amplifier will become less sensitive at other frequencies.

The bandwidth of a tuned circuit such as the parallel RLC network is defined as the amount of frequency deviation from the resonance frequency fowhere the powerdelivered to the circuit is one-half that delivered at resonance. It can be shown fromequation (1) and (2) that:

Q is known as the quality factor of the parallel RLC network. We observe that the higher the Q of a tuned network, the smaller will be the bandwidth. A high Q tuned network is synonymous with low loss. In the case of the parallel RLC network, the larger the value of R, the smaller the loss and hence a higher Q is obtained.

Figure 4 – Frequency response of the RLC tank network with different value of R.

3.2 RF Amplifier Design Considerations

Great care must be taken in designing an amplifier that could function properly inthe megahertz range. The frequencies between 1MHz to 300MHz are collectively knownas radio frequency (RF). Frequencies higher than 300MHz are categorized as microwavefrequency (MF). An RF amplifier is an amplifier that would work well into the radiofrequency region. RF amplifier is difficult to design, as most of the components will notapproach their ideal characteristics at radio frequency. A number of considerations indesigning an RF amplifier:

• Parasitic elements such as stray capacitance and inductance associated with the components. Parasitic elements exist for all practical components such as resistors, capacitors and inductors. It also exists between the conductors used to connect the components. The parasitic element of a practical transistor is illustrated in Figure 5.
• Stability of the amplifier. The parasitic inductance and capacitance associated with the components used to construct the amplifier will result in unwanted feedback. This could result in oscillation within the circuit, rendering the circuit useless.
• The characteristic of the transistor depends on operating frequency. Particularly the input impedance and the small signal current gain βof the transistor decreases as the operating frequency is increased. This would decrease the overall gain of the circuit at radio frequency. This effect is evident for the un-tuned amplifier frequency response as shown in Figure 2. The drop in voltage gain as frequency is increased is due to the decrease in βof the transistor.

Figure 5 – Parasitic elements associated with a practical transistor.

3.3 Transistor at High Frequencies

At low frequencies, it is assumed that the transistor responds instantly to changes of input voltage or current. Actually, such is not the case, because the mechanism of the transport of charge carriers from Emitter to Collector is essentially one of diffusion. The common-emitter circuit is the most important practical configuration. Hence, we seek a CE model that will be valid at high frequencies. Such a circuit is called the hybrid-π, or the Giacoletto model, it is shown in Figure 6. The hybrid-πmodel gives a reasonable compromise between accuracy and simplicity up to microwave frequencies. A detailed explanation of the model can be found in Gray & Meyer, chapter 1, MillmanHalkias, chapter 11 or Collins, chapter 10.

Figure 6 – The hybrid-πmodel for a transistor in CE configuration.

4.0 Tuned RF Class-A Amplifier

Most RF Class-A amplifiers are of the tuned type. The reasons for using tuned amplifier are as follows:

• Higher voltage gain – The gain is dependent on the value of the load resistor and the collector biasing resistor. If we could increase the value of this resistance, higher gain would be obtained. However, by increasing the value of RC, we interfere with the biasing point of the transistor. If RC is too large, we might even bias the transistor into saturation. By using parallel RLC network, the transistor will see a large resistance of R at the resonance frequency while at dc the collector resistance is zero.
• Less susceptible to noise – If the gain of the amplifier is high, electrical noise in the input will be amplified to a perceptible level. A highly tuned amplifier will reject noise power that is not within its bandwidth. Since the noise power within the bandwidth is very small, a high signal-to-noise ratio at the output can be attained.
• Enable transformer coupling and impedance transformation – Sometimes the load can be coupled to the amplifier through mutual inductance. Thus, the inductor L in the RLC network can be magnetically coupled to another inductor, forming a transformer. The transformer has the advantage of being able to reflect a low impedance load into a high impedance value at the RLC network. Usually transformer-coupling circuit is used to cascade two single stage amplifiers together to obtain larger gain. At radio frequency, the input impedance of the transistor is small due to Ceand CC in the hybrid-πmodel of the transistor. Thus, a transformer is used to reflect the small input impedance of the second stage into large impedance at the output of the first stage.

Analysis of tuned RF amplifier proceeds in the sequence as in the analysis for unturned amplifier. Instead of using RL for the load, the load is complex by virtue of the load inductor and load capacitor.

Figure 7 – Using transformer to connect two single stage amplifiers together.

In particular, at resonance, the complex load will become real. For instance the impedance of the parallel RLC network becomes R at resonance f = fo. The voltage gain for the tuned amplifier at resonance is given by:

5.0 The Experiment

Figure 8 – Class-A tuned transistor amplifier.

1. Connect the Class-A RF amplifier circuit as shown in Figure 8 using the prototype board (or breadboard as it is usually known among the electronic hobbyist community).

2. Be extra careful when building the circuit for high frequency operation. Keep the connecting wires on the prototype board as short as possible to reduce unwanted inductance and capacitance in your circuit. An example of the circuit construction is shown in Figure 9. Note the pin assignment on the general purpose NPN switching transistor 2N3904 in Figure 9.

DC biasing measurement [15 marks]

3. Temporarily remove the transistor from the prototype board. Use the digital multimeter to measure the exact resistance of RB1 and RB2. Reinsert the transistor after the resistance measurement is done. Calculate the voltage at the base of the transistor

using the resistive voltage divider formula . Then calculate the voltage at the emitter of the transistor VE using VE=VB-0.7.

4. Connect the circuit to a 5V-power supply. Do not connect the function generator to the input yet. Measure the voltages at VC , VB and VE at C, B, E pins of the transistor using the digital multi-meter.

Figure 9 – Photograph of the experiment setup.

5. Verify that VB-VE ≈0.7V.

6. Check if the calculated and measured VB and VE agree. If the values do not agree, can you explain why?

7. Estimate IC using .

8. Evaluate the transconductance gm of the transistor at T=25°Cusing

AC Measurement [15 marks]

9. Now connect the function generator to the input of the circuit as shown in Figure 9. Set the function generator to output a sine wave at a frequency of about 5MHz.

10. .Set the oscilloscope to dual trace mode. Using Channel 2 of the oscilloscope, probe the base of the transistor. Using Channel 1of the oscilloscope, probe the load resistor RL as shown in Figure 10. Set the trigger of the scope to internal, with the triggering signal from Channel 1. Adjust the trigger level on the oscilloscope to obtain a stable waveform. Also, adjust the vertical scale and horizontal scale of the oscilloscope to obtain the waveform as in Figure 11.

11. Adjust the output level of the function generator so that the peak-to-peak voltage swing is approximately 36mV or ΔVB = ±18mV. You would have to use the built-in attenuator of the function generator. Select attenuation of –40dB. Actually, we should keep the peak-to-peak voltage below 30mV. This will result in almost perfect sine wave obtained at the output (no harmonic distortion). However, for the current setup, large amount of noise is observed for signal voltage below 30mV, hence the compromise of 36mV.

Figure 10 – Probing locations on the circuit.

12. Now sweep the frequency of the function generator from 1MHz to 10MHz. Record the peak-to-peak voltage at the input (Channel 2) and output (Channel 1) and determine the magnitude of the voltage gain AV as a function of frequency.

Figure 11 – Sample waveforms as observed on Channel 1 and Channel 2 of the oscilloscope.

13. Plot a curve of AV versus frequency; estimate the resonance frequency of the amplifier from the curve. The input and output waveform should be approximately 180°out of phase during resonance. Furthermore the voltage gain AV is maximum when resonance occur, with the theoretical resonance frequency being approximated by equation (2), repeated here:

The above equation is accurate if the series resistance of the inductor L is negligible, or the Q of the inductor is high. At L = 4.7μH, the series resistance of the inductor is small and hence can be omitted.

14. Practical Hints - Usually there will be a significant deviation of the calculated resonance frequency to the measured resonance frequency. This is due to stray capacitance between the transistor collector and ground/VCC rails on the prototype board and the tolerance of the inductor LC and capacitor CC. Acceptable resonance frequency is between 5.5MHz to 7.5MHz. Sketch the input and output waveform at resonance frequency on your lab report.

15. Use the digital multi-meter to measure the actual resistance of the load resistor RL. Substitute the value of RL, gm (calculated in 8.) and AVat resonance into equation (6) to estimate the value of rce. Ignore base spreading resistance rb’b.

(7)

16. From the value of the estimated rce, find the Early voltage VA of the transistor2N3904 using.

Using the Hybrid-πparameters to Estimate Voltage Gain [15 marks]

17. Now change the load resistor RL to 470Ω.

18. Set the function generator output frequency at the resonance frequency measuredearlier. Adjust the output level of the function generator so that VB peak-to-peak isapproximately 36mV.

19. Measure VL peak-to-peak and determine the voltage gain of the circuit.

20. Measure the exact value of RL using the digital multi-meter. Using the value of rce,gm calculated earlier, determine the voltage gain of the amplifier using equation (6).Again, ignore the base spreading resistance rb’b. Compare the voltage gain AVobtained through measurement with the value obtained using equation (6).

21. Repeat 17. To 20. For RL=1470Ω. (1000Ωand 470Ωin series).Records the readings of VL, VB and AV at different RL(470Ω, 1000Ω, and 1470Ω) and comments.

22. Practical Hints - At radio frequencies above 10MHz, the typical voltage gain of a tuned Class A amplifier will be smaller than the value calculated using equation (6) due to reactance from capacitance Cc, Ceand stray capacitance between the leads of the transistor. Thus to have a reasonable gain at higher frequency, we could increase the value of RL, the parallel resistance of the RLC tank network or increase the transconductance gm. Usually RL is fixed, as it is the load resistor, the only recourse available is to increase gm. The gm can be increased by increasing the dc collector current IC. Hence, for this experiment you will notice that IC is in the range of 6-7mA. Typical IC for small signal amplifier is approximately 0.1-1mA. Usually in calculating the voltage gain of a transistor amplifier, rb’band rceare ignored. The base spreading resistance rb’bis more or less independent of the transistor biasing and is small enough to be omitted for practical application. However, rce, which is due to Early effect, is inversely proportional to IC. For a transistor with VA = 30V, rce=30kΩat IC=1mA and is big enough to be ignored. At IC=6mA, rceis only 5kΩ, its effect must be included in the AV formula of (6). The preceding steps illustrate a simple approach to estimating rce.

23. Finally, the circuit of Figure 8 using 2N3904 is capable of operating up to a frequency of 120MHz, although the effective voltage gain will drop to around -5.

Maximum Output Swing of the Tuned Class-A Amplifier [15 marks]

24. Change the load resistor RL to 1kΩagain. With the function generator output frequency at the resonance frequency, slowly increase the output level of the function generator until the output voltage waveform is clipped at the lower level (Figure 12). Determine the maximum output voltage swing and the lower voltage threshold.

25. Compare the lower voltage threshold with the emitter voltage VE measured in the dc measurement. This level should be slightly above VE (about 0.1V-0.3V). The reason this clipping occurs is when VC (the transistor collector voltage) approaches VE, VCE is reduced. As VCE approaches 0V, the transistor shifts from forward active region to saturation region. When the transistor saturates the impedance between collector and emitter is low, thus the output voltage at collector is effectively shorted to the ground through the bypass capacitor CE (Figure 8). This results in the clipping of the output voltage.

Figure 12 – Clipping at the output.

26. Therefore to allow maximum swing, the dc level VE of a Class A amplifier must be kept as low as possible, or alternatively a higher operating voltage can be used to shift the dc collector voltage higher.

Discussion [40 marks]:

1) During DC biasing measurement, if CEchanges from 0.1 µF to 1 µF, analyze the difference of the readings in VE (assume CE is ideal capacitor, no parasitic resistance or inductance)?

2) The input and output (VB and VL) waveform should be approximately 180°out of phase during resonance. Is VB leading VL or in the reverse way? Explain why.

3) Based on the information obtained in Step 21, evaluate the efficiency ηof the Class A amplifier with respect to different values of RL at 470Ω, 1000Ω and 1470Ω.

4)Analyze the power dissipation PDeffect of the Class A amplifier by comparing different values of RL= 470Ω, 1000Ω, and 1470Ω?

Hints: ; ; ;VL is the peak AC output voltage