Experiment4 - EEE3007C0011 Electronics1Lab...

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Unformatted text preview: EEE3007C­0011 Electronics 1 Lab Monday 9:00 ­ 11:50 AM Experiment #4 Transistor AC Amplifiers By: Due Date: March 14, 2016 Objective: The objective of this experiment is to study AC transistor amplifiers. Specifically, the effects of resistor sizes on the Q­point, capacitors and their placement, and how to vary the maximum unclipped output. Equipment: ● ● ● ● ● ● ● Oscilloscope: Tektronix DPO 4034 Digital Oscilloscope Function Generator: Tektronix AFG3022 Dual Channel Function Generator Power Supply: Agilent E3630A 2N2222 Bipolar Transistors Breadboard Capacitors available in the laboratory Resistors available in the laboratory 1. Pre­Lab: As stated in the lab manual, the values to be used in the circuit shown in Fig. 1 are: V​ = 12V, R​ = 6.2kΩ, R​ = 1.8kΩ, R​ = 2.2kΩ, C​ =​ 1µF, C​ = 1µF, and C​ = 100µF. CC ​ C ​ E ​ L ​ B ​ ​ L​ E​ 1. In order to reach the desired I​ (≈ 1 mA) and maintain good bias stability, R​ and R​ CQ​ 1​ 2 were calculated to be 121.65kΩ and 35kΩ, respectively. 2. Using these values for R​ and R​ , the Q­point, with C​ included, was calculated to be 1​ 2​ E​ (3.988V, 1mA), and the maximum unclipped output voltage equal to 1.62V. 3. Using the same values of R​ and R​ , this time without C​ included, the Q­point remained 1​ 2​ E​ the same. However, the maximum unclipped output voltage increased to 3.42V. 4. In order to shift the operating point to the center of the AC load line, the values of R​ and 1​ R​ would need to be changed. It was calculated that with C​ included, the new values of 2​ E​ R​ and R​ were 109.6kΩ and 36.14kΩ, respectively. The resulting calculated maximum 1​ 2​ unclipped output voltage was equal to 2.81V. 5. Without C​ included, R​ and R​ were calculated to be 118.5kΩ and 35.26kΩ, E​ 1​ 2 ​ respectively. The maximum unclipped output voltage was 3.71V. 6. Finally, R​ was changed to 3.2kΩ, resulting in I​ equal to 1.035mA, which shows very C​ CQ​ little change to I​ . The fact that I​ was unchanged means that V​ will then be CQ​ CQ​ CE​ increased. Essentially, this shifts the AC load line. The next step was to confirm the calculated results above, using computer simulation. Figures P.1 – P.6 below show the simulation results of each of the steps above. Figure P.1 Using the values of R​ and R​ calculated in step 1 above, simulation results showed I​ to 1​ 2​ CQ​ be equal to 1.043 mA. Figure P.2 The simulation shows the Q­point in step 2 to be (3.644V, 1.043mA). Figure P.3 The simulation results show the Q­point to remain unchanged without C​ included. E​ Figure P.4 Figure P.4 shows V​ equal to 2.45V, which would correspond to the maximum CE​ unclipped output voltage, since the Q­point is at the center of the AC load line. The values of R​ 1 and R​ correspond to those found in step 4. 2​ Figure P.5 Figure P.5 shows V​ equal to 3.35V, which would correspond to the maximum CE​ unclipped output voltage, since the Q­point is at the center of the AC load line. The values of R​ 1 and R​ correspond to those found in step 5. 2​ Figure P.6 The simulation results with R​ equal to 6.2kΩ shows that I​ is equal to 1.079 mA. C​ CQ​ Figure P.6.1 The simulation results with R​ changed to 3.2kΩ shows that I​ is increased to 1.082mA. C​ CQ​ It is noticed that I​ is altered by a very small amount, and also that I​ is increased as the value CQ​ CQ​ of R​ decreases. C ​ 2. Experiment: In order to verify the validity of the simulation results, experiments were performed by constructing the circuit in Figure 1 and using the values of R​ = 121.65 kΩ and R​ = 35 kΩ such 1​ 2​ that I​ ~ 1 mA. For a sinusoidal signal frequency of 5 kHz, the maximum unclipped output was CQ​ measured, to be 1.52V on an oscilloscope, and is shown in Figure 2.1, corresponding to step 1 in the prelab. This measurement was repeated with C​ removed, resulting in a maximum unclipped E​ output voltage of 3.44V, and is displayed in Figure 2.2. Figure 2.1 The measured unclipped waveform for R​ = 121.65 kΩ and R​ = 35 kΩ with C​ in. The 1​ 2​ E​ unclipped voltage is 1.52 V. Figure 2.2 The measured unclipped waveform for R​ = 121.65 kΩ and R​ = 35 kΩ with C​ in. The 1​ 2​ E​ unclipped voltage is 3.44 V​ . PP​ Further measurements were taken using the calculated R​ and R​ such that the operating 1​ 2​ point is at the center of the AC load line. Figure 2.3 shows the maximum unclipped output voltage measurement, Figure 2.4 the measurement with C​ removed, and Figure 2.5 with R​ = E​ C​ 3.2 kΩ and C​ removed. Additionally, when R​ was changed, I​ was measured to be 0.95 mA. E​ C​ CQ​ Figure 2.3 The measured unclipped waveform for R​ = 109.6 kΩ and R​ = 36.14 kΩ with C​ in. The 1​ 2​ E​ maximum unclipped voltage is 3.28V​ . PP​ Figure 2.4 The measured maximum unclipped waveform for R​ = 118.5 kΩ and R​ = 35.27 kΩ with 1​ 2​ C​ not included. The maximum unclipped voltage is 3.84 V​ E​ pp Conclusion: In conclusion, we have not only observed the use of transistor circuits as AC amplifiers, but we have further delved into what conditions best optimize the performance of this amplifier. First, based off of the circuit conditions, optimization occurs by using R​ = 109.6 kΩ and R​ = 1​ 2​ 36.14 kΩ such that the operating point is at the center of the AC load line, leading to symmetric signal amplification over a better range before clipping occurs (see Figures 2.1 and 2.3). Furthermore, it has been observed that including C​ results in a lower unclipped output voltage, E​ but does not affect the value of V​ (see Figures 2.2 and 2.4). Lastly, upon changing R​ from 6.2 CE ​ C​ kΩ to 3.2 kΩ, the DC operating point (I​ ) is seen to decrease slighty when measured with the CQ​ DMM (from 1.02mA to 0.95mA). Changing the value of R​ hardly affects I​ , however, does C​ CQ​ affect V​ . R​ and R​ are responsible for altering the value of I​ . Discrepancies between CE​ 1​ 2​ CQ​ simulation and experimentation arise from the variation of values in the transistors. References: th​ Microelectronics­Circuit Analysis and Design, D. A. Neamen, McGraw­Hill, 4​ Edition, 2007, ISBN: 978­0­07­252362­1 ...
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