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Experiment #7: Design of Common Base Amplifiers Friday Group Dr. Somnath Ari Mahpour 10‐30‐09 Teddy Ariyatham Jayson De La Cruz Table of Contents Objective ......................................................................................................................................... 3 Tools ............................................................................................................................................... 3 Theory ............................................................................................................................................. 4 Preliminary Calculations ................................................................................................................. 5 Part 1 ....................................................................................................................................... 5 Part 2 ....................................................................................................................................... 5 Part 3 ....................................................................................................................................... 5 Discussion and Results ................................................................................................................... 7 Part 3 ........................................................................................................................................... 7 Part 4 ........................................................................................................................................... 8 Part 5 ........................................................................................................................................... 9 Part 6 ......................................................................................................................................... 10 Conclusion .................................................................................................................................... 11 Objective The purpose of this laboratory experiment was to design a common base amplifier using a bipolar junction transistor. Prior to doing so, it was imperative to follow the collector and base currents and beta values through the curve tracer. In previous experiments we had exposure to the curve tracer so this did not become an obstacle in our experiment. Using a series of methods such as the ten points (provided by the professor) and notes from the corresponding lecture class, the design was very straight forward, enabling the preliminary calculations portion to move through quickly. Tools - Oscilloscope - Functional generator - Power supply - Transistor: Q2N2222A - Capacitors: 10µF - Resistors: (Specific for each circuit design) Theory Using the curve tracer, the student is expected to find the corresponding beta and current values that match up to the bipolar junction transistor that is used in the lab. In this case, the students are instructed to use a 2N2222A transistor and must trace their own part. Using another student’s values could potentially cause trouble since each and every transistor was not created equally. All corresponding beta and current values can be different for each transistor since the fabrication process is not always exact. When tracing ones own transistor, experimental errors stay at a minimum (if none at all). Design calculations must be done by hand and then verified using PSPICE. Both methods are necessary to ensure that the circuit will function correctly. Preliminary Calculations Part 1 (Refer to calculations sheet) Part 2 (Refer to calculations sheet) Part 3 (Refer to calculations sheet) Figure 7.1a: PSPICE Model 2.0V 1.0V 0.0V -1.0V -2.0V -3.0V 4.00ms 4.05ms V(VSIG:+) V(C3:1) 4.10ms 4.15ms 4.20ms 4.25ms 4.30ms Time Figure 7.1b: PSPICE Simulation 4.35ms 4.40ms 4.45ms 4.50ms Discussion and Results Part 3 VIN (V) VOUT (V) AV Theoretical Experimental Percent Error 0.06 0.06 4.24 5.24 70.6 70.6 0.00% Comparing our experimental values with our PSPICE values, we found our calculated resistances to be very accurate. This means that our circuit values were calculated and implemented perfectly (with a zero percent error). You can see that our experimental output values are very similar, if not identical to our PSPICE output waveform, Figure 7.3. Our measured Av (output gain) matches our PSPICE output gain perfectly. Part 4 VIN (V) VOUT (V) Theoretical Experimental Percent Error 0.1 0.1 5.48 5.76 5.11% 2.0V 1.0V -0.0V -1.0V -2.0V -3.0V -4.0V 4.00ms 4.05ms V(VSIG:+) V(C3:1) 4.10ms 4.15ms 4.20ms 4.25ms 4.30ms 4.35ms 4.40ms 4.45ms Time Figure 7.4b: PSPICE Simulation We used PSPICE to measure the output voltage at which our Vout begins to show distortion, meaning the maximum voltage our amplifier is able to drive the output. In both our experimental and PSPICE circuits, we were able to drive the load resistor to around 5.8V. This surpasses the specified output swing of at least 3V. 4.50ms Part 5 Theoretical Experimental Percent Error V1 (V) 4.24 4.24 Resistance (Ω) 130 130 V2 (V) 2.17 2.24 3.23% Zi (Ω) 86.3 86.3 In experiment 6, we learned that we could essentially guess and check with various input resistors to find out what the input impedance of our circuit was. This was done by inserting an additional input resistor (Rn) with a value such that our Vn will be half the input voltage (Vin) of the original circuit. We know this because whenever a voltage is passed through 2 equivalent resistors in series, the voltage is divided in half. In Figure 7.5b, we see that our original PSPICE input voltage is 4.24V, and when a 130Ω resistor is inserted, the voltage across that resistor becomes 2.24V. Therefore we can conclude that our input impedance (Zin) is approximately 130Ω. This coincides with the generalization that common base amplifiers have low input impedance, which explains why they are commonly used as the 2nd stage in multi‐ stage amplifiers. 2.0V 1.0V 0V -1.0V -2.0V 4.00ms 4.05ms V(VSIG:+) V(C3:1) 4.10ms 4.15ms 4.20ms 4.25ms 4.30ms Time Figure 7.5b: PSPICE Simulation 4.35ms 4.40ms 4.45ms 4.50ms Part 6 Theoretical Experimental Percent Error V1 (V) 4.24 4.24 Resistance (Ω) 3000 3000 V2 (V) 2.09 2.4 0.01% Zo (Ω) 7670 7670 0.00% As with finding input impedance, we use a similar guess and check method to find output impedance. This is done by first measuring the output voltage with an open (infinite) load resistance. Next, we increase the original load resistance such that the output voltage (open load) is divided in half. Above in Figure (INSERT FIGURE HERE), we can see that inserting a 3kΩ resistor changes our output voltage from 4.24V to 2.4V. Therefore our output impedance (Zout) is approximately 3kΩ. Note: PSPICE did not work properly with respect to finding V2, therefore, the experimental V2 was used to calculate the Zi. 1.0V -0.0V -1.0V -2.0V 4.00ms 4.05ms V(VSIG:+) V(C3:1) 4.10ms 4.15ms 4.20ms 4.25ms 4.30ms Time Figure 7.6: PSPICE Simulation 4.35ms 4.40ms 4.45ms 4.50ms Conclusion In this laboratory experiment we modeled a common base amplifier with little errors and ease of design. The most important factor to the beginning of this laboratory experiment was to ensure that the proper curve tracer values were found. In addition, the ten steps were very helpful in designing the circuit in a fast manner. In real world applications people find common bases to be an integral part of the basic amplifier. They are the fastest and consist of a very high bandwidth. In future experiments, the laboratory manual will take the student through more complicated amplifiers that require a bit more skill to design.