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Berkeley ELENG 105 - Single Stage BJT Amplifiers: Common Emitter

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ObjectiveMaterialsProcedureBiasing for Maximum GainAmplifier Properties (a.k.a. Two-Port Parameters)Emitter DegenerationThe World's Worst Speaker AmplifierUNIVERSITY OF CALIFORNIA AT BERKELEYCollege of EngineeringDepartment of Electrical Engineering and Computer SciencesEE105 Lab Exp er imentsExperiment 4: Single St age BJT Amplifiers: CommonEmitter1 ObjectiveDesign and analysis of analog amplifiers is not often done on the individual trans istor level, but instead bygrouping together known configurations of single transistor amplifiers to create a new amplifier with just theright properties for the application. Those properties are input impedance, gain, and output impedance.Before analyzing and building these robust multi-transistor amplifiers, however, you must first understandthe basics of the single transistor amplifier case.The objective of this lab is to familiarize you specifically with the s ingle stage BJT commo n emitter (CE)amplifier and all its wonderful properties. The next lab will cover the s ame properties of the common baseand common collector (a.k.a. emitter follower) cases. By the end of this lab and the next you will beginto see why these different configurations are useful and how they might be fit together to achieve specificproperties.2 MaterialsComponent Quantity2N4401 NPN BJT 18 Ω speaker 11 kΩ resistor 110 kΩ resistor 110 µF capacitor 110 kΩ potentiometer 1Table 1: Components use d in this lab3 Procedure3.1 Biasing for Maximum GainThe properties of a common emitter amplifier a re greatly affected by the DC bias point a nd specific biasingcircuitry. In this case we are using a voltage sourc e and a pull up resistor to bias the collector and theDC o ffset on the function generator to bias the base. In multi-transistor amplifiers, however, a transistor isgenerally bias e d using other transistors to acheive the greatest small signal gain.1. Build the circuit shown in Figure 1, a simple common emitter amplifier with no load attached. LetRC= 1 kΩ and VCC= 5 V. Note: You may want to us e the parameter analyzer test fi xture since you’lluse ICS to analyze the circuit.2. Using ICS, perform a sweep of VINfrom 0 V to 1 V and graph VOU Tvs. VIN. Using the plot, findthe DC bia s of the amplifier that a chieves the maximum gain. What is the gain? What are VINandVOU Tat the point of maximum gain?13 PROCEDURE 2−+VIN−vin+IINRCVCC+vout−Figure 1: CE amplifier3. Using a load line for the pull up resistor on a BJT I-V curve, explain why a BJ T has very low gain ifit is not biased in the forward active region.3.2 Amplifier Properties (a.k.a. Two-Port Parameters)The operational cha racteristics of a n amplifier can be quantified by three properties: input impedance, gain,and output impedance. These three properties are called the two-port parameters of the amplifier. The goalnow is to measure and to learn about these properties for the CE amplifier so that we may later on use thisgeneral knowledge in our high level design of multistage amplifiers.1. Sweep VINfrom 0 V to 1 V and plot IINvs. VINusing ICS. What is the input resistance when VINis biased to achieve the maximum gain?2. Using the function generator, apply a 20 mV peak-to-peak sine wave at the input with a DC offsetcorresponding to the DC bias po int giving the maximum gain. Measure the peak-to-peak amplitudeof the signal at the output us ing the oscillo scope. What is the gain measured with the o scilloscope? Isthe g ain measured with the oscilloscope roug hly the same as the gain you measured with ICS?3. Increase the amplitude of the input signal until the output waveform appears to be clipped (flattened)on the top and bottom. Why does clipping happen at the top? Why does clipping happen at thebottom? You may find graphs useful in your expla nation. Approximately how large can the outputwave be without clipping? The maximum amplitude without clipping is called the output voltageswing.4. Attach a 10 µF AC coupling capacitor and a 10 kΩ potentio meter at the output as shown in Figure 2.The potentiometer is used to simulate a loa d for the amplifier to drive. Why is the capacitor necessary?Hint: Consider the DC bias.5. Apply the same 20 mV peak-to-peak signal at the input and adjust the resistance of the potentiometeruntil the amplitude of the signal across the potentiometer is exactly half of the amplitude of the signalwith no load attached. The res istance of the potentiometer at this point will be equa l to the outputresistance.3.3 Emitter DegenerationEmitter degeneration refers to the place ment of a resistor on the emitter of a common emitter amplifier.Here we will analyze the effect of this resistance on the common emitter amplifier.1. Build the circuit in Figure 3, let VCC= 5 V, RC= 10 kΩ, and RE= 1 k Ω .3 PROCEDURE 3−+VIN−vin+RCVCCCRL+vout−Figure 2: CE amplifier with load attached−+VIN−vin+RCVCC+vout−REFigure 3: CE amplifier with emitter degeneration2. Bias the circuit for maximum gain. What is the DC operating point? What is the gain, Av, at thispoint? Is it more or less than the gain found without the degenera ting resistor? Give an explanationfor what’s going on in the circuit that causes this change in gain.3. Measure the input and output impeda nce s.4. Showing all steps, c alculate the theoretical values of the input and output impedances as well as thegain. Use β = 270, which is about what you should’ve mea sured in Exper iment 3.5. Why might emitter degeneration be useful?3.4 The World’s Worst Speaker AmplifierThis part will demonstrate the capabilities of your CE amplifier on a physically o bservable load.1. Apply a 1 kHz, 10 mV amplitude sine wave directly to the two terminals of the speaker. Qualitativelyobserve the volume.2. Build the circuit shown in Figure 2. Let RC= 1 k Ω and C = 10 µF.3. Bias VINfor maximum gain (you should have the bias voltage from a previous part) and apply a1 kHz, 10 mV amplitude sine wave at the input. Attach the speaker to the output and qua lita tivelyobserve the volume. Is it louder, quieter, or just as lo ud as when the speaker is direcly hoo ked up tothe function generator? Explain this


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Berkeley ELENG 105 - Single Stage BJT Amplifiers: Common Emitter

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