ObjectiveMaterialsProcedureGenerating a differential signalDifferential pair with resistive loadDifferential pair with active loadSPICE AnalysisUNIVERSITY OF CALIFORNIA AT BERKELEYCollege of EngineeringDepartment of Electrical Engineering and Computer SciencesEE105 Lab Exp er imentsExperiment 10: Differential Amplifiers1 ObjectiveDifferential amplifiers are designed to amplify the difference between two signals. Differential amplifiers arethereby able to reduce noise that is common to both inputs, only amplifying the differential signal thatwe’re interested in. We can quantify the differential-mode versus co mmo n-mode gain in a quantity calledthe c ommon-mode rejection ratio (CMRR). Differential amplifiers also lend themselves to use in feedback,though we will not explore that usage in this lab. A typica l differential amplifier with a single-ended outputthat you are fa milia r with is the op-amp.2 MaterialsFor this lab, assume all NPN transistors are identical 2N3904 BJTs and all PNP transistors are identical2N3906 BJTs.Component QuantityLM741 op-amp 12N3904 NPN BJT 42N3906 PNP BJT 21 kΩ res istor 25.1 kΩ r e sistor 210 kΩ resistor 20.1 µF capacitor 1Table 1: Components used in this labComponent IS(A) VA(V)2N3904 NPN BJT 6.734 × 10−1574.032N3906 PNP BJT 1.41 × 10−1518.7Table 2: Transistor pro perties3 Procedure3.1 Generating a differential signalBefore building a differential amplifier, we’d like to be able to generate a differential signal. This requiresinverting an analog signal. One way we can do this is by using an op-amp in negative feeback, as shown inFigure 1.13 PROCEDURE 2−+1kΩvin1kΩvoutVCC−VCCFigure 1:Inverting amplifier1. Construct the circuit in Figure 1. Use the LM741 o p-amp. The pin layout for the LM741 op-amp isin Figure 2. Note: If your LM741 doesn’t have a notch as shown in the figure, check for a small dot.This dot labels pin 1.Figure 2: LM741 pin layout2. Apply a 30 mV amplitude, 1 kHz sine wave to the input. Display the input and output on the oscillo-scope. The output should be the inver se of the input.3.2 Differential pair with resistive load1. Construct the circuit in Figure 3 using 2N3904 transistors for the NPN BJTs. Use R1= 10 kΩ,R2= R3= 5.1 kΩ, and VCC= 9 V. This is the same circuit you analyzed in the prelab.−VCCVCCQ1R1Q2Q3R2Q4R3vin+vin−+ vout−Figure 3: Differential pair with res istive load3 PROCEDURE 32. Ground the inputs and measure IC1, IC2, IC3, and VOU T,DC. How do these values compare to whatyou’d e xpe c t from hand calculations?3. Apply a 30 mV amplitude, 1 kHz sine wave to vin+and ground vin−. Use the oscilloscope to displaythe input waveform at vin+and the output waveform at vout+and sketch the result. If the input signalis noisy, us e the avera ging feature of the oscilloscope to get a more accurate result.4. Use the oscilloscope to measure the peak-to-pe ak voltages of vin+and vout+.5. Use the oscilloscope to display vout+and vout−. Do they appear as you’d expect?6. Use the oscilloscope to display vout+− vout−. Measure the peak-to-pe ak voltag e of the signal andcalculate the differential gain of the circuit. Does this match the gain you calculated in the prelab?7. Apply a 30 mV amplitude, 1 kHz sine wave to both vin+and vin−. Use the oscilloscope to display theoutput waveform at vout+and vout−. What do you see at the output? Why?8. Use the inverting amplifier you built to apply a 20 mV amplitude, 1 kHz differential sine wave to theinputs (that means a 10 mV a mplitude s ine wave to vin+and the inverted sine wave to vin−). Measurethe peak-to -p eak voltage of the differential input and output with the oscilloscope. Does the gainmatch your prelab calculations? Does it match the gain you observed in step 3.2.6?3.3 Differential pair with active load−VCCVCCQ1R1Q2Q3Q5Q4Q6vin+vin−voutFigure 4:Differential pair with ac tive load1. Construct the circuit in Figure 4 using 2N3904 transistors for the NPN BJTs and 2N3906 transistorsfor the PNP BJTs. Use R1= 10 kΩ and VCC= 9 V.2. Apply a 30 mV amplitude, 1 kHz sine wave to vin+and ground vin−. Use the oscilloscope to displaythe output waveform at voutand sketch the result. Why isn’t the output sinusoidal?3. We’d like to reduce Routby loading the amplifier with a small resistor. Attach a load to the amplifieras shown in Figure 5. Use CL= 0.1 µF and RL= 5 kΩ.4. Calculate the differential gain for the amplifier with the new load resistance .5. Apply a 20 mV amplitude, 1 kHz sine wave to vin+and ground vin−. Use the oscilloscope to displayvin+and vout. Sketch vout. What is the measured differential ga in of the circuit? How doe s thiscompare with your hand calculations? Does the gain match the differential gain you measured in step3.2.6? Should it?3 PROCEDURE 4−VCCVCCQ1R1Q2Q3Q5Q4Q6vin+vin−CLRL+vout−Figure 5: Differential pair with reduced output resistance3.4 SPICE Analysis1. Write a netlist for the circuit in Figure 3. Apply a differentia l input of amplitude 20 mV, fre quency1 kHz as you did in step 3.2.8. Hint: Generate a 20 mV amplitude, 1 kHz sine wave and use dependentsources to generate the non-inverted and inverted 10 mV amplitude sine waves.2. Use SPICE to find IC1, IC2, IC3, a nd VOU T,DC. Co mpare these values with your calculations from theprelab and measurements in lab.3. Plot the differential input and differential output signals in Awaves. Print the plot and a ttach it toyour lab worksheet. Use the plot to calculate the gain. Does it match your hand calculations ? Does itmatch your
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