UNIVERSITY OF CALIFORNIA AT BERKELEY College of Engineering Department of Electrical Engineering and Computer Sciences EE105 Lab Experiments Experiment 7 Frequency Response 1 Objective You have already seen the performance of several BJT amplifiers These were designed to operate well at certain small signal frequencies and indeed there is a frequency limit imposed by the parasitic capacitances of BJT devices In this lab you will observe how an amplifier responds to different input frequencies making use of the National Instrument Bode Analyzer software This lab will help familiarize you with the native capacitances in a transistor and how the frequency response is affected by these capacitances and external loads as well 2 Materials Component 2N4401 NPN BJT 10 k resistor 1 k resistor 51 resistor 1 nF Capacitor Quantity 1 1 1 1 1 Table 1 Components used in this lab 3 3 1 Procedure Frequency Response of Common Emitter Amplifier VCC 5 V RC IBIAS vOUT VBIAS vin RS Figure 1 Common emitter amplifier test setup 1 3 PROCEDURE 2 1 Construct the common emitter amplifier as shown in Figure 1 Use a RC 10 k and RS 51 2 Use a function generator to generate a 25 mV peak to peak 1 kHz sinusoidal signal with a DC offset of 580 mV This signal is both VBIAS and vin combined and will be referred to as vIN Note You may notice some nonlinear effects on the output waveform This is due to the high input signal amplitude that we are using We choose this amplitude to avoid noise from the oscilloscope messing up our Bode plot measurements in a later step 3 What is IBIAS and the DC voltage at VOUT 4 Using the oscilloscope plot input vIN on Channel 2 and output vOUT on Channel 1 Make sure to transfer the waveforms over to the Lab Report 5 What are the magnitude and phase of vout vin measured from the oscilloscope 6 Instead of using the oscilloscope to measure the magnitude and phase of vout vin at other frequencies manually let s use NI Bode Analyzer software to automate the process To avoid errors from the software make sure you follow the instructions below to set up the software Make sure that vIN is on Channel 2 and vOUT is on Channel 1 Connect the Trigger Output from the function generator to the External Trigger port of the oscilloscope Open NI Bode Analyzer exe from the computer desktop When both the function generator and the oscilloscope are turned on click Refresh under Resource Name The function generator is configured to a higher GPIB address than the oscilloscope Select the GPIB device with a higher GPIB address for the function generator and select the GPIB device with a lower GPIB address for the oscilloscope Set the stop frequency to 2 MHz for this measurement You can leave the starting frequency at its default value Set the amplitude of the input signal to 25 mV and the DC offset to 290 mV Note We set the DC offset to 290 mV because the function generator outputs an offset that is twice as large as the value entered here Hit Run The software will start sweeping the input across different frequencies to generate a Bode plot 7 After the software is done producing the Bode plot you can drag the cursor on the plot to read out the magnitude and phase at different frequencies Drag the cursor to around 1 kHz What are the magnitude and phase obtained with the software How different is this measurement compared to the one obtained with the oscilloscope 8 Now drag the cursor to a point where the gain decreases from its DC value by 3 dB This is the dominant pole of the amplifier What is the pole frequency What is the phase at this frequency Is the phase consistent with the magnitude Recall that at the 3 dB point the phase should be drop from its DC value by 45 degrees 9 You can export the plot by right clicking on the plot and then select Export Simplified Image You can also export the data points as a csv which can be opened in Microsoft Excel file by clicking the Export button Print the plot out and turn it in with your Lab Report You do not have to find the second pole because it occurs at a frequency higher than what is measureable by our equipment 3 3 PROCEDURE VCC 5 V RC IBIAS vOUT CM VBIAS vin RS Figure 2 Miller capacitor test setup 3 2 Miller Effect 1 The Miller effect can be best examined by introducing a Miller capacitor across the amplifier Let s take a look at the impact of the Miller effect by adding a 1 nF capacitor for CM as shown in Figure 2 2 Repeat the above procedures on using the NI Bode Analyzer to find the frequency response of this amplifier but set the stop frequency to 500 kHz Attach the Bode plot to the Lab Report 3 How does the dominant pole of this amplifier compare to the dominant pole of the previous amplifier Is this expected 4 In this amplifier we are using a 1 nF capacitor to simulate a large base collector capacitor C If we are to design an amplifier with high bandwidth is a transistor with a large C desirable 3 3 Output Capacitance 1 Let s examine the impact of output capacitance on a common emitter amplifier Take out the Miller capacitor CM from Section 4 2 and place it at the output as shown in Figure 3 VCC 5 V RC IBIAS vOUT VBIAS vin RS CM Figure 3 Output capacitance test setup 2 Repeat the above procedures on using the NI Bode Analyzer to find the frequency response of this amplifier but set the stop frequency to 500 kHz Attach the Bode plot to the Lab Report 3 4 PROCEDURE 3 How does the dominant pole of this amplifier compare to the dominant poles of the previous two amplifiers Is this expected 3 4 Common Collector Amplifier VCC 5 V VBIAS vin RS vOUT RE 1k Figure 4 Common collector amplifier test setup 1 The common collector amplifier is a wide bandwidth amplifier Let s examine the frequency response of this amplifier by building the common collector amplifier as shown in Figure 4 Configure the NI Bode Analyzer with an input signal of amplitude 1 V and DC offset of 1 V which is 2 V effective DC offset for vin and VBIAS 2 Repeat the above procedures on using the NI Bode Analyzer to find the frequency response of this amplifier Attach the Bode plot to the Lab Report However keep in mind that the breadboard has a parasitic capacitance that will start deforming the signal when the signal frequency goes beyond approximately 1 MHz You can observe this effect at the output of the amplifier using the oscilloscope Therefore the experiment in finding the dominant pole of this amplifier is only an approximation Note You may not be able to find the pole with the equipment
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