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 2 1 1 1 1 Table 1 Components used in this lab 3 Procedure 3 1 Frequency Response of Common Emitter Amplifier 1 Construct the common emitter amplifier as shown in Figure 1 Use a 10 k resistor for RC and a 51 resistor for RS 2 Use a function generator to generate a sinusoidal signal with amplitude of 25 mV frequency of 1 kHz and DC offset of 580 mV This signal is both VBIAS and vin combined We call this signal VBIAS together with vin 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 the noise from the oscilloscope messing up our Bode plot measurement 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 is the gain magnitude and phase of vOUT vIN measured from the oscilloscope 1 3 2 PROCEDURE VCC 5 V RC IBIAS vOUT VBIAS vin RS Figure 1 Common emitter amplifier test setup 6 Instead of using the oscilloscope to measure both the gain magnitude and phase of vOUT vIN at other frequencies manually by examining the waveforms 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 stopping frequency at 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 much Hit Run The software will start sweeping the input across different frequencies to generate a Bode plot 7 After the software is done plotting the Bode plot you can drag the cursor on the plot to read out the gain magnitude and phase at different frequencies Drag the cursor to around 1 kHz What is the gain 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 by 3 dB This will be the dominant pole of this 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 is 45 away from the phase before the 3 dB point 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 csv 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 is at too high of a frequency that our equipments cannot handle 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 gain Let s take a look at the impact of 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 stopping frequency at 500 kHz Attach the Bode plot to the Lab Report 3 How is the dominant pole of this amplifier compared 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 CBC If we are to design an amplifier with high bandwidth is a transistor with high CBC 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 also set the stopping frequency at 500 kHz Attach the Bode plot to the Lab Report 3 4 PROCEDURE 3 How is the dominant pole in this amplifier compared to the dominant pole 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 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
View Full Document
Unlocking...