Summer 2007 EE100 EE43 Lab 6 EECS 100 43 Lab 6 Frequency Response 1 Objective In this lab you will learn about the concept of gain bandwidth product of an op amp 2 Equipment a Breadboard b Wire cutters c Wires d Oscilloscope e Function Generator f Power supply g AD827 op amp NOTE THIS DOES NOT HAVE THE SAME BEHAVIOUR AS THE LMC6482 SO PLEASE MAKE SURE TO GET THE CORRECT OP AMP FROM YOUR TA h Various connectors for the power supply function generator and oscilloscope 3 Theory a Concept of Frequency Response As you learned from class EVERY electronics component does not behave the same at all frequencies That is their response differs according to the frequency of your input signal In other words EVERY electronics component has a FREQUENCY RESPONSE The algorithm you use to measure frequency response in lab is a Input a sinusoid of a known amplitude frequency and phase b Measure the amplitude frequency and phase difference at the output Plot the gain versus frequency on one plot1 and phase versus frequency on another plot c Change the frequency to a higher value go back to step a If you have a linear system then the sinusoidal output frequency is the same as the sinusoidal input frequency In this lab we will explore a very important frequencyresponse related property of op amps the gain bandwidth product 1 If you plot gain in dB linear scale versus the frequency log scale you get the Bode magnitude plot University of California Berkeley Department of EECS Summer 2007 EE100 EE43 Lab 6 b Concept of Gain Bandwidth product of an op amp Probably the most important feature of an op amp is its gain bandwidth product and the fact that it is a constant2 You know the definition of gain ratio of the output variable to the input variable There is no universal agreed upon definition of bandwidth For this lab we will define the bandwidth as the frequency at which the gain decreases by 3 dB You should now complete Prelab Task 1 If the gain bandwidth product of an op amp is constant and is 5 MHz gain is dimensionless it means a If the op amp is wired up for unity gain voltage follower the gain drops by 3 dB or 1 the amplitude in volts decreases by a factor of at a frequency of 5 MHz 2 b If you have the op amp wired for a gain of 10 then the gain drops by 3 dB at a frequency of 0 5 MHz Notice that the gain bandwidth product is 10 0 5 MHz 5 MHz c If you have the op amp wired for a gain of 20 then the gain drops by 3 dB at a frequency of 0 25 MHz Notice that the magnitude of the gain is what matters for the gain bandwidth product 20 x 0 25 MHz 5 MHz Why is the gain bandwidth product so important Answer it tells you for what frequency values your op amp acts in gain mode before the components inside the opamp start functioning differently your op amp starts loosing gain You can find the gain bandwidth product of op amps in datasheets this is probably the most important factor after power requirements that you want to look at while selecting op amps for your design Ask yourself this question what is the maximum operational frequency of your circuit If this frequency is less than the bandwidth of the selected op amp then your op amp is fine for the design 2 The fact that the gain bandwidth product is constant is not a physical law rather a consequence of the design A mathematical explanation of this property is beyond the scope of this class But it is interesting to note that not all op amps have this property current feedback op amps don t have this property University of California Berkeley Department of EECS Summer 2007 EE100 EE43 4 PRELAB Lab 6 NAME SECTION Please turn in INDIVIDUAL COPIES of the prelab They are due 10 MINUTES after start of lab NO EXCEPTIONS a TASK 1 You should start understanding the nomenclature of frequency response the most important of which is the decibel Read Appendix D from your textbook it describes the decibel Now in the function plot below zoom in to view the signal what is the bandwidth of the signal Figure 1 Bode magnitude plot of a complex valued function The bandwidth is defined as the frequency at which the signal gain drops by 3 dB PRELAB COMPLETE TA CHECKOFF University of California Berkeley Department of EECS Summer 2007 EE100 EE43 Lab 6 5 REPORT NAME S SECTION a TASK 1 Build the following AD827 circuits on the breadboard and power up the opamp PLEASE BE VERY CAREFUL WHILE HANDLING THIS OP AMP DOUBLE CHECK POWER CONNECTIONS IF IN DOUBT ASK THE TA WE DON T HAVE A LOT OF THESE OP AMPS SO DON T BLOW THEM UP 4 0 3 V1 7V U1 FunctionGenerator 0 OPAMP 5T VIRTUAL R2 0 R1 1 2 5 10k 1k 0 V2 7 V Figure 2 Non inverting amplifier 1 4 0 3 V1 7V U1 FunctionGenerator 0 OPAMP 5T VIRTUAL 0 R2 1k R1 1 2 5 5k 0 V2 7 V Figure 3 Non inverting amplifier 2 University of California Berkeley Department of EECS Summer 2007 EE100 EE43 Lab 6 For each circuit 1 Compute the gain 2 Use 1 Vpp sine wave 1 Vpp as measured on the scope as input to your circuit and measure the bandwidth 3 Determine the gain bandwidth product Circuit Non inverting amplifier 1 Non inverting amplifier 2 Gain Bandwidth Gain Bandwidth product Table 1 Gain Bandwidth product comparison TURN IN ONE REPORT PER GROUP AT THE END OF YOUR LAB SESSION THERE IS NO TAKE HOME REPORT 6 REVISION HISTORY Date Author Summer 2007 Bharathwaj Muthuswamy University of California Berkeley Revision Comments Typed up source documentation organized lab report typed up solutions Department of EECS
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