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PSPICE Homework 1 – Op Amps rev7 PSPICE homework assignments provide you with an opportunity to express your creativity in designing a circuit to meet specific specifications. Problem 1: Two-Stage Amplifier Design 1. Need to design and construct a two-stage amplifier with a total voltage gain of A = A1 ∙ A2 = +X. where X is the sum of digits of your student Z number. What is the total voltage gain of your circuit? 2. You should do it in two different ways both leading to the same performance: a) Using two inverting amplifiers, and b) Using two non-inverting amplifiers. Let A1 be the gain of the first amplifier and A2 be the gain of the second amplifier. The output of Stage 1 becomes the input of Stage 2. Choose any suitable values |A1|> 1 and |A2| >1that you wish. (Note that each of the two gains must be strictly larger than 1. No buffers allowed in this design. Assume that the available batteries are +15V and -15V. Use the µA741 part in the EVAL library. 3. Using the PSPICE cursor to check whether the overall gain that you get and each stage’s gain are exactly as planned. Simulate the Transient response of the circuit and observe the output for a VSIN input of amplitude 0.1V and frequency of 1 KHz (in Transient). Avoid resistors less than 1kΩ. No clipping should appear on the output waveform. Hints: a) Whenever you construct a non-inverting op-amp leave it at its original position - no need to "mirror vertically". However - don't forget to double check that pin 7 receives a positive DC voltage, and pin 4 receives a negative DC voltage, b) In the inverting amplifier you have to mirror the op-amp vertically, but again verify that correct DC voltages reach pins 4 and 7, c) The gain may not be exactly +X because the 741 op-amp is not "ideal". It is only "almost ideal". Checking the peak-to-peak output amplitude will show you that the error is very small, d) Use multiple plots or multiple Y axes, to see each signal at full scale, (e) Initially when you set the simulation parameters for the Transient / Time response leave the “Maximum Step Size” entry blank and observe how wrong the simulation results look (i.e. the signals are supposed to look smooth, but if the step size is wrong they may have corners), (f) Now set the maximum step size to be 1/1000 of the input signal’s period and see the correct results. (g) An example of a two-stage amplifier is shown in Figure 1. Figure 1: Example of two-stage amplifier - Transient Simulation vout R1 7.5k vin U2 uA741 3 2 7 4 6 1 5 + - V+ V- OUT OS1 OS2 vcc vcc 0 vee 0 vee 0 V1 15Vdc R4 27k 0 U1 uA741 3 2 7 4 6 1 5 + - V+ V- OUT OS1 OS2 V6 FREQ = 1000 VAMPL = 0.1V VOFF = 0V R3 1k V2 15Vdc vee 0 vcc R2 15k vout1 Type your first and last name on each schematic and simulation plot 2 | P a g e Problem 2: Level Shifting Design 1. Create a sinusoidal signal V(in) with the following parameters –VOFF=(last digit of your Z number), AMPLITUDE=2.5V, FREQUENCY=5 KHz. What is your VOFF? 2. Design a level shifting circuit such that the lower peak of V(in) is shifted to 8V, and the higher peak of V(in) is shifted to -4V. 3. Show by means of PSPICE Time Response simulation that your circuit really works. [Continue to use the two 15V batteries]. Make sure to explain every step in your design. Hint: Follow the step-by-step design procedure in the lecture notes (Unit 2 Part 2). You may even use the notes as a template so that you don’t have to retype the formulas. Just put in the numbers relevant to your design. Problem 3: Low-Pass Filter Design and Testing 1. Let the desired signal be an AC with amplitude of 3.2 V and frequency =(first four numerical digits of your Z number in Hz). What is the frequency of your desired AC signal? 2. Let the “noise” signal be AC with amplitude of 2V and frequency of 15 KHz. 3. Need to design an op amp low-pass filter (as in the lecture notes). 4. You need to choose the cutoff frequency f=fc. (Hint: fc must be slightly larger than the desired signal’s frequency [1.5 to 2 times larger may be a good choice] but sufficiently smaller than the noise frequency. You may have to experiment with a few possible choices]. Test the filter both in AC Sweep (asking “Is f = fc located as designed?”) and in Transient (as in the demonstrated example in the lecture notes, asking “Is the amplitude of the output noise small enough relative to the amplitude of the desired output signal?”). [Continue to use the two 15V batteries]. Homework Solution Format: Each simulation solution must be submitted neatly edited and should include the following items: 1) Some calculations (in case of a design exercise) predicting approximately the expected outcome. Always explain your design considerations. 2) Printout of the circuit diagram – As the author, print your name on each circuit diagram sheet. 3) Output printouts – Be selective and use only the most relevant output. Don’t dump on the grader your entire collection of computer printouts. In particular, never submit graphs that you cannot explain. Print your name on every result sheet that you submit. 4) Annotations to the results: PSPICE allows you to put comments and annotations on all output graphs and circuit schematics. It is highly recommended (for best readability of your work) to include notes and computations directly on the output graph pages themselves. 5) Brief conclusions – Did the circuit work as expected? If the results are far from your handcalculation prediction, where is the difference coming from?

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