New Mexico Tech EE321L - Analog Electronics Fall 2013 Laboratory Exercise 1 Amplifiers, Input/ Output Resistance, Cascades and Frequency Response Pre-Lab 1. Find the LM741 data sheet, for example on www.digikey.com. 2. Design the amplifier in question 1. 3. Design the amplifiers in question 5, derive an expression for Avo of the cascade, and compute the expected value. 4. Design the amplifiers in question 6. 5. Determine what bandwidth is and how to compute it based on the Gain-Bandwidth-Product of the amplifier. Input and Output Resistance 1. Using a LM 741 op-amp, build an inverting voltage amplifier with a gain of approximately 100, and input and output resistances of 1 kΩ (Hint: the circuit is a regular inverting amplifier followed by a series resistor). Measure the resistors and compute an actual expected gain, Avo. 2. Attach the function generator directly to the input, and the output directly to a voltage-meter of the oscilloscope input. Plot the output as a function of the input (use the function generator’s DC Offset by selecting the buttons: Waveform -> More -> DC -> Parameter, and then entering the values you desire) for several different input voltages. Start at and increment by 10mV. What does this imply about the source (function generator) and load (meter or oscilloscope) resistances? NOTE: Press the channel button and set output to “on” to output a signal. It is good habit to set the output to “off” when adjusting the circuit or no longer using the waveform. 3. Measure the input and output resistance of the amplifier using DC Signals and a 1 kΩ potentiometer as part of a voltage divider. You can use the function generator’s DC offset with the potentiometer in series at the input. Repeat with the potentiometer in series with the output. Set the function generator to have a DC voltage of 80 mV. Adjust the potentiometer until the potentiometer’s output voltage is half the input voltage. Once finished remove the potentiometer and measure its resistance value. Compare the results to the actual value of the 1 kΩ resistors. Is the 50 Ω output resistance from the function generator negligible? NOTE: The function generator describes all voltages, including DC, as peak-peak values, therefore if you view the 80mV signal on the oscilloscope, it will display a voltage around 160 mV. Hint: Make sure to place the potentiometer as “r1” or the voltage divider will not act as desired. New Mexico Tech EE321L - Analog Electronics Fall 2013 Frequency Response 4. Measure the frequency response of the amplifier. Use a small-amplitude sinusoidal input, gradually increasing the frequency until the gain has dropped significantly (by at least a factor of 10.) Plot the gain and phase difference between input and output as a function of frequency. Compare to your expectations. Is the low frequency gain what you expect? What is the bandwidth? Compare your bandwidth values by searching the datasheet for its declared bandwidth and/or Gain-Bandwidth-Product (GBWP). Once found, divide the GBWP (should be between 0.5Mhz and 1.5MHz) by the gain you are using. Increase the Frequency until a gain of 1 is reached. Does this value agree with the datasheet? Would you want to use the LM741 for audio amplification knowing that the phase of low frequencies and high frequencies are not constant? NOTE: At high frequencies you must use a small amplitude input signal. If you notice any distortion of the output causing it to look triangular, reduce the input signal amplitude. Cascaded Amplifiers 5. Build a two-stage amplifier cascade consisting of two inverting amplifier, each with a gain of 10 and each with input and output resistances of 1k. Measure Avo of the cascade. 6. Modify your two-stage amplifier circuit (keeping the feedback resistance values identical) to produce an Avo of approximately 100. Measure the new gain and confirm it is 100. 7. What is the bandwidth of the cascade? You can measure this without plotting the full frequency response. Compare this measure bandwidth of the two-stage amplifier with the theoretical value you computed based on the bandwidth of the single-stage amplifier.