Brandeis University Physics 29a
Spring 2017 Kevan Hashemi

Lab 9: Operational Amplifiers and Feedback

Notes: Use your bench-top power supply to provide ±10 V for your op-amp circuits. Leave your transistor radio set up. You will need the radio for Part 3.

Part 1: Build the following non-inverting amplifier. Use R1 = 10 kΩ and R2 = 100 kΩ. Op-amp U1 can be the uA741 (original and classic), EL2044 (fast and rugged), or TL081 (low input bias current). Their data sheets are on the 29A web page. We give the DIP-8 pinout below.

The gain of an ideal op-amp is A = VOUT/(VIN+−VIN−), where VOUT is its output and VIN+ and VIN− are its non-inverting and inverting inputs respectively. What is the theoretical gain, G = Y/X, for this amplifier, assuming A is large? Apply a 1 Vpp sinusoid to X. Measure and plot the gain versus frequency from 10 Hz to 1 MHz. Over what frequency range is your measured gain constant to within ±20%?

Part 2: Connect X to 0 V. Measure Y with a precision of ±1 mV. What is the input offset voltage of your op-amp? Connect X to 0 V with a 1 MΩ resistor. Measure Y once more. What is the input bias current of your op-amp?

Part 3: Remove the speaker transformer from your transistor radio and replace it with a resistor of your own choosing. Couple the audio output from your radio into your non-inverting op-amp amplifier with the help of a capacitor and another resistor, both of your own choosing. Connect the speaker transformer and speaker to the output of your non-inverting amplifier. Is your radio louder?

Part 4: Derive an equation for Y as a function of X for the following circuit. You may assume A is large.

Build the above circuit with values of R1 and R2 that give a gain of −22. Apply a 1-kHz, 1-Vpp sinusoid to X and measure the amplitude at Y. Does your calculated gain match your measured gain?

Part 5: In the following circuit, R1 = R3 and R2 = R4. Derive an equation for Y as a function of X and W. You may assume A is large.

Arrange your equation in the form d(WX) + c(W+X)/2. We call d the differential gain and c the common mode gain. In your solution, the common mode gain should be zero. Build the circuit with resistors of your own choosing. Devise a way to measure c and d with the help of your function generator. Do your calculations and measurements agree?