Lab exercise 6

Operational amplifiers


Operational amplifiers (opamps) are one of the most widely used circuits in electrical engineering. This lab will introduce several uses for opamps including amplifiers and filters.


It is possible to use standard wav files as input to LTspice simulations. This allows you to input voice, music or ECG signals into your simulated filters before you build the filters and play the wav files through the computer into your actual circuit. You can get some ECG files from, but I have already downloaded them and added noise to them, which you will need to remove with a filter.

To add a WAV file input to a LTspice simulation, right click a voltage source, then in the DC value field enter: wavefile="ecgwav_noise.wav"chan=1. Note that every time you open the voltage module, the string is erased and has to be re-entered, BUT if you right-click the text instead, you can edit it (see below). More information on using wav files:
wav source

To save a waveform from a simulation as a WAV file, add a spice directive to the simulation by menu Edit>Spice directive..., then in the edit box enter .wave "filename" nbits, sample_rate V(node_name). Where nibts is usually 16 bits and the sample rate is 8000, 16000, or 44100. An example might be .wave "loon.wav" 16 16000 V(Vout).

The file VCVS_bandpass_filter.asc displays the frequency response of a bandpass filter with cutoff frequencies at about [16 1600] Hz, marked by the white arrows. The vertical scale (on the left) is a log scale where 20db represents a factor of 10. More precisely displayed_db= 20*log10(amplitude_voltage_out/amplitude_voltage_in) at each different frequency. The dashed line is the phase shift of the sine wave as it passes through the two filters measured in degrees on the right-hand scale. Note for that correct operation R9=R8, C3=C4, C1=C2 and R3=R5.


  1. Build the following noninverting amplifier circuit and show that the gain=11. At what (fairly high) frequency does the gain start to drop below the gain at low frequency? Plot log(gain) versus frequency. What happens to the phase-shift at high frequencies. Use the signal generator as the sinewave source.

October 24, 2011 Copyright Cornell university