Lab

Lab 4 - EE420L

Authored by Rodolfo Gutierrez

gutie284@unlv.nevada.edu

2/24/2016

Op-amps II, gain-bandwidth product and slewing

Estimate, using the datasheet, the bandwidths for non-inverting op-amp topologies having gains of 1, 5, and 10.

For the non inverting topology we have

BW * Gain = 1.3MEG Hz

At gain = 1

   BW = 1.3MEG / 1 = 1.3 MEG Hz

At Gain = 5

    BW = 1.3MEG / 5 = 260k Hz

At Gain = 10

    BW = 1.3MEG / 10 = 130k Hz

Experimentally verify these estimates assuming a common-mode voltage of 2.5 V.

Your report should provide schematics of the topologies you are using for experimental verification along with scope pictures/results.
Associated comments should include reasons for any differences between your estimates and experimental results.

We can measure the bandwidth by finding the output voltage at 3db. To put simply we measure the output at a low frequency then look for the output response at higher frequencies. This is done with vout * 0.707.

Gain 1

Gain 5

Gain 10

	low frequency Pk-Pk vout	high frequency Pk-Pk vout	bandwidth
Gain 1	237 mV	168 mV	750 kHz
Gain 5	980 mV	693 mV	160 kHz
Gain 10	2.1 V	1.48 V	77 kHz

We see that the measured bandwidth is much lower than the calculated bandwidth. This is probably due to the room temperature. We know from the open loop frequency response graph given from the data sheet we should expect a range for the devices bandwidth, using the lower range we see that the unity frequency is about 900 kHz. Using that we get 900kHz for the gain of 1, 180kHz for the gain of 5, and 90kHz for the gain of 10. With our recorded values we can state that the op-amps bandwidth is operation in the lower range.

Repeat these steps using the inverting op-amp topology having gains of -1, -5, and -10.

For the inverting topology we have

BW * (1+R2/R1) = 1.3MEG Hz

At gain = -1

   BW = 1.3MEG / (1+1) = 650k Hz

At Gain = -5

    BW = 1.3MEG / (1+5) = 216.66k Hz

At Gain = -10

    BW = 1.3MEG / (1+10) = 118.1k Hz

Gain -1

Gain -5

Gain -10

	low frequency Pk-Pk vout	high frequency Pk-Pk vout	bandwidth
Gain -1	664 mV	469.4 mV	300 kHz
Gain -5	2.08 V	1.47 V	90 kHz
Gain -10	2.71 V	1.91 V	75 kHz

Once again we see that the theoretical bandwidth is much higher than the measured bandwidth. If we use the same assumption from the last experiment we get the following. At unity gain = 900kHz you will have 450kHz for the gain of -1, 180kHz for the gain of -5, and 90k for the gain of -10. With the measured results from the inverting topology we can confirm that the bandwidth is operating at the lower range given from the datasheet.

Design two circuits for measuring the slew-rate of the LM324. One circuit should use a pulse input while the other should use a sinewave input.

Provide comments to support your design decisions.
Comment on any differences between your measurements and the datasheet’s specifications.

To find the slew rate the gain of 10 topology was used, this is so that we can see the raising voltage. Then the frequency was greatly increased until the output voltage resembles a sawtooth wave form, with this we take the raising voltage divided by the rise time. By using the cursor function from the oscilloscope we are able to measure the raise time of vout. For the square wave we see vout raises 200 mV, at raise time = 1.04 us we have 0.192 V/us. For the sine wave we have the same voltage but a rise time of 200ns, giving us about about 0.2 V/us.

We find that the measured value is within the ballpark range from the datasheet result.

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