Day 14.5: Cascaded Op Amp Circuits

Cascaded Op Amp Circuits

 We started right off with a lab. In this lab, we designed a simple temperature measurement system which outputs a DC voltage which indicates temperature. Our system used a thermistor to indicate the temperature; the electrical resistance of the thermistor changes as the temperature changes. We used a Wheatstone bridge circuit to convert this resistance change to a voltage change.

Here we designed our circuit on the white board. We have our Wheatstone bridge with a difference circuit in the middle. We connected the 10kΩ pot up in series with a 3.6kΩ resistor in order to balance the bridge with the thermistor, which had a max resistance of 10.78kΩ. We designed the difference circuit to have a gain of 4 to amplify our voltage difference (V_diff = .567) by 4.

For the difference circuit, we used 2-12kΩ and 2-3kΩ resistors in order to get our gain of 4 that we required.

Here is a closer look at our Wheatstone bridge. See above for exact resistance values.

Here you can see our circuit in action. We hooked it all up, balanced the Wheatstone bridge, and with much difficulty, finally got our circuit to spit out the value of above 2V we were looking for!

A picture of our Wheatstone bridge balancing our circuit. 0V.

 Discussion: We were successful in building a circuit that converted a change of resistance (from the thermistor) to a change in voltage (by use of the Wheatstone bridge) that can be detected a voltmeter. Our circuit performed perfectly, just as we had expected. With the gain of 4, we should have expected that our voltage value would be 0.567V • 4 = 2.27V, which is only 15% off. This is not an unreasonable percentage off to be.

Classwork: We were instructed to find i_o as a function for v_s. After that, we were able to find the V_o. We had to do each op amp separately first.