Day 12 - Cascade Op-Amp Circuits

Topics Discussed

On day 12 of the ENGR 44 course, we discussed cascading operational amplifiers. This would essentially allow us to combine a series of amplifiers together in one circuit in order to achieve far more adjustability and precision in a circuit. What we essentially uncovered is the fact that these cascaded circuits could be analyzed the same way as previous and more simple op-amp circuits. We simply had to take each op-amp as a separate piece of the circuit, recognizing that the output voltage of one op-amp could be the input of the other. We also had a brief introduction to DAC (Digital to Analog Converters).

Temperature Measurement System Design Lab

In the temperature measurement system design lab, we were required to design a circuit using a thermistor, a wheatstone bridge, and a difference amplifier to gain an output voltage that was temperature dependent. We had created a circuit similar to this in a previous class meeting, however, in this case we wanted more precision in terms of how sensitive the thermistor would be to the slightest temperature change. Our goal was to have an output voltage that would increase in the positive direction by at least 2V. We created a schematic for the circuit (Fig. 1).

Fig. 1

Firstly, we had to create a wheatstone bridge which would have 4 resistors of the same value. However, we found that in our class-setting, this was unachievable due to the lack of extremely precise resistors which would be required. Therefore, we included a potentiometer in our wheatstone bridge that would allow us to essentially "dial-in" and achieve an exact resistor value that would give us a zero voltage difference across the two ends of the bridge. We found that our room temperature resistance for our thermistor was 10.7k ohms and the body temperature resistance was 6.6k ohms. Once this was completed, we created the circuit (Fig. 2).

Fig. 2

We found that our output voltage became extremely sensitive, and we had a range of greater than 2V. A video demonstration of the entire circuit working is shown below.

Summary
We were able to successfully create the difference amplifier that would be dependent on the temperature applied to the thermistor in this lab. One of the fundamental aspects of this lab was balancing the wheatstone bridge, in that it was an imperfect piece of equipment that we were able to dial in to get precise values. This process of creating this circuit showed us the capabilities of using op amps in practical designs to get results that are precise and responsive.