Operational amplifier rise time

In order to determine the thermal time constant of the microhotplate in dynamic measurements, a square-shape voltage pulse was applied to the heater. The pulse frequency was 5 Hz for uncoated and 2.5 Hz for coated membranes. The amplitude of the pulse was adjusted to produce a temperature rise of 50 °C. The temperature sensor was fed from a constant-current source, and the voltage drop across the temperature sensor was amplified with an operational amplifier. The dynamic response of the temperature sensor was recorded by an oscilloscope. The thermal time constant was calculated from these data with a curve fit using Eq. (3.29). As already mentioned in the context of Eq. (3.37), self-heating occurs with a resistive heater, so that the thermal time constant has to be determined during the cooHng cycle. 

In Section 3B-4 we discussed the relationship between the rise time and the bandwidth of an operational amplifier. These variables are also u.sed to characterize the capability of complete instruments to transduce and transmit informaliim, because... 

Calculate the slew rale and the rise time for an operational amplifier with a 50-Mf fz bandwidth in which the output changes by 10 V. 

FIGURE 3-9 Response of an operational amplifier to a rapid step change in input voltage. The slope of the changing portion of the output signal is the slew rate, and the time required for the output to change from 10% to 90% of the total change is the rise time. 

As previously shown, the gel effect is a result of increasing viscosity, too, leading to higher heat production rates. In consequence it must be understood, that both effects amplify each other. The heat production rate may rise easily by a factor between two and three due to the gel effect, while at the same time the accompanying removal power is diminished to 1/3 down to 1/5 of its original value. Therefore the cooling must be designed in such a way that it can remove 6 to 10 times the power initially necessary to ensure isothermal operation. 

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