Download Hw4-1

Name: ____________________________________________ Student # ________
Homework for Chapter 4-1 and 2
Questions of the Day:



What is an Op-Amp, and how is it used?
What is the eqivalent circuit model for an op-amp?
What is negative feedback, and why is it used in OpAmp Circuits?
Problems:
Electrical Engineering is all about ‘what can you do to a voltage’ (or current, or resistance…). Op amp circuits let us do
many different things to our voltages, currents, and resistance – such as amplifying (or reducing), comparing, adding,
differencing, etc. In the next several homework assignments, we will explore several of these different types of
circuits and how they are used in various applications.
1. Op amps can amplify small voltage differences. See Problem 4.8 from your Ulaby textbook. Do this problem
first using the op amp model in Figure 4-4 (like the solution manual) and then repeat using the simplified ideal
op amp equations in Table 4-2. You should get the same answers.
2. Op amps can be used to detect small changes in resistance. Here is an example of a Rain or Moisture sensor
from1. You will be building something similar in Lab 4. Using the ideal op amp equations in Table 4-2, find an
equation for the op amp output voltage for the Rain Activated Logic circuit. Although he has given you possible
values of resistance and supply voltage (+6V), leave your answer as a function of the variables
R1,R2,Rsensor,and Vcc, so you can use it later in the lab. Hint: The circuit you should solve looks like this:
3. Op amps can be used to amplify small differences in resistance. See the Thermistor Amplifier Circuit below.
(This is often called a Comparator.) This circuit is like an inverting amplifier. The ‘nominal’ value of Vo = -R3/R2,
but it will change slightly as the thermistor resistance RT changes with temperature. Solve for Vo. Treat R3 as a
constant = 100kohm. Treat R1 as a constant 10kohm. Treat the thermistor as RT = R1+ΔR1 .
4. The Temperature Switch below is very similar to the Thermistor Amplifier you calculated above. Repeat your
calculations for Vo for the Temperature Switch.
1
Forrest M. Mims III, ‘Science & Communication Circuits & Projects’, Master Publishing, Inc.,2007