Operational-Amplifier Review

This part of your lab is to provide a review of op-amps, and to give you an opportunity to use the power supply circuit you made in a real application. Recall that an op-amp is a three-terminal integrated circuit. Inside the op-amp is a fairly complicated circuit which typically consists of more than thirty transistors. Later in this course you will be constructing your own op-amps out of transistors. However, for now we will treat the op-amp as a black-box with the following equivalent circuit:

  

Figure 1.7: Operational Amplifier Equivalent Circuit

Fig. 1.7 shows that the op-amp can be described as a voltage controlled voltage source where the output voltage is proportional to the voltage across the input resistance R_in. Recall that most op-amps have extremely high input resistance, low output resistance, and extremely high gain. For example, the ubiquitous 741 op-amp has $R_{in}=2M\Omega,~R_{out}=40\Omega$, and gain A_v=200,000. For most applications, it's appropriate to treat an op-amp as ideal. For an ideal op-amp $R_{in}=\infty$, R_out=0, and $A_v=\infty$.Under the ideal approximation, using an op-amp in circuit design is extremely simple.

An ideal op-amp without feedback is simply a comparator, where the output switches to the positive rail if V⁺ > V^-, and switches to the negative rail if V⁺ < V^-.

Once feedback is connected, the infinite gain forces V⁺=V^-. As a result, we can find the closed loop voltage gain very easily.