Eng. Moriz

Friday, February 5, 2010

Operational Amplifier Circuits

We have built voltage and current amplifiers using transistors. Circuits of this kind with nice properties (high gain and high input impedance, for example), packaged as integrated circuits (ICs), are called operational amplifiers or op amps. They are called ``operational'' amplifiers, because they can be used to perform arithmetic operations (addition, subtraction, multiplication) with signals. In fact, op amps can also be used to integrate (calculate the areas under) and differentiate (calculate the slopes of) signals.

Figure 22: A circuit model of an operational amplifier (op amp) with gain $A$ and input and output resistances $R_{in}$ and $R_{out}$.
\includegraphics{lab4-opamp.eps}

A circuit model of an operational amplifier is shown in Figure 22. The output voltage of the op amp is linearly proportional to the voltage difference between the input terminals $v_+ - v_-$ by a factor of the gain $A$. However, the output voltage is limited to the range $-V_{CC} \leq v \leq V_{CC}$, where $V_{CC}$ is the supply voltage specified by the designer of the op amp. The range $-V_{CC} \leq v \leq V_{CC}$ is often called the linear region of the amplifier, and when the output swings to $V_{CC}$ or $-V_{CC}$, the op amp is said to be saturated. The output ranges of the amplifiers we built as part of Lab 3 were similarly limited by the supply voltage.

An ideal op amp has infinite gain ($A = \infty$), infinite input resistance ( $R_{in} = \infty$), and zero output resistance ($R_{out} = 0$). You should use these two assumptions to analyze the op amp circuits covered in the assignments below. A consequence of the assumption of infinite gain is that, if the output voltage is within the finite linear region, we must have $v_+ = v_-$. A real op amp has a gain on the range $10^3$-$10^5$ (depending on the type), and hence actually maintains a very small difference in input terminal voltages when operating in its linear region. For most applications, we can get away with assuming $v_+ \approx v_-$.

Figure 23: (a) Schematic symbol for an op amp. (b) Connection diagram for the LM741 and LF411 8 pin dual inline packages (DIPs). We will not make use of the null (LM741) / balance (LF411) pins. Pins labeled NC are not connected to the integrated circuit.
\scalebox{1.2}{ \includegraphics{lab4-opschem.eps} }
We will use two operational amplifiers in our laboratory exercises, the LM741, a general purpose bipolar junction transistor (BJT) based amplifier with a typical input resistance of 2 M$\Omega $, and the LF411, with field effect transistors (FETs) at the inputs giving a much larger input resistance ( $10^{12}~\Omega$). Detailed data sheets for these devices are available for dowload at the National Semicondictor web site (www.national.com ). Of the two, the LF411 comes closest to satisfying our two assumptions associated with ideal op amp behavior. It costs more than the LM741 (a whopping $0.61 vs. $0.23 as of spring 2001). The schematic symbol for an op amp and the connection diagram for the chips, called dual inline packages (DIPs), we will be using are shown in Figure 23
Posted by at

No comments:

Post a Comment

Subscribe to: Post Comments (Atom)