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What is an operational amplifier?

A voltage amplifier is a special circuit that accepts an input voltage, $V_{\rm in}$ and outputs a voltage, $V_{\rm out} = A V_{\rm in}$ that is proportional to the input voltage. The proportionality factor $A$ is called the gain of the amplifier. If $A>1$, then the amplifier actually does amplify the input voltage. If $A < 1$, then the amplifier attenuates the input voltage.

An operational amplifier or op-amp is a special integrated circuit that accepts two input voltages, $V^+$ and $V^{-}$. The op-amp's output is a single voltage (relative to ground), such that

V_{\rm out} = A (V^+ - V^-)

and such that $A$ is a very very large number. In other words, an operational amplifier is an integrated circuit that behaves like a high-gain difference amplifier. It amplifies the difference between two input voltages.

The symbol for an operational amplifier is a triangle that has two inputs and a single output. This symbol is shown below in figure 2. The input with a positive sign is called the non-inverting terminal and the input with the negative sign is called the inverting terminal. In addition to the two inputs and single output, the op-amp must have two supply voltages. These are shown be the two extra lines coming out of the top and bottom of the triangle in figure 2. The output voltages generated by the op-amp will be confined to lie within these two supply voltages. To function properly the top supply voltage should be at least 7-9 volts and the bottom supply can be anything less than or equal to 0 volts.

Figure 2: Opamp symbol
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As mentioned above the op-amp is an IC that acts as a high-gain difference amplifier. The gain is, in fact, very large, somewhere on the order of $10^5-10^7$. In addition to this the op-amp circuitry is designed so that the device has a very high input resistance and very low output resistance. This means that we can model the op-amp using a dependent voltage controlled voltage source. A dependent source is a voltage/current source whose value is a function of some other voltage/current in the circuit. Your textbook should discuss these idealized circuit elements in more detail.

Dependent voltage sources are a very good approximation for the op-amp's behavior. In other words, the op-amp is a circuit that has been engineered to be well approximated by an idealized circuit element . This means that we can use op-amp models in a reliable manner to predict the behavior of op-amp circuits with high confidence that our analytical predictions will be duplicated by the physical device. This simple fact makes the op-amp one of the most useful building blocks in analog circuit design.

To operate properly, the op-amp must be supplied a voltage that is larger than the range of differential input voltages. These other voltages are called supply voltages and they are denoted as $V_{\rm supply}$ and $V_{\rm gnd}$ in figure 2. In practice there are two types of op-amps. Double side op-amps have supply voltage of $\pm V_s$ volts (where $V_s$ is some positive voltage between 9 and 15 volts). This means that the output of the op-amp can swing between these positive and negative supply voltages. A single sided op-amp has a supply voltage of $+V_s$ volts and ground. This means that the output can only swing from 0 to $V_s$ volts. In our labs we'll be using a single sided op-amp known as the LM660.

The op-amp you've been supplied with in your kit is a standard single-sided quad op-amp (LM660). By single sided, we mean that the supply voltages are $+V_s$ volts and ground (rather than $\pm V_s$ volts). By quad, we mean that there are 4 op-amps on a single chip. The pin-out for the LM660 is shown below in figure 3.

Figure 3: Pin out for LMC660 Quad Op-amp

next up previous
Next: What is a buffering Up: Background Previous: What is the reference
Michael Lemmon 2009-02-01