6.3 Op Amp Input-Output Characteristic

David J. McLaughlin

6.3 Op Amp Input-Output Characteristic

Consider the ideal op amp having open loop gain $A=10^{6}$ with unipolar $12V$ power supply shown in Fig. 6.11.

Let us consider the relationship between the output voltage $V_{o}$ and the differential input voltage $V_{i}$ where we understand $V_{i}=V_{+}-V_{-}$ . The circuit will be capable of providing output voltages between $0V$ and $12V$ . This range is referred to as the voltage swing. The output voltage $V_{o}$ will be $AV_{i}=10^{6}V_{i}$ over the linear range of operation, ie, when the op amp is not saturated. This requires $V_{i}<12\mu V$ as discussed earlier in this chapter. For a differential input voltage $V_{i}>12\mu V$ the output voltage is clipped or pegged at the power supply voltage rail of $12V$ . Likewise, for inputs $V_{i}<0V$ , the output voltage is pegged to $0V$ . A plot characterizing the relationship between $V_{i}$ and $V_{o}$ is shown in Fig. 6.12.

Note that the slope of the $V_{i}$ vs $V_{o}$ curve is $\frac{12V}{12\mu V}=10^{6}=A$ for the linear operating region (that is, inputs between $0$ and $12\mu V$ ), while the slope is 0 in the saturated regions (input voltage $<0$ or $>12\mu V$ ). An op amp having a larger open loop gain would have a steeper slope in the linear region and achieve saturation for smaller input voltages. If we consider an ideal op amp having gain $A=\infty$ , the linear slope would be infinite, meaning that output voltage would saturate at the positive voltage rail whenever the input voltage $V_{i}$ is and positive value, whereas the output would be zero whenever $V_{i}$ is negative. The input-output relationship for this case is shown in Fig. 6.13.

We now consider an op amp again having open loop gain $A=10^{6}$ but with bipolar power supply voltages $\pm 12V$ shown in Fig. 6.14.

The output voltage is capable of a voltage swing between $- 12 V$ and $+ 12 V$ , corresponding to input voltages $-12\mu V$ to $+12\mu V$ . The op amp saturates at $-12V$ output for $V_{i}< -12\mu V$ and $+12V$ for $V_{i}>12\mu V$ as shown in the input-output relationship plotted in Fig. 6.15.

Finally, a bipolar op amp circuit having infinite open loop gain, $A=\infty$ has the input-output relationship shown in Fig. 6.16.

As a practical matter, the output voltage of an op amp will typically be 1-2 volts lower than the voltage supply rail due to voltage drop across components within the op amp device. (This is analogous to the voltage drop inside a battery resulting from the battery’s internal resistance.) An op amp having unipolar 6 V supply might produce an output voltage ranging from 1 V(low) to 5 V (high). As we shall see in the next section, the op amp comparator with high vs low output (digital output) is an important op amp application in which the precise value of the output voltage is not important. On the other hand, there are many applications where the precise value of the output voltage is important. A bipolar circuit with ± 12V rails would therefore achieve maximum output voltage swings of ±10 or ±11 volts. If the circuit application required output voltage swings as high as ±12V, then a ±15V power supply could be used.

Another important practical topic for op amps is output current. Op amps are not designed to produce a significant amount of output current. The devices we will be workin with are capable of sourcing only a few 10’s of mA. There will be many applications where substantially more current is needed; consider the small DC motor in the ECE361 kit, for example. This motor draws ~ 100-300 mA when unloaded mechanically, and the current draw increases to over 1A when the motor is mechanically loaded down. Driving a motor with a circuit that involves an op amp requires an additional device – a current amplifier of some sort – between the op amp output and the motor power supply terminals. We will examine how transistors serve this role in avsubsequent chapter.

6.3 Op Amp Input-Output Characteristic

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