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GOALS
- Discuss the operation of the operational amplifier (op amp).
- List the major types of connections for operational amplifiers.
- Connect a level detector circuit using an op amp.
- Connect an oscillator using an op amp.
The operational amplifier, like the 555 timer, has become a very common component in industrial electronic circuits. The operational amplifier, or op amp, is used in hundreds of applications. Different types of op amps are available for different types of circuits. Some op amps use bipolar transistors for input while others use field effect transistors. The advantage of field effect transistors is that they have an extremely high input impedance that can be several thousand megohms. As a result of this high input impedance, the amount of cur rent needed to operate the amplifier is small. In fact, op amps that use field effect transistors for the inputs are generally considered to require no input current.
The ideal amplifier would have an input impedance of infinity. With an input impedance of infinity, the amplifier would not drain power from the signal source; therefore, the strength of the signal source would not be affected by the amplifier. The ideal amplifier would also have zero output impedance. With zero output impedance, the amplifier could be connected to any load resistance without causing a voltage drop inside the amplifier. If it had no internal voltage drop, the amplifier would utilize 100% of its gain. Finally, the ideal amplifier would have unlimited gain.
This would enable it to amplify any input signal as much as desired.
FIG. 1 The 741 operational amplifier.
FIG. 2 The offset null connection.
FIG. 3 Inverting output.
Fig. 4 Noninverted output.
Although the ideal amplifier does not exist, the op amp is close. In this Section, the operation of an old op amp, the 741, is described as typical of all operational amplifiers. Other op amps may have different characteristics of input and output impedance, but the basic theory of operation is the same for all of them.
The 741 op amp uses bipolar transistors for the in puts. The input impedance is about 2 megohms, the output impedance is about 75 ohms, and the open loop, or maximum gain, is about 200,000. The 741 is impractical for use with such a high gain, so negative feedback (discussed later) is used to reduce the gain. For example, assume that the amplifier has an output voltage of 15 volts. If the input signal voltage is greater than 1/200,000 of the output voltage, or 75 microvolts,
15 _____ _ .000075 200,000
the amplifier will be driven into saturation, at which point it will not operate.
The 741 operational amplifier is usually housed in an eight-pin, in-line, integrated circuit package (FIG. 1). The op amp has two inputs, the inverting input and the noninverting input. These inputs are connected to a differential amplifier that amplifies the difference between the two voltages. If both of these inputs are connected to the same voltage, say by grounding both inputs, the output should be 0 volts. In actual practice, however, unbalanced conditions within the op amp may cause a voltage to be produced at the output. Since the op amp has a very high gain, a slight imbalance of a few microvolts at the input can produce several millivolts at the output. To counteract any imbalance, pins #1 and #5 are connected to the offset null, which is used to produce 0 volts at the output. These pins are adjusted after the 741 is connected in a working circuit. To make the adjustments, a 10 kilohm potentiometer is connected across pins #1 and #5, and the wiper is connected to the negative voltage (FIG. 2).
Pin #2 is the inverting input. When a signal voltage is applied to this input, the output is inverted. For example, if a positive AC voltage is applied to the inverting input, the output will be a negative voltage (FIG. 3).
Pin #3 is the noninverting input. When a signal voltage is applied to the noninverting input, the output voltage is the same polarity. For example, if a positive AC voltage is applied to the noninverting input, the output voltage will be positive also (FIG. 4).
Operational amplifiers are usually connected to above and belowground power supplies. Although there are some circuit connections that do not require an above and below ground power supply, these are the exception instead of the rule. Pins #4 and #7 are the volt age input pins. Pin #4 is connected to the negative, or below ground, voltage and pin #7 is connected to the positive, or above ground, voltage.
The 741 operates on voltages that range from about 4 volts to 16 volts. Generally, the operating voltage for the 741 is 12 to 15 volts plus and minus. The 741 has a maximum power output rating of about 500 milliwatts.
Pin #6 is the output and pin #8 is not connected.
As stated previously, the open loop gain of the 741 operational amplifier is about 200,000. Since this amount of gain is not practical for most applications, something must be done to reduce the gain to a reason able level. One of the great advantages of the op amp is the ease with which the gain can be controlled (FIG. 5). The amount of gain is controlled by a negative feedback loop. This is accomplished by feeding a portion of the output voltage back to the inverting input. Since the output voltage is always opposite in polarity to the inverting input voltage, the amount of output voltage fed back to the input tends to reduce the input voltage. Negative feedback affects the operation of the amplifier in two ways: it reduces the gain, and it makes the amplifier more stable.
The gain of the amplifier is controlled by the ratio of resistor R2 to resistor R1. If a noninverting amplifier is used, the formula is used to calculate the gain. If resistor R1 is 1 kilohm and resistor R2 is 10 kilohms, the gain of the amplifier
is 11.
R1 _ R2
___
_ _ R1
If the op amp is connected as an inverting amplifier, the input signal will be out of phase with the feedback voltage of the output. This will cause a reduction in the input voltage applied to the amplifier and in the gain.
The formula is used to compute the gain of an inverting amplifier. If resistor R1 is 1 kilohm and resistor R2 is 10 kilohms, the gain of the inverting amplifier is 10.
As a general rule, the 741 operational amplifier is not operated above a gain of about 100 because it tends to become unstable at high gains. If more gain is de sired, it is obtained by using more than one amplifier (FIG. 6). The output of one amplifier is fed into the input of another amplifier.
Another general rule for operating the 741 op amp is that the total feedback resistance (R1_R2) is kept at more than 1,000 ohms and less than 100,000 ohms.
These rules apply to the 741 operational amplifier but may not apply to other operational amplifiers.
FIG. 5 Negative feedback connection.
FIG. 6 Two operational amplifiers are used to obtain a higher gain.
Basic Circuits
Op amps are generally used in three basic circuits that are used to build other circuits. One of these basic circuits is the voltage follower. In this circuit, the output of the op amp is connected directly back to the inverting input (FIG. 7). Since there is a direct connection between the output of the amplifier and the inverting input, the gain of this circuit is 1. For ex ample, if a signal voltage of 0.5 volts is connected to the noninverting input, the output voltage will be 0.5 volts also. You may wonder why anyone would want an amplifier that doesn't amplify. Actually, this circuit does amplify something. It amplifies the input impedance by the amount of the open loop gain. If the 741 has an open loop gain of 200,000 and an input impedance of 2 megohms, this circuit will give the amplifier an in put impedance of 200 k_2meg, or 400,000megohms.
This circuit connection is generally used for impedance matching purposes.
FIG. 7 Voltage follower connection.
FIG. 8 Noninverting amplifier connection.
FIG. 9 Inverting amplifier connection.
The second basic circuit is the noninverting amplifier (FIG. 8). In this circuit, the output voltage has the same polarity as the input voltage. If the input voltage is positive, the output voltage will be positive also. The formula is used to calculate the amount of gain in the negative feedback loop.
The third basic circuit is the inverting amplifier (FIG. 9). In this circuit, the output voltage is opposite in polarity to the input voltage. If the input signal is positive, the output voltage will be negative at the same instant in time. The formula is used to calculate the amount of gain in this circuit.
FIG. 10 Inverting level detector.
FIG. 11 Adjustable inverting level detector.
FIG. 12 Noninverting level detector.
Circuit Applications
The Level Detector
The operational amplifier is often used as a level detector or comparator. In this type of circuit, the 741 op amp is used as an inverted amplifier to detect when one voltage becomes greater than another (FIG. 10).
This circuit does not use above and belowground power supplies. Instead, it is connected to a power supply that has a single positive and negative output.
During normal operation, the noninverting input of the amplifier is connected to a zener diode that produces a constant positive voltage at the noninverting in put of the amplifier. This constant positive voltage is used as a reference. As long as the noninverting input is more positive than the inverting input, the output of the amplifier is high.
A light-emitting diode (LED), D1, is used to detect a change in the polarity of the output. As long as the output of the op amp is high, the LED is turned off.
When the output of the amplifier is high, the LED has equal voltage applied to its anode and cathode. Since both the anode and cathode are connected to_12 volts, there is no potential difference and, therefore, no cur rent flow through the LED.
If the voltage at the inverting input becomes more positive than the reference voltage applied to pin #3, the output voltage will fall to about_2.5 volts. The out put voltage of the op amp will not fall to 0 or ground in this circuit because the op amp is not connected to a voltage that is below ground. To enable the output voltage to fall to 0 volts, pin #4 must be connected to a voltage below ground. When the output drops, a potential of about 9.5 volts (12 - 2.5 = 9.5) is produced across R1 and D1. The lowering of potential causes the LED to turn on, which indicates that the op amp's output has changed from high to low.
In this type of circuit, the op amp appears to be a digital device in that the output seems to have only two states, high and low. But, the op amp is not a digital device. This circuit only makes it appear to be digital. In FIG. 10, there is no negative feedback loop connected between the output and the inverting input.
Therefore, the amplifier uses its open loop gain, which is about 200,000 for the 741, to amplify the voltage difference between the inverting input and the non-inverting input. If the voltage applied to the inverting input becomes 1 millivolt more positive than the reference voltage applied to the noninverting input, the amplifier will try to produce an output that is 200 volts more negative than its high state voltage (0.001_200,000_200).
The output voltage of the amplifier cannot be driven 200 volts more negative, though, because only 12 volts are applied to the circuit. Therefore, the output voltage reaches the lowest voltage it can and goes into saturation. This causes the op amp to act like a digital device.
If the zener diode is replaced with a voltage divider as shown in FIG. 11, the reference voltage can be set to any value by adjusting the variable resistor. For example, if the voltage at the noninverting input is set for 3 volts, the output of the op amp will go low when the voltage applied to the inverting input becomes greater than _3 volts. If the voltage at the noninverting input is set for 8 volts, the output voltage will go low when the voltage applied to the inverting input becomes greater than _8 volts. In this circuit, the output of the op amp can be manipulated through the adjustment of the noninverting input.
In the two circuits just described, the op amp's out put shifted from a high level to a low level. There may be occasions, however, when the output must be changed from a low level to a high level. This can be accomplished by connecting the inverting input to the reference voltage, and the noninverting input to the voltage being sensed (FIG. 12). In this circuit, the zener diode is used to supply a positive reference voltage to the inverting input. As long as the voltage at the inverting input is more positive than the voltage at the non inverting input, the output voltage of the op amp will be low. If the voltage applied to the noninverting input be comes more positive than the reference voltage, the out put of the op amp will become high.
Depending on the application, this circuit could cause a small problem. As stated previously, since this circuit does not use an above and below ground power supply, the low output voltage of the op amp is about _2.5 volts. This positive output voltage could cause any other devices connected to the op amp's output to be on when they should be off. For instance, if the LED shown in FIG. 12 is used, it will glow dimly even when the output is in the low state.
FIG. 13 Below ground power connection permits the output voltage to become negative.
FIG. 14 A zener diode is used to keep the output turned off.
One way to correct this problem is to connect the op amp to an above and below ground power supply as shown in FIG. 13. In this circuit, the output volt age of the op amp is negative or below ground as long as the voltage applied to the inverting input is more positive than the voltage applied to the noninverting input.
When the output voltage of the op amp is negative with respect to ground, the LED is reverse biased and cannot operate. If the voltage applied to the noninverting input becomes more positive than the voltage applied to the inverting input, the output of the op amp will become positive and the LED will turn on.
Another method of correcting the output voltage problem is shown in FIG. 14. In this circuit, the op amp is connected again to a power supply that has a single positive and negative output. A zener diode, D2, is connected in series with the output of the op amp and the LED. The voltage value of diode D2 is greater than the output voltage of the op amp in its low state, but less than the output voltage of the op amp in its high state. For instance, assume that the value of zener diode D2 is 5.1 volts. If the output voltage of the op amp in its low state is 2.5 volts, diode D2 will not conduct. If the output voltage becomes _12 volts when the op amp switches to its high state, diodeD2will turn on and con duct current to the LED. The zener diode, D2, keeps the LED completely off until the op amp switches to its high state, providing enough voltage to overcome the reverse voltage drop of the zener diode.
In the preceding circuits, an LED was used to indicate the output state of the amplifier. Keep in mind that the LED is used only as a detector, while the output of the op amp can be used to control almost anything. For example, the output of the op amp can be connected to the base of a transistor as shown in FIG. 15.
The transistor can then control the coil of a relay which could, in turn, control almost anything.
FIG. 15 The operational amplifier supplies the base current for a
switching transistor.
FIG. 16 A simple square wave oscillator.
The Oscillator
The operational amplifier can be used as an oscillator. The simple circuit shown in FIG. 16 produces a square wave output. However, this circuit is impractical because it depends on a slight imbalance in the op amp, or random circuit noise, to start the oscillator. A voltage difference of a few millivolts be tween the two inputs is all that is needed to raise or lower the output of the amplifier. For example, if the inverting input becomes slightly more positive than the noninverting input, the output will go low or become negative. When the output is negative, capacitor CT charges through resistor RT to the negative value of the output voltage. When the voltage applied to the inverting input becomes slightly more negative than the voltage applied to the noninverting input, the output changes to a high, or positive, value of voltage. When the output is positive, capacitor CT charges through resistor RT toward the positive output voltage.
This circuit would work well if there were no imbalance in the op amp and if the opamp were shielded from all electrical noise. In practical application, however, there is generally enough imbalance in the amplifier or enough electrical noise to send the op amp into saturation, which stops the operation of the circuit.
The problem with this circuit is that a millivolt difference between the two inputs is enough to drive the amplifier's output from one state to the other. This problem can be corrected by the addition of a hysteresis loop connected to the noninverting input as shown in FIG. 17. Resistors R1 and R2 form a voltage divider for the noninverting input. These resistors generally have equal value. To understand the circuit operation, assume that the inverting input is slightly more positive than the noninverting input. This causes the output voltage to be negative. Also assume that the out put voltage is _12 volts as compared to ground. If resistors R1 and R2 have equal value, the noninverting input is driven to _6 volts by the voltage divider. Capacitor CT begins to charge through resistor RT to the value of the output voltage. When capacitor CT has been charged to a value slightly more negative than the _6 volts applied to the noninverting input, the op amp's output rises to _12 volts above ground. When the out put of the op amp changes from_12 volts to_12 volts, the voltage applied to the noninverting input changes from _6 volts to _6 volts. Capacitor CT now begins to charge through resistor RT to the positive voltage of the output. When the voltage applied to the inverting input becomes more positive than the voltage applied to the noninverting input, the output changes to _12 volts.
The voltage applied to the noninverting input is driven from _6 volts to _6 volts, and capacitor CT again begins to charge toward the negative output voltage of the opamp.
The addition of the hysteresis loop has greatly changed the operation of the circuit. The voltage differential between the two inputs is now volts instead of millivolts. The output frequency of the oscillator is determined by the values of CT and RT. The period of one cycle can be computed by using the formula T _ 2RC.
FIG. 17 A square wave oscillator using a hysteresis loop.
FIG. 18 Output of an oscillator.
The Pulse Generator
The operational amplifier can be used as a pulse generator. The difference between an oscillator and a pulse generator is the period of time the output is on compared to the period of time it is low or off. For in stance, an oscillator is generally considered to produce a waveform that has positive and negative pulses of equal voltage and time (FIG. 18). The positive value of voltage is the same as the negative value, and the positive and negative cycles are turned on for the same amount of time. This waveform is produced when an oscilloscope is connected to the output of a square wave oscillator.
FIG. 19 Output of a pulse generator.
FIG. 20 Pulse generator circuit.
If the oscilloscope is connected to a pulse generator, however, a waveform similar to the one shown in FIG. 19 will be produced. The positive value of volt age is the same as the negative value, just as it was in FIG. 18, but the positive pulse is of a much shorter duration than the negative pulse.
The 741 operational amplifier can easily be changed from a square wave oscillator to a pulse generator (FIG. 20). The pulse generator circuit is the same basic circuit as the square wave oscillator with the addition of resistors R3 and R4, and diodes D1 and D2. This circuit permits capacitor CT to charge at a different rate when the output is high, or positive, than when the output is low, or negative. For instance, assume that the voltage of the op amp's output is _12 volts. When the output voltage is negative, diode D1 is reverse biased and no current can flow through resistor R3. Therefore, capacitor CT must charge through resistor R4 and diode D2, which is forward biased. When the voltage applied to the inverting input becomes more negative than the voltage applied to the noninverting input, the output voltage of the op amp rises to _12 volts. When the output voltage is _12 volts, diode D2 is reverse biased and diode D1 is forward biased. Therefore, capacitor CT begins charging to ward the _12 volts through resistor R3 and diode D1.
The amount of time the output of the op amp is low is determined by the value of CT and R4, and the amount of time the output remains high is determined by the value of CT and R3. The ratio of the amount of time the output voltage is high to the amount of time it is low can be determined by the ratio of resistor R3 to resistor R4. A typical 741 operational amplifier is shown in FIG. 21.
FIG. 21 Typical 741 Operational Amplifier
QUIZ
1. When the voltage connected to the inverting input is more positive than the voltage connected to the noninverting input, will the output be positive or negative?
2. What is the input impedance of a 741 operational amplifier?
3. What is the average open loop gain of the 741 operational amplifier?
4. What is the average output impedance of the 741 operational amplifier?
5. Operational amplifiers are commonly used in what three connections?
6. When the operational amplifier is connected as a voltage follower, it has a gain of 1 (one). If the input voltage is not amplified, what is?
7. Name two effects of negative feedback.
8. Refer to FIG. 8. If resistor R1 is 200 ohms and resistor R2 is 10 kilohms, what is the gain of the amplifier?
9. Refer to FIG. 9. If resistor R1 is 470 ohms and resistor R2 is 47 kilohms, what is the gain of the amplifier?
10. What is the purpose of the hysteresis loop when the op amp is used as an oscillator?