OP-AMP EXPERIMENTS
INTEGRATED CIRCUITS AND OPERATIONAL AMPLIFIERSAn integrated circuit is defined as a combination of interconnected circuit elements inseparably associated on or within a continuous semiconductor (often called a chip).
A number of important electronic devices, such as diodes and transistors, are separate devices that are individually packaged and interconnected in a circuit with other devices to form a complete, functional unit. Such devices are referred to as discrete components. In an IC, however, many transistors, diodes, resistors and capacitors are fabricated on a single tiny chip of semiconductor material and packaged in a single case to form a functional circuit. An IC is thus treated as a single device.
Operational amplifiers (Op-amps) are integrated electronic devices. In our laboratory course, we will be concerned with what the circuit does more from an external viewpoint than from an internal, component-level viewpoint.
The operational amplifier is an electronic circuit element designed to be used with other circuit elements to perform a specified signal processing
operation. It is basically a “solid-state device” with several circuits within a single package capable of sensing and amplifying dc and ac input signals. (“Solid state” gets its name from path that electrical signals take through solid pieces of
semiconductor material. Prior to the use of solid state devices, electricity passed through various elements inside of a heated vacuum tube.)
Early op-amps were constructed with vacuum tubes and worked with high
voltages. Today’s op-amps are linear integrated circuits that use relatively low dc supply voltage and are reliable and inexpensive.
1. OP-AMP BASICS
SYMBOL AND TERMINALSThe schematic diagram for a standard op-amp is represented as a triangle as shown in Figure 1.1.
The inverting input is represented by a minus sign. The voltage at this input will cause the output voltage to be inverted by180°. The non-inverting input is
represented by a plus sign. The voltage at this input will cause the voltage at the output to be in phase. The output terminal is at the apex of the triangle. Power supply leads are shown above and below the triangle. The dual (±) power supply connections enable the output to swing both positive and negative. These dc voltages must always be connected even though they may not be indicated on a schematic diagram. Other leads coming out of the op-amp may be used for frequency compensation or nulling components. These leads are also left off the schematic symbol for simplicity. Thus the simplified standard op-amp symbol is:
CIRCUIT FUNCTION OF THE OP-AMP
The circuit function of the op-amp is that it senses the difference between voltage signals applied at its two input terminals (vnon-in - vin), multiply this by a number A
(or Av, called the differential gain or voltage gain) and cause the resulting voltage
A(vnon-in – vin) to appear at the output terminal.
THE IDEAL AND PRACTICAL OP-AMP
To illustrate what an op-amp is, we consider its ideal characteristics. A practical op-amp, of course, falls short of these ideal standards, but it is much easier to understand and analyze the device from an ideal point of view.
Characteristics of an ideal op-amp are:
Infinite voltage gain and infinite bandwidth
Infinite input impedance (open) so that it does not load the driving source Zero output impedance
These characteristics are illustrated in Figure 1.3.
Although modern IC op-amps approach parameter values that can be treated as ideal in many cases, the ideal device can never be made. Any device has
limitations, and the IC op-amp is no exception. Op-amps have both voltage and current limitations. Peak to peak output voltage, for example, is also limited by internal restrictions such as power dissipation and component ratings.
Very high input impedance, which produces negligible current at the inputs Very high voltage gain, which is useful for amplifying very small signals Very low output impedance, so that it is affected very little by other circuit
loads
These characteristics are illustrated in Figure 1.4.
INTERNAL BOLCK DIAGRAM OF AN OP-AMP
A typical op-amp is made up of three types of amplifier circuits as shown in block diagram (Figure 1.5).
THE 741 OP AMP
The 741 operational amplifier is one of the commonly used integrated-circuit op-amps. It has eight pin connections as shown in Figure 1.6.
The lead identification shown in the Figure 1.6 is usually self-explanatory. The
positive supply voltage is connected to the +V terminal, and the negative supply voltage is connected to the –V terminal. Input and output terminals are clearly indicated. The balance terminals (sometimes designated “Offset Null”) are connected to a potentiometer for null adjusting. Terminals marked “NC” (no connection) are included for physical ruggedness of the package.
OP-AMP INPUT SIGNAL MODES Single-ended input
When an op-amp is operated in the single-ended mode, one input is grounded and the signal voltage is applied only to the other input, as shown in Figure 1.7. In the case where the signal voltage is applied to the inverting input as in Figure 1.7a, an inverted, amplified signal voltage appears at the output. In the case where the signal is applied to the noninverting input with the inverting input grounded, as in Figure 1.7b, a noninverted, amplified signal voltage appears at the output.
Differential input
In the differential mode, two opposite-polarity (out-of-phase) signals are applied to the inputs, as shown in Figure 1.8. This type of operation is also referred to as double-ended. The amplified difference between the two inputs appears on the output.
Common-mode input
In the common-mode, two signal voltages of the same phase, frequency and amplitude are applied to the two inputs, as shown In Figure 1.9. When equal input signals are applied to both inputs, they cancel, resulting in a zero output voltage.
This action is called common-mode rejection. Its importance lies in the situation where an unwanted signal appears commonly on both op-amps inputs.
Common-mode rejection means that this unwanted signal will not appear on the output and distort the desired signal. Common-mode signals (noise) generally are the result of the pick-up of radiated energy on the input lines, from adjacent lines, the 60 Hz power line, or other sources.
INPUT/OUTPUT VOLTAGE POLARITY
An important function to remember about an op-amp is the relationship of input voltage polarity to output voltage polarity. Figure 1.10 illustrates this relationship, where the noninverting input is at 0V or ground. If the inverting input is more positive than the noninverting input, the output will be at negative voltage
potential. Similarly, if the inverting input is more negati ve than noninverting input, the output voltage will be at a positive potential. This relationship remains even if both input voltages are positive or negative.
OP-AMP GAIN
Ideally, the gain of an op-amp should be infinite, however, practically, the gain may exceed 200,000 in the open-loop mode. In the open-loop mode, there is no
feedback from the output to the input and voltage gain (Av) is maximum, as
shown in Figure 1.11a.
The open-loop voltage gain, AOL, of an op-amp is the internal voltage gain of the
device and represents the ratio of output voltage to input voltage when there are no external components. The open-loop voltage gain, also referred to as large-signal voltage gain, is not a well-controlled parameter. In a practical circuit, the slight voltage difference at the inputs will cause the output voltage to attempt to swing to the maximum power-supply level. The maximum voltage at the output will be about 90% of the supply voltage because of the internal voltage drops of the op-amp. The output is said to be at saturation and can be represented (for either polarity) by +Vsat and –Vsat. As an example, an op-amp circuit in the
open-loop mode using a ±15V supply would have its output swing from +13.5 to -13.5. With this type of circuit the op-amp is very unstable and the output will be 0V for a 0V difference between the inputs, or the output voltage will be at either
extreme, with a slight voltage difference at the inputs. The open-loop mode is found primarily in voltage comparators and level-detector circuits.
The versatility of the op-amp is demonstrated by the fact that it can be used in so many types of circuits in the closed-loop mode, as shown in Figure 1.11b.
External components are used to feedback a portion of the output voltage to the inverting input. This feedback stabilizes most circuits and can reduce the noise level. The voltage gain (Av) will be less than maximum gain in open-loop mode.
Closed-loop gain must be controlled to be of any value in a practical. By adding resistor Rin to the inverting input as shown in Figure 1.11c, the gain of the
op-amp can be controlled. The resistance ratio of Rf to Rin determines the voltage
gain of the circuit and can be found by the formula v f in R A R
If both Rin and Rf are the same value, the Av equals 1, or unity gain as shown in
Figure 1.11d. In this noninverting configuration, the voltage out equals the voltage in and Av equals +1.
OP-AMP FREQUENCY RESPONSE
The gain of an op-amp decreases with an increase in frequency. The gain given by manufactures is generally at zero hertz or dc. At very low frequencies, the open-loop gain of an op-amp is constant, but starts to taper off at about 6Hz or so at a rate of -6 dB/octave or -20db/decade (an octave is a doubling in
frequency, and a decade is ten-fold increase in frequency). This decrease continues until the gain is unity, or 0dB. The frequency at which the gain is unity is called the unity gain frequency. The unity gain point occurs at 1MHz. The unity gain frequency establishes the reference point at which many op-amps are specified by manufacturers.
Figure 1.12 shows a voltage-gain versus frequency-response curve. In the open-loop mode, the gain falls off very rapidly as frequency increases. When the frequency increases tenfold, the gain decreases by 10. The breakover point occurs at 70.7% of the maximum gain. The frequency bandwidth is normally considered at the point where the gain falls to the breakover point. Therefore, the open-loop bandwidth is about 10 Hz for this example. Op-amps usually require degenerative feedback in amplifier circuits, and this feedback increases
bandwidth of the circuit. For a closed-loop gain of 100, the bandwidth has increased to about 10 kHz. Lowering the gain to 10 increases the bandwidth to about 100 kHz.
The gain-bandwidth product is equal to the unity-gain frequency. It not only tells us the upper useful frequency of a circuit, but allows us to determine the
bandwidth for a given gain. For example (referring to Figure 1.12, which shows a frequency-response curve for a frequency-compensated op-amp, such as the 741), if you multiply the gain and bandwidth of a specific circuit, the product will equal the unity-gain frequency:
gain - bandwidth product = gain bandwidth = unity - gain frequency GBP = 100 10 kHz = 1000000 Hz (1MHz)
or
GBP = 10 100 kHz = 1000000 Hz (1MHz)
Therefore, if we wanted to know the upper frequency limit or bandwidth of a circuit with gain of 100, we would divide the unity-gain frequency by gain:
unity - gain frequency bandwidth = gain 1000000 BW = = 10kHz 100 OFFSET NULLING
Ideally the output voltage of an op-amp should be zero when the voltages at both inputs are the same or zero. If the two input terminals of the op-amp are tied together and connected to ground, it will be found that a finite dc voltage exists at the output (Figure 1.13a). This is the output dc offset voltage (VOO). In a critical
circuit, this offset can cause error voltages at output. If we divide the output dc offset voltage by the gain AOL, we obtain the input offset voltage VIO. The latter
may be represented by a voltage source connected in series with one of the input leads of an ideal op-amp, which would cause the output dc voltage to be reduced to zero as shown in Figure 1.13b.
Most integrated circuit op-amps provide a means of compensating for offset voltage. An external potentiometer is connected to one of the inputs and then it is adjusted to bring back the output voltage to zero when the voltage difference at the inputs is zero. This method is called “offset nulling” or “input offset voltage compensation”. Many op-amps have offset nulling pins, as shown in Figure 1.14. The ends of the potentiometer are connected to these pins with the viper
attached to the –V supply. Often null circuits are used with an op-amp but are not shown on the schematic diagram.
2. OP-AMP PARAMETERS
The following parameters are useful to know when working with op-amps. INPUT PARAMETERS:
Differential input voltage
The difference of voltage between the two inputs is called differential input voltage.
Input offset voltage (VIO)
The ideal op-amp produces zero volts out for zero volts in. In a practical op-amp, however, a small dc voltage, Vout (error), appears at the output when no
differential input voltage is applied. The input offset voltage, VIO, is the differential
values of input offset voltage are in the range of 2mV or less. In the ideal case, it is 0 volts.
Input bias current (IB)
In order for the (real) op-amp to operate, its two input terminals have to be applied by finite dc currents, termed the input bias currents. By definition, the input bias current, IB, is the average of both input currents. Ideally, the two input
bias currents are equal. Input offset current (IIO)
Ideally, the two input bias currents are equal, and thus their difference is zero. In a practical op-amp, however, the bias currents are not exactly equal. The input offset current, IIO, is the difference of the input bias currents (expressed as an
absolute value).
Common-mode input voltage range
All op-amps have limitations on the range of voltages over which they will operate. The common-mode input voltage range is the range of input voltages which, when applied to both inputs, will not cause clipping or other output
distortion. Many op-amps have common-mode input voltage ranges of ±10V with dc supply voltages of ±15V.
Input resistance (ZI)
This is the resistance “looking in” at either input with the remaining input grounded.
OUTPUT PARAMETERS: Output offset voltage (VOO)
Output offset voltage, VOO, is a slight unwanted voltage at the output when the
voltage between inputs is zero. Ideally, VOO should be zero.
Output short-circuit current (IOSC)
The maximum output current that the op-amp can deliver to a load is called output short-circuit current, IOSC.
Output voltage maximum swing (±VOm ax)
Depending on the load resistance, output voltage maximum swing, ±VOmax, is the
maximum peak output voltage that the op-amp can supply without saturation or clipping.
Output resistance (ZO)
This is the resistance “looking into” the op-amp’s output. DYNAMIC PARAMETERS:
Open-loop voltage gain (AOL)
Ratio of the output voltage to the differential input voltage in a differential
amplifier without the external feedback is called open-loop voltage gain, AOL, or
differential gain. Slew rate (SR)
The maximum rate of change of the op-amp’s output voltage under large signal conditions is called slew rate, SR.
SR =ΔVout Δt where ΔV = +Vout max-(-Vmax)
Δt is the time interval required for the output voltage to go from its lower limit to its upper limit.
The unit of slew rate is volts per microsecond (V/µs).
Slew rate tells how fast the op-amp can react to changes at input. It reflects the op-amp’s ability of handling varying signals. If one tries to drive the output at a rate of voltage change greater than the slew rate, the output would not be able to change fast enough and would not vary over the full range expected resulting in signal clipping or distortion. In any case, the output would not be an amplified duplicate of the input signal if op-amp slew rate is exceeded.
Consider the unity gain follower circuit in Figure 2.1 and let the input voltage V±
be the step voltage of height V (shown in Figure 2.2a). When the op-amp is slew rate limited (or slewing) it is not capable of responding to its input signal without distortion and the output appears as shown in Figure 2.2b.
If sinusoidal waveform is applied at the inputs of the unity gain follower, the op-amp slew rate limiting causes nonlinear distortion as shown in Figure 2.3.
OTHER PARAMETERS AND DEFINITIONS: Supply current
This is the current the op-amp will draw from the power supply. Common-mode voltage (VCM)
Common-mode voltage is an unwanted, but unavoidable voltage on both inputs, such as 60-cycle hum.
Common-mode gain (ACM)
Ideally, an amp provides zero gain for common-mode signals but practical op-amps do exhibit a very small common-mode gain, (ACM), which is defined as the
ratio of the common-mode output voltage to the common-mode input voltage. Common-mode rejection ratio (CMRR)
Common-mode rejection ratio, (CMRR), is a measure of the ability of the op-amp to reject signals that are simultaneously present at both inputs. It is the ratio of the open-loop voltage gain, AOL, to the common-mode gain, ACM.
OL
CM A CMRR =
A
The higher the CMRR, the better. A high value of CMRR means that the open-loop gain, AOL, is high and the common mode gain ACM, is low and the
performance of the op-amp in terms of rejection of common mode signals is better.
Power supply voltage rejection ratio (PSRR)
The ratio of the change in the power supply voltage to the resulting change in input offset voltage is called power supply voltage rejection ratio, (PSRR). Variation in power supply voltage will also affect the input offset voltage. Power supply decoupling
Capacitors in the range 0.1 to 1.0 µF connected from the power supply voltages to ground to bypass voltage variations to ground provide the power supply decoupling.
Input protection
Diodes, zener diodes, and/or resistors are used at the inputs to protect the op-amp from excessively large input voltages.
Latch-up is a condition where a large input signal causes the output to remain in +Vsat or –Vsat. Diodes and resistors used in the output circuit can prevent this.
Output protection
A low-value resistor connected in series with the output of an op-amp to limit current during a short-circuit condition provides output protection. Some op-amps have the protection built in.
3. PRACTICAL OP-AMP CIRCUITS
(DESIGN USING OP-AMP)
One of the early applications of operational amplifiers was to build circuits that performed mathematical operations. Indeed, the operational amplifier takes its name from this important application. Many of the op-amp circuits that perform mathematical operations are used so often that they have been given names (e.g. summing amplifier, difference amplifier, integrator, differentiator etc). Op-amp can be connected in a large number of circuits to provide various operating characteristics. Some of the basic applications are discussed below:
“Open-loop mode” circuits “Basic linear amplifier” circuits
“Integrator”, “Differentiator” and “Square wave generator” circuits OPEN-LOOP MODE CIRCUITS:
Comparator is a circuit that compares two input voltages and produces an output in either of two states indicating the greater than or less than relationship of the inputs. In this application, the op-amp is used in the open-loop
configuration, with the input voltage on one input and a reference voltage on the other.
The polarity of the voltage at the output of an op-amp depends on the
relationship of the polarity between the voltages at the inputs. The inverting ( -) input is referenced to the noninverting (+) input. When the inverting (-) input is more positive than the noninverting (+) input, the output will be negative and when the inverting (-) input is more negative than the noninverting (+) input, the output will be positive. Without a feedback path, the output will either be at +Vsat
or –Vsat. Figure 3.1 shows a comparator.
A comparator circuit can be used for: Zero-level detection
Nonzero-level detection Zero-level detector
The inverting (-) input is grounded to produce a zero level and the input signal voltage is applied to the noninverting (+) input. Because of the high open-loop voltage gain, a very small voltage difference between the two inputs drives the amplifier into saturation, causing the output voltage to go to its limits.
Figure 3.2b shows the result of a sinusoidal input voltage applied to the
noninverting (+) input of the zero-level detector. When the sine wave is positive, the output is at its maximum positive level. When the sine wave crosses zero, the amplifier is driven to its opposite state and the output goes to its maximum
negative level. Thus the zero-level detector can be used as a squaring circuit to produce a square wave from a sine wave.
Nonzero level detector
An op-amp comparator can be used to detect a positive voltage level as shown in Figure 3.3a. It is inverting input sensor. The reference voltage at the noninverting input is found by the formula
ref 3
2 3
R
V = +V
When the voltage at the inverting input is below Vref, the output is at +Vsat. When
the voltage at the inverting input increases above Vref, the output swings to –Vsat.
When Vref is at the inverting input as shown in Figure 3.3b, it becomes a
noninverting input sensor. The output will swing to +Vsat the instant the voltage at
the noninverting input is greater than Vref.
If point A is moved to –V power supply the circuits will detect a negative voltage. Figure 3.4a shows the arrangement with a sinusoidal input voltage applied to noninverting input of the nonzero-level detector.
BASIC LINEAR AMPLIFIER CIRCUITS:
Linear applications are those in which the output signal is directly proportional to the input signal.
Negative feedback is one of the most useful concepts in electronics, particularly in op-amp linear applications. Negative feedback is the process whereby a portion of the output voltage of an amplifier is returned to the input with a phase angle that opposes (or subtracts from) the input signal.
The usefulness of an op-amp in an open-loop mode (i.e. without negative
feedback) is severely restricted and is generally limited to comparator and other nonlinear applications. As the inherent open-loop voltage gain of a typical op-amp is very high therefore, an extremely small input voltage drives the op-op-amp into its saturated output states and the op-amp becomes nonlinear. With negative feedback, the closed-loop voltage gain can be reduced and controlled so that the op-amp can function as a linear amplifier. Negative feedback is illustrated in Figure 3.5.
The inverting input effectively makes the feedback signal 180° out of phase with the input signal. The “negative feedback network” closes the loop around the op-amp. The gain of op-amp in such configurations is called the closed loop gain. Inverting Amplifier
An op-amp connected as an inverting amplifier with a controlled amount of voltage gain is shown in Figure 3.6.
The input signal is applied through a series input resistor Rin to the inverting
input. Also the output is fed back through Rf to the same input. The noninverting
input is grounded.
The gain of the circuit is calculated by the formula Av= -Rf/Rin (the minus sign
indicates only that the polarity of the output voltage is opposite to the polarity of the input voltage) or can be found by Av= -Vout/Vin.
The junction of Rf and Rin at the inverting input is about the same voltage as the
noninverting input and is referred to as virtual ground.
To reduce the offset bias currents, the noninverting input is not directly grounded but a resistor Rn is used. Rn is equal to the value of Rin and Rf in parallel
(Rn=RinRf/Rin+Rf).
When inverting amplifier is used for ac signals, capacitors are used at the input and output terminals, to block any dc voltage from the circuit which might cause distortion. The frequency response of an op-amp circuit depends on its gain. The lower the gain, the wider the frequency response.
Noninverting Amplifier
An op-amp connected as a noninverting amplifier with a controlled amount of voltage gain is shown in Figure 3.7.
The input signal is applied to the noninverting input. The output is applied back to the inverting input through the feedback circuit (closed loop) formed by the input resistor Rin and the feedback resistor Rf.
The gain of the circuit is calculated by the formula Av= Rf/Rin+1 or Av= Vout/Vin.
When the noninverting amplifier is used for ac signals, capacitors are used at the input and output terminals, to block any dc voltage form the circuit that might cause distortion. Even though the input voltage changes, an amplifier’s gain remains the same. A noninverting amplifier is used for high input impedance, where Rin cannot be made larger, because of affecting the gain of the circuit and
creating more noise.
Voltage followers (or Source followers)
Voltage followers are special cases of the noninverting and inverting amplifiers. A noninverting amplifier with Rf=0 and Rin=∞, becomes a noninverting voltage
follower as shown in Figure 3.8a. It has a gain of 1 because of the zero resistance feedback loop. It is referred to as voltage follower since the output “follows” the input and is in phase with the it. Because of gain of 1, this circuit is also named as the unity gain amplifier. The impedance to this circuit can be made very high.
An inverting amplifier with Rf=Rin becomes an inverting voltage follower as shown
in Figure 3.8b.
The gain of this circuit is 1 (Av= -Rf/Rin) and the output voltage is 180° out of
phase with the input voltage. The input impedance to this circuit is lower, being limited by the value of Rin.
Voltage followers are used to match circuit impedances and act as buffer amplifiers, isolating one circuit from another.
If more than one input is used on an inverting amplifier, it becomes a summing circuit or adder as shown in Figure 3.9.
The output voltage is the algebraic sum of the inputs, but inverted, and can be found by the formula
out f 1 f 2 f n 1 2 n R R R V V V ... V R R R
where Rn and Vn are the number of input resistors and input voltages. The output
voltage is weighted sum of the input signals (V1, V2,….Vn). This circuit is
therefore called weighted summer. Each summing coefficient may be
independently adjusted by adjusting the corresponding “feed-in” resistors (R1 to
Rn).
When all the resistors in the summing amplifier are of the same value, the circuit becomes unity gain summing amplifier and the formula for Vout simplifies to
Vout
V1V2... V n
When all input resistors are of the same value with Rf a larger value, the circuit
becomes summing amplifier with gain. Vout is given by
Vout Rf
V1 V2 ... Vn
R
where R is the value of each equal-value input resistor.
When in the summing amplifier with gain, the ratio Rf/R is set equal to the
reciprocal of the number of inputs (n), the circuit becomes “averaging amplifier”. Hence the summing amplifier produces the mathematical average of the input voltages when Rf/R=1/n.
When different weights are assigned to each input of a summing amplifier, by adjusting the values of the input resistors, the circuit becomes scaling adder. In this circuit, some inputs influence the output voltage more than the others. The weight of a particular input is set by the ratio of Rf to the resistance Rx for that
input (Rx = R1, R2,….,Rn). For example, if an input voltage is to have a weight if 1,
then. Or, if a weight of 0.5 is required, Rx=2Rf. The smaller the value of input
resistance Rx, the greater the weight, and vice versa.
The input currents and current through Rf add up to zero at the inverting input,
referred to as the current summing point. The summing amplifier can also be used as an audio signal mixer.
Both inputs are used (or active) for a difference amplifier or subtractor, as shown in Figure 3.10. The output voltage is found by the formula
2 1
2 out 1 2 1 3 4 1 R R R V V V R 1 R R If all resistors are equal, the formula simplifies to Vout=V2-V1; however, the
polarity of the output voltage depends on the relationship of the inverting and noninverting inputs polarities, similar to a comparator circuit.
A difference amplifier may have gain or use scaling input arrangement where one input has more influence on the output.
DIFFERENTIATOR, INTEGRATOR AND SQUARE WAVE GENERATOR CIRCUITS:
Op-amp Differentiator
An op-amp differentiator simulates mathematical differentiation, which is a
process of determining the instantaneous rate of change of a function. The basic op-amp differentiator, shown in Figure 3.11, is similar to the basic inverting amplifier, except that the input element is a capacitor. This circuit produces output that is proportional to the rate of change of the input voltage and is given by
Vout R Cf dVin d t
The product RfC is called the time constant and it should be approximately equal
to the period of the input signal to be differentiated.
Op-amp Integrator
An op-amp integrator simulates mathematical integration, which is basically a summing process that determines the total area under the curve of a function.
The basic op-amp integrator, shown in Figure 3.12, is similar to the basic
inverting amplifier, except that the feedback element is a capacitor. This circuit is said to be inverse of the differentiator circuit, which is consistent with the
mathematical operation of differentiation and integration.
The output voltage of the integrator, as a function of time, is given by t out in 0 1 V V dt RC
The product RC is the time constant and, as with the differentiator circuit, it is made approximately equal to the period of the input signal to be integrated.
Op-amp square wave generator
An op-amp can be constructed to produce a square-wave generator as shown in Figure 3.13. Resistors R2 and R3 form a voltage divider from the output of the
op-amp to ground and determine the ±Vref. Assume, initially, that Vout is at +Vsat.
Capacitor C1 begins to charge through R1 to +Vsat. The instant the voltage on the
capacitor is greater than +Vref at the noninverting input, the output switches to
–Vsat. The capacitor now charges toward –Vsat and the instant it is greater than
–Vref, the output switches back to +Vsat and the process begins again. The square
wave output at Vout is ±Vsat in amplitude. The amplitude of VC1 is ±Vref and can be
found by the formula
ref 3
sat
2 3 R +V +V R R and ref 3
sat
2 3 R -V -V R R
If R3 is 86% of R2, the approximate output frequency can be found by the formula
out 1 1 1 f 2R C
BEFORE STARTING THE EXPERIMENTS
Inverter check: Before starting the experiments, check that your IC is
working properly. This can be easily done by connecting the op-amp in an inverting unity gain amplifier (as shown in figure below) and checking the output signal on scope for any suitable input signal.
(It is better to do a quick inverter check than to waste time experimenting with a damaged IC.)
Power supply range: Op-amps are designed to be powered from
voltage supply which is typically in the range of ±5 to ±15 volts. To avoid damaging the op-amp use ±12 volts for voltage supply in the experiment.
Power supply polarity: Never reverse power supply polarity to the
op-amps. Applying a negative voltage to the “+V” pin and a positive to the “-V” pin, even momentarily will result in destructive current flow through the op-amp!
Power and signal sources: After wiring the circuit, connect or turn on
the power and signal sources to the breadboard last!
Planning the experiment: Plan your experiment beforehand. Know
what type of results you are expected to observe. Don’t mindlessly take data unless you have a good idea of what should be observed. You can analyze things before doing the lab or as you go along.
4. EXPERIMENTS
EXPERIMENT 1To demonstrate the basic operation of an op-amp as a comparator circuit. Apparatus: dual 12V power supply, digital voltmeter, 741 op-amp,
10kpontentimeters, 10kresistors, bread board for constructing circuit Procedure: Circuit is constructed as shown in Figure 4.1. V1 and V2 are set to
definite values and the corresponding values of Vout are recorded, indicating
polarity. EXPERIMENT 2 Observations V1 (V) V2 (V) Vout (V) +1 0 -1 0 0 +1 0 -1 +2 +1 +1 +2 +1 -1 -1 +1 -1 -2 -2 -1
To demonstrate the operation of an op-amp inverting amplifier with dc and ac voltages and calculate gain of the circuit.
Apparatus: dual 12V power supply, digital voltmeter, 741 op-amp, oscilloscope, AC signal generator, 10 kpotentiometer, breadboard for constructing circuit, resistors (4.7kkkkkkF capacitors
Procedure: For dc amplifier, circuit is constructed as shown in Figure 4.2a. For different values of Rin, Rf and Vin (as shown in data table), Vout is measured. Gain
is calculated by the formulae: Av=-Rf/Rin and Av=Vout/Vin.
Observations and Calculations Rin (k) Rout=Rf (k) Vin (V) Vout (V) Av=-Rf/Rin Av=Vout/Vin 10 47 +1 10 100 +1 10 22 +1 4.7 47 -1 22 47 -1 10 47 -1
For ac amplifier, circuit is constructed as shown in Figure 4.2b. For
Rf=100kthe frequency generator is set at Vin=1Vp-p and Vout is measured for
different frequencies (f). Gain is calculated by: Av=Vout/Vin. Graph is plotted
Observations and Calculations
Rf=100k Rf=47k f (kHz) (for Vin at 1 Vp-p) Vout (Vp-p) Av=Vout/Vin Vout (Vp-p) Av=Vout/Vin 0.1 0.15 0.2 0.5 1 1.5 2 5 10 15 20 50 100 150 200 500 1000 1500 2000
EXPERIMENT 3
To demonstrate the operation of an op-amp non-inverting amplifier with dc and ac voltages and calculate gain of the circuit.
Apparatus: dual 12V power supply, digital voltmeter, 741 op-amp, oscilloscope, AC signal generator, 10 kpotentiometer, breadboard for constructing circuit, resistors (4.7kkkkkkF capacitors
Procedure: For dc amplifier, circuit is constructed as shown in Figure 4.3a. For different values of Rin, Rf and Vin (as shown in data table), Vout is measured. Gain
is calculated by the formulae: Av=Rf/Rin+1 and Av=Vout/Vin.
Observations and Calculations Rin (k) Rf (k) Vin (V) Vout (V) Av=Rf/Rin+1 Av=Vout/Vin 10 47 +1 10 100 +1 10 22 +1 4.7 47 -1 22 47 -1 10 47 -1
For ac amplifier, circuit is constructed as shown in Figure 4.3b. For Rf=100kgain is calculated by the formula: Av=Rf/Rin+1. The frequency
generator is set at 1kHzand Vout is measured for different input voltages Vin(Vp-p).
Vout is also calculated by the formula: Vout=AvVin.
Observations and calculations Vin (Vp-p) Vout (Vp-p) (measured) Vout=AvVin (Vp-p) (calculated) 0.1 0.2 0.5 1.0 1.5 EXPERIMENT 4
To demonstrate the operation of op-amp voltage followers, and to show the difference between the inverting and non-inverting types.
Apparatus: dual 12V power supply, digital voltmeter, 741 op-amp, oscilloscope, AC signal generator, breadboard for constructing circuit, 1F capacitors, resistors (4.7kkk
Procedure: Circuit is constructed as shown in Figures 4.4a and 4.4b. The signal generator is set for 1kHz at 2Vp-p for Vin. Vout is measured and the output
waveform is drawn for both the circuits.
EXPERIMENT 5
To demonstrate how an op-amp can be used to sum algebraically various input voltages.
Apparatus: dual 12V power supply, digital voltmeter, 741 op-amp, resistors (4.7kkk 10 kpotentiometers, breadboard for constructing circuit Procedure: Circuit is constructed as shown in Figure 4.5a, using all 10k resistors. V1 and V2 are set at different voltages and corresponding Vout is
measured. Vout is also calculated by the formula: Vout= -(V1+V2). Same procedure
is repeated after changing Rf to 22k but with Vout calculated by:
Observations and calculations
Rf=10k Rf=22k
Input voltage Vout algebraic sum (inverted) Input voltage Vout algebraic sum (inverted)
V1 V2 Calculated Measured V1 V2 Calculated Measured
(V) (V) (V) (V) (V) (V) (V) (V) +1 +2 +1 +2 +1 -2 +1 -2 +2 +1 +2 +1 +2 +1 +2 +1 -2 -2 -2 -2
Circuit shown in Figure 4.5b is constructed by using the voltage divider circuits of the first part. V1 and V2 are set at different voltages and corresponding Vout is
EXPERIMENT 6
To demonstrate how an op-amp can be used to find the algebraic differences between two input voltages.
Apparatus: dual 12V power supply, digital voltmeter, 741 op-amp, kresistors 10 kpotentiometers, breadboard for constructing circuit Procedure: Circuit is constructed as shown in Figure 4.6. V1 and V2 are set at
different voltages and corresponding Vout is measured. Vout is also calculated by
the formula: Vout= -(V2 -V1).
Observations and calculations
Input voltage Vout algebraic sum (inverted) V1 (V) V2 (V) Calculated (V) Measured (V) +1 +2 +1 -2 +2 +1 +2 -1 -2 -2
EXPERIMENT 7
To demonstrate how an op-amp can sense a specific voltage level and how to calculate the reference voltage (Vref).
Apparatus: dual 12V power supply, digital voltmeter, 741 op-amp, resistors (kk 10 kpotentiometer, breadboard for constructing circuit
Procedure: Circuit is constructed as shown in Figure 4.7a. Wiper of R1 is placed
at gound and Vref is calculated by the formula: Vref= (+V)(R3)/(R2+R3). Using the
voltmeter Vout and Vref are measured. R1 is adjusted until Vout changes and the
new reading is recorded. Wire at point A is removed from the +V supply and connected to the –V supply and the above steps are repeated to detect negative supply. Same procedure is repeated to detect voltages for circuit shown in Figure 4.7b.
Observations and calculations
Input voltage Vout algebraic difference (inverted) V1 (V) V2 (V) Calculated (V) Measured (V) +2 +4 +4 +2 +4 -2 -2 +4 -4 +2
To detect positive voltage
Vref = ---V
Vout = ---V when Vin is less than Vref
Vout = ---V when Vin is greater than Vref
To detect negative voltage
Vref = ---V
Vout = ---V when Vin is less than Vref
EXPERIMENT 8
To show how an op-amp can be used as a square-wave generator and how to calculate its output frequency.
Apparatus: dual 12V power supply, digital voltmeter, 741 op-amp, oscilloscope, breadboard for constructing circuit, capacitors(0.1F, 0.02F, 0.05F), resistors (22k k 4.7kkk
Procedure: Circuit is constructed as shown in Figure 4.8. Vref is calculated using
the formulae: +Vref=(+Vsat)(R3)/(R2+R3) and –Vref=(-Vsat)(R3)/(R2+R3). Using the
oscilloscope fout,+Vsat, -Vsat, +Vref and –Vref are measured and waveforms at Vref,
Vout and V1 are drawn. Frequency of the generator is calculated by: fout=1/2R1C1.
These steps are repeated for different values of R1and C1.
Observations and calculations
To detect positive voltage
Vref = ---V
Vout = ---V when Vin is less than Vref
Vout = ---V when Vin is greater than Vref
To detect negative voltage
Vref = ---V
Vout = ---V when Vin is less than Vref
Observations and calculations
R1 (k) C1 F) fout (Hz) Calculated fout(Hz) Measured 10 0.05 22 0.05 4.7 0.05 10 0.02 10 0.1 REFERENCES:
Electronic devices by Thomas L. Floyd
Electronic devices and circuit theory by Robert L. Boylestad and Louis Nashelsky
Introduction to electric circuits by Richard C. Dorf and James A. Svoboda Microelectronic circuits by Adel S. Sedra and Kenneth C. Smith
Operational Amplifiers (Electronic Technology Series) by Heathkit educational systems
