Experiment 1.2: Characterization of op-amp
►1.17◄
EXPERIMENT–1.2
CHARACTERIZATION OF OP-AMP
1.2.1 OBJECTIVE
1. To sketch and briefly explain an operational amplifier circuit symbol and identify all
terminals
2. To list the amplifier stages in a typical op-amp and briefly discs each stage.
3. To explain the negative feedback control in op-amp circuits.
4. To discuss the op-amp modes and most important op-amp parameters.
5. To measure the input bias current, input offset current, input offset voltage, input and
output voltage ranges, the slew rate and bandwidth of op – amp.
1.2.2
HARDWARE REQUIRED
a. Power supply
:
Dual variable regulated low voltage DC source
b. Equipments
:
CRO, AFO, DMM (Digital Multimeter), DRBs
c. Resistors
:
d. Semiconductor
:
IC741 op-amp
e. Miscellaneous
:
Bread board and wires
1.2.3
PRE LAB QUESTIONS
1. Determine the output voltage of an op-amp for the input voltages of Vi1=150µV and
Vi2=140µV. The amplifier has a differential gain of Ad=4000 and the value of CMRR is 100.
2. Calculate the output voltage of an inverting amplifier for values of VS=1V, Rf=500K and
R1=100K.
3. Calculate the output voltage of a non-inverting amplifier for values of VS=1V, Rf=500K and
R1=100K.
4. Calculate the output offset voltage of the circuit in Fig (a). The op-amp spec lists VIO=1.2mV.
5. Calculate the offset voltage for the circuit in fig (a) for op-amp spec listing IIO=100nA.
6. Calculate the total offset voltage for the circuit of fig (a) for an op-amp with specified values of
VIO=1.2mV and IIO=100nA.
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Experiment 1.2: Characterization of op-amp
►1.18◄
150k
+VCC
2k
V1
uA741
Vo
+
-VCC
Fig (a)
7. Calculate the input bias current at each input of an op-amp and input offset current having
specified values of IIO=5nA and IIB=30nA.
8. For an op-amp having a slew rate of 2 V/µs, what is the maximum closed loop voltage gain that
can be used when the input signal varies by 0.5V in 20µs.
9. How long does it take the output voltage of an op-amp to go from -10V to +10V if the slew rate
is 0.5V/µs.
10. Determine the input bias current and input offset current, given that the input currents of an opamp are 8.3µA and 7.9µA.
1.2.4 THEORY
An op-amp is a high gain, direct coupled differential linear amplifier choose response
characteristics are externally controlled by negative feedback from the output to input, op-amp has
very high input impedance, typically a few mega ohms and low output impedance, less than 100Ω.
Op-amps can perform mathematical operations like summation integration, differentiation,
logarithm, anti-logarithm, etc., and hence the name operational amplifier op-amps are also used as
video and audio amplifiers, oscillators and so on, in communication electronics, in instrumentation
and control, in medical electronics, etc.
1.2.4.1 Circuit symbol and op-amp terminals
The circuit schematic of an op-amp is a triangle as shown below in Fig. 1-2-1 op-amp has
two input terminal. The minus input, marked (-) is the inverting input. A signal applied to the minus
terminal will be shifted in phase 180o at the output. The plus input, marked (+) is the non-inverting
input. A signal applied to the plus terminal will appear in the same phase at the output as at the
input. +VCC denotes the positive and negative power supplies. Most op-amps operate with a wide
EC0222 Electronic Circuits Lab Manual
Experiment 1.2: Characterization of op-amp
►1.19◄
range of supply voltages. A dual power supply of +15V is quite common in practical op-amp
circuits. The use of the positive and negative supply voltages allows the output of the op-amp to
swing in both positive and negative directions.
+Vcc
inverting input
offset null
non-inverting
input
output
uA741
+
offset null
-Vcc
Fig 1-2-1 op-amp circuit symbol
1.2.4.2 Negative feedback control
The basic circuit connection using an op-amp is shown below in fig. 1-2-2
Rf
+VCC
R1
-
uA741
Vo
+
-
Vs
-VCC
Fig 1-2-2 op-amp circuit connection in the inverting mode
An input signal, Vs is applied through resistor R, to the minus input. The output is then connected
back to the same minus input through resistor Rf. The plus input is connected to ground since the
signal is essentially applied to the minus input the resulting output is opposite in phase to the input
signal Note that the output is feedback to the minus input terminal (inverting input terminal) in
order to provide negative feedback for the amplifier. This circuit arrangement is called inverting
amplifier.
For this amplifier, the output can be defined as
VO = −(
Rf
R1
)V S
(1-2-1)
The minus sign indicates that the sign of the output is inverted as compared to the input. The
equation for gain of this amplifier is
Gain = −(
Rf
)
R1
(1-2-2)
EC0222 Electronic Circuits Lab Manual
Experiment 1.2: Characterization of op-amp
►1.20◄
It is also possible to operate the op-amp as a non-inverting amplifier by applying the signal to the
plus input (non-inverting input terminal), as shown below in fig. 1-2-3.
Rf
+VCC
R1
-
uA741
Vo
+
Vs
-VCC
Fig 1-2-3 op-amp circuit connection in the non-inverting mode
Note that the feedback network is still connected to the inverting input. For this amplifier circuit,
the output of the amplifier is defined by
VO = (1 +
Rf
)VS
R1
Gain = 1 +
and its gain is
(1-2-3)
Rf
(1-2-4)
R1
1-2-4-3 The op-amp transfer characteristics
The transfer characteristics of a typical op-amp are sketched in fig. 1-2-4 and it shows three
regions of operation, namely the linear region, the negative saturation region and the positive
Vo
saturation region.
+Vcc
+ve saturation
+Vsat
range of input for
linear operation
----------------------------------
------------------- linear
region
------------------------------------------------------------------------------
(V1 - V2)
-Vsat
-ve s aturation
-Vcc
Fig 1-2-4 op-amp transfer characteristics
EC0222 Electronic Circuits Lab Manual
Experiment 1.2: Characterization of op-amp
►1.21◄
In the linear region, the output voltage VO is linearly related to the difference in the input voltage
(V1-V2). The supply voltage limits the maximum value of the output voltage. The OUTPUT voltage
is normally 2 to 3 volts lower than the power supply voltage, ie., |VO| < |VCC|
Also,
VO = A (V1-V2)
(1-2-5)
Therefore
| (V1-V2)| < |VCC/A|
(1-2-6)
For VCC = 15V and A = 105, |V1-V2| < 150µV.
Thus, for very high gain op-amps, the input voltages V1 and V2 are almost equal. Unequal
input voltages characterize the operation in saturation region. If V1 > V2 by 150µV, Vo will be
saturated at a positive voltage and if V1 < V2 by the same amount, Vo will be saturated at a negative
voltage –Vsat. Although the op-amp has distinct non-linear characteristics bias as a linear devices
under certain conditions and the principles of linear circuit theory can be used to design and
analyses op-amp is operated in the linear region. Since the magnitude of the input voltage for linear
operations is quite small, op-amps are seldom used in open-loop configuration. Feedback from
output to inverting (-) terminal tends to extend the range of input for linear operation.
1-2-4-4 Equivalent circuit of op-amp
In the linear region of operation, the op-amp can be modeled as a VCVS. Fig. 1-2-5 shown
an equivalent circuit of op-amp.
Fig 1-2-5 op-amp equivalent circuit
EC0222 Electronic Circuits Lab Manual
Experiment 1.2: Characterization of op-amp
►1.22◄
Here Rid is the differential input resistance, AVid is the Thevenin voltage source and RO is the
Thevenin equivalent output resistance looking back into output terminals. The output voltage VO is
VO = AVid = A (V1-V2)
(1-2-7)
where A is the open-loop voltage gain of the op-amp, Vid is the differential input voltage, and V1
and V2 are the voltages w.r.t. ground potential at the non-inverting and the inverting input terminals
respectively.
Thus the op-amp amplifies the difference between the two input voltages. The input voltages
V1 and V2 can be cither ac or dc voltages.
In the open-loop configuration, no connection exists between the output and input terminals.
When connected in an open-loop configuration, the op-amp works as a high gain amplifier. Any
input signal slightly above zero volts drives the output VO to saturation. For this reason, the op-amp
is seldom used in open-loop configuration for linear applications. The property of op-amp output
saturating under open-loop configuration is used in non-linear circuit applications of op-amp as a
voltage comparator.
1-2-4-5 The ideal op-amp
The ideal behavior of an op-amp implies that
a. The output resistance is zero
b. The input resistance seen between the two input terminals (called the differential input
resistance) is infinity.
c. The input resistances seen between each input terminal and the ground (called the common
mode input resistance) are infinite.
d. op-amp has a zero voltage offset ie., for V1 = V2 = 0, output voltage VO = 0
e. Common mode gain AC is zero.
f. Differential mode gain, Ad is infinity.
g. Common Mode Rejection Ratio (CMRR) is infinity
h. Bandwidth is infinite, ie., Ad is real and constant.
i. Slew rate is infinite.
j. Since VO = Ad (V1 – V2) and Ad = ∞
V1-V2 = VO/Ad = 0
ie., V1 = V2
EC0222 Electronic Circuits Lab Manual
Experiment 1.2: Characterization of op-amp
►1.23◄
The above condition implies that the inverting and non-inverting terminals are at the
same potential because of the very high (infinite) gain property. This condition along
with the condition i1 = i2 = 0 are the keys to the simplified analysis of the op-amp circuits.
1-2-4-6 op-amp input modes and CMRR
In op-amp, a number of input signal combinations are possible:
•
If an input signal is applied to either input with the other input connected to ground, the
operation is referred to as single – ended.
•
If two opposite polarity input signals are applied, the operation in referred to as doubleended.
•
If the same input is applied to both inputs, the operation is called common mode.
Differential gain, Ad
U1
V2
uA741
V1
Vo
+
V1 and V2 are the two input signals and VO is the output. In an ideal op-amp, VO is proportional to
the difference between the two signal voltages.
VO ∝ (V1-V2)
(1-2-8)
From equation 1-2-8 we can write,
VO = Ad (V1-V2)
(1-2-9)
Where Ad is the constant of proportionality. Ad is the gain with which differential amplifier the
difference between two input signals. Hence, Ad is called differential gain of the differential
amplifier. The difference between the two inputs, V1 – V2 is generally called difference voltage and
denoted as Vd.
VO = AdVd
(1-2-10)
Hence, the differential gain can be expressed as
Ad =
VO
Vd
(1-2-11)
EC0222 Electronic Circuits Lab Manual
Experiment 1.2: Characterization of op-amp
►1.24◄
Common mode gain, AC
If we apply two input voltages which are equal in all respects to the differential amplifier,
ie., if V1=V2, then ideally the output voltage, VO = Ad(V1-V2) must be zero. But the output voltage
of the practical differential amplifier not only depends on the difference voltages, but also depends
on the average common level of the two inputs. Such an average level of the two input signals is
called common mode signal denoted as VC.
VC =
(V1 + V2 )
2
(1-2-12)
Practically, the differential amplifier produces the output voltage proportional to such common
mode signal, also. The gain with which it amplifier the common mode signal to produce the output
is called as common mode gain of the differential amplifier denoted as Ac.
VO = AC VC
(1-2-13)
So the total output of any differential amplifier can be expressed as
VO = AdVd+ACVC
(1-2-14)
Common Mode Rejection Ratio (CMRR)
In an ideal different amplifier, Ad is infinite while AC must be zero. However, in a practical
differential amplifier; Ad is very large and AC is very small. ie., the differential amplifier provides
very large amplification for difference signals and very small amplification for common mode
signals.
Many disturbance signals/noise signals appear as a common input signal to both the input
terminals of the differential amplifier. Such a common signal should be rejected by the differential
amplifier. “The ability of a differential amplifier to reject a common-mode signal is expressed by a
ration called Common Mode Rejection Ratio, denoted as CMRR”.
CMRR is defined as the ratio of the differential voltage gain Ad to common mode voltage
gain Ac.
CMRR =
Ad
AC
(1-2-15)
Ideally AC is zero. Hence, the ideal value of CMRR is ∞.
EC0222 Electronic Circuits Lab Manual
Experiment 1.2: Characterization of op-amp
►1.25◄
1-2-4-7 op amp internal circuit
Commercial integrated circuit OP-amps usually consists of your cascaded blocks as shown in figure
1-2-6 shown below.
V2
V1
Differential
Amplifier
Differential
Amplifier
Buffer and
Level
Translator
Output
driver
Vo
Fig 1-2-6 Internal block schematic op-amp
The first two stages are cascaded difference amplifier used to provide high gain. The third stage is a
buffer and the last stage is the output driver. The buffer is usually an emitter fallowing whose input
impedance is very high so that it prevents loading of the high gain stage. The output stage is
designed to provide low output impedance. The buffer stage along with the output stage also acts as
a level shifter so that output voltage is zero for zero inputs.
1-2-4-8 op-amp characteristics
An ideal op-amp draws no current from the source and its response is also independent of
temperature. However, a real op-amp does not work this way. Current is taken from the source into
op-amp inputs. Also the two inputs respond differently to current and voltage due to mismatch in
transistors. A real op-amp also shifts its operation with temperature. These non-ideal characteristics
are:
1. Input bias current
2. Input offset current
3. Input offset voltage
4. Thermal drift
5. Slew rate
6. input and output voltage ranges
Input bias current
The op-amp’s input is a differential amplifier, which may be made of. BJT or FET. In either
case the input transistors must be biased into this linear region by supplying currents into the bases.
In an ideal op-amp, no current is drawn from the input terminals. However, practically, input
terminals conduct a small value of dc current to bias the input transistors when base currents flow
through external resistances, they produce a small differential input voltage or unbalance; this
EC0222 Electronic Circuits Lab Manual
Experiment 1.2: Characterization of op-amp
►1.26◄
represents a false input signal. When amplified, this small input unbalance produces an offset in the
output voltage.
The input bias current shown on data sheets is the average value of base currents entering
into the terminals of an op-amp.
+
−
(I + I B )
IB = B
2
(1-2-16)
For 741, the bias current is 500nA or less. The smaller the input bias current, the smaller the offset
at the output voltage.
Input offset current
The input offset current is the difference between the two input currents driven from a common
source
|IOS| = IB+ + IB-
(1-2-17)
It tells you how much larger one current is than the other. Bias current compensation will work if
both bias currents IB+ and IB- are equal. So, the smaller the input offset current the better the OP
amp. The 741 op-amps have input offset current of 20nA.
Input offset voltage
Ideally, the output voltage should be zero when the voltage between the inverting and noninverting inputs is zero. In reality, the output voltage may not be zero with zero input voltage. This
is due to un-avoidable imbalances, mismatches, tolerances, and so on inside the op-amp. In order to
make the output voltage zero, we have to apply a small voltage at the input terminals to make output
voltage zero. This voltage is called input offset voltage .i.e., input offset voltage is the voltage
required to be applied at the input for making output voltage to zero volts. The 741 op-amp has
input offset voltage of 5mV under no signal conditions. Therefore, we may have to apply a
differential input of 5mV, to produce an output voltage of exactly zero.
Thermal drift
Bias current, offset current and offset voltage change with temperature. A circuit carefully
mulled at 25oC may not remain so when the temperature rises to 35oC. This is called drift often,
offset current drift is expressed in n A/oC and offset voltage drift in mV/oC. These indicate the
change is offset for each degree celsius change in temperature. There are very few techniques that
can be used to minimize the effect of drift.
EC0222 Electronic Circuits Lab Manual
Experiment 1.2: Characterization of op-amp
►1.27◄
Slew rate
Among all specifications affecting the ac operation of the op-amp, slew rate is the most
important because it places a severe limit on a large signals operation. Slew rate is defined as the
maximum rate at which the output voltage can change. The 741 op-amp has a typical slew rate of
0.5 volts per microsecond (V/µs). This is the ultimate speed of a typical 741; its output voltage can
change no faster than 0.5V/µs. If we drive a 741 with large step input, it takes 20µs (0.5 V/µsX10V)
for the output voltage to change from 0 to 10V.
Band width
Slew rate distortion of a sine wave starts at a point where the initial slope of the sine wave
equals the slew rate of the op-amp. The maximum frequency at which the op-amp can be operated
without distortion is
f max =
SR
(2πV P )
(1-2-18)
where SR=slew rate of op-amp, VP= peak voltage of output sine wave. As an example, if the output
sine wave has a peak voltage of 10V and the op-amp slew rate is 0.5 V/µs, the maximum frequency
for large signal operation is
f max =
0.5V / µs
= 7.96 KHz
2π × 10V
Frequency ƒmax is called bandwidth of op-amp. The 741 op-amp has a bandwidth of approximately
8 KHz. This means the undistorted band width for large signal operation is 8 KHz.
Input and output voltage ranges
Maximum positive and negative input voltage applied to the op-amp for undistorted output
gives the input voltage range. Maximum positive and negative undistorted output voltage of the opamp gives the output voltage range.
1-2-4-9 OP amp applications
1) Signal conditioners
(a) Linear – eg. Adder, subtractor, differentiator, integrator, V-I converter, etc.
(b) Non-Linear – eg., log amplifier, anti-log amplifier, multiplier, divider, etc.
2) Signal Processors
(a) Linear – eg., voltage follower, instrumentation amplifier, etc.
(b) Non-Linear – eg., log amplifier, anti-log amplifier, multiplier, divider, etc.
EC0222 Electronic Circuits Lab Manual
Experiment 1.2: Characterization of op-amp
►1.28◄
1-2-5 EXPERIMENT
Use op-amp dc power supply voltages ±15V wherever not specified
1. Input bias current and input offset current
+Vcc
220k
Vo
uA741
+
220k
-Vcc
Fig 1-2-7 Input bias and input offset current
DC voltage at
the noninverting
terminal
V+
Input bias
DC voltage at
the inverting
terminal
IB
+
V+
=
220 K
IB
−
Input offset
current
V−
=
220 K
+
−
(I + I B )
IB = B
2
V-
current
IOS = |IB+ - IB-|
Table 1-2-1
1.1 Connect the circuit of figure 1-2-7.
1.2 Using a DMM, measure the dc voltage at the (-) terminal & record the values in Table 1-2-1.
1.3 By ohm’s law, calculate the input currents; IB+ and IB-. Average these values to find out the
input Bias current. Also, find the difference between these two currents to know the input
offset current. Record these values in Table 1-2-1.
2. Input offset voltage
100k
+Vcc
100
Vo
uA741
+
100
-Vcc
Fig 1-2-8 Input offset voltage
EC0222 Electronic Circuits Lab Manual
Experiment 1.2: Characterization of op-amp
Vout
►1.29◄
Vin = Vout/1000
Table 1-2-2
2.1 Connect the circuit of Figure 1-2-8.
2.2 Measure the DC output voltage at pin 6 using multimeter and record the result in Table 2.
2.3 Calculate the input offset voltage using the formula
Vi = Vout / 1000
and record the value in table 1-2-2.
3. Slew rate and bandwidth
+Vcc
-
+
1Vpp
20KHz
uA741
Vo
-Vcc
Fig 1-2-9(a) Slew rate and bandwidth
Fig 1-2-9(b )Model graph
EC0222 Electronic Circuits Lab Manual
Experiment 1.2: Characterization of op-amp
∆V
∆T
SR = ∆V/∆T
►1.30◄
BW
Table1-2- 3
3.1 Connect the circuit of Figure 1-2-9(a).
3.2 Using an AFO, provide a 1V peak to peak square wave with a frequency of 25 KHz.
3.3 With an oscilloscope, observe the output of OPAMP. Adjust the oscilloscope timing the
get a couple of cycles.
3.4 Measure the voltage change ∆V and time change ∆T of the output waveform. Record the
results in Table 1-2-3.
3.5 Calculate the slew rate using the formula
SR = ∆V / ∆T
3.5 Using the circuit of figure 3, set the AFO at 1KHz. Adjust the signal level to get 20V
peak – to – peak (20 VPP) out of the op-amp.
3.6 Increase the frequency and watch the waveform somewhere above 10 KHz, slew rate
distortion will become evident. That maximum frequency ƒ max at which the op-amp
can be operated is called bandwidth of an op-amp record the value in Table 1-2-3.
4. Input and output voltage ranges
4.1 Assemble the voltage follower circuit as shown in Figure 1-2-10 with R1 = R2 = 100 kΩ. Use
op-amp dc power supply voltages of ±9 V.
R2
+Vcc
-
Vs
R1
+
uA741
Vo
-Vcc
Fig 1-2-10 Circuit to find the input voltage range
4.2 Apply ±5 V, 100 Hz sinusoidal input, Vs. Observe on a CRO the voltages at the non-inverting
input and output pins simultaneously. Increase the signal amplitude until distortion is observed
EC0222 Electronic Circuits Lab Manual
Experiment 1.2: Characterization of op-amp
►1.31◄
at the peak value of the output. Measure the positive and negative input voltage peak values.
This gives the op-amp input voltage range.
4.3 Change the circuit of Figure 1-2-10 to an inverting amplifier. Connect R1 between the source
and inverting input. Ground the non-inverting input. Choose R1 = 10 kΩ, R2 = 100 kΩ. Repeat
observations of step 3.2 starting with ±0.5 V, 100 Hz sinusoidal input. Measure the positive and
negative output voltage peak values. This gives the op-amp output voltage range.
1-2-6 POSTLAB QUESTIONS
Check your understanding by answering these questions
1. The input stage of a 741 op-amp is a --------------2. The output stage of a 741 op-amp is a -------------3. The input bias current of an op-amp is the --------------------- of the two input base currents under
no-signal condition.
4. The input ----------------- current is the difference of the two input base currents.
5. The input ---------------- voltage is the differential input voltage needed to null or zero the
quiescent output voltage.
6. The CMRR of an op-amp is the ratio of ------------- voltage gain to ---------- voltage gain.
7. A 741 has a slew rate of ------------- V/µs.
8. The bandwidth is the ----------------- undistorted frequency out of an op-amp. It depends on the ----------- rate of the op-amp and the ---------------------- of the output signal.
9. Identify the type of input mode for each op-amp in fig (b)
Vin1
uA741
Vo
Vin2
uA741
Vin
Vo
Vin
uA741
Vo
Fig (b)
EC0222 Electronic Circuits Lab Manual