op-amp

Experiment 1.2: Characterization of op-amp 1.17

EC0222 Electronic Circuits Lab Manual

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.



V1

150k

+VCC

+

-Vo

2k

-VCC

uA741

-

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 op-

amp 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



non-invertinginput

output-

-Vcc

inverting input

+ offset null

+Vcc

offset null

uA741

-

+VCCR1

+Vo

-VCC

-uA741

Rf

Vs

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.

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

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

Sf

O VRR

V )1

(−= (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

)1

(RRfGain −= (1-2-2)



+VCCR1

Rf

-Vo

Vs

-uA741

+

-VCC

+ve saturation

linear region----------------------------------

-----------------------------------

Vo

range of input forlinear operation

+Vcc

+Vsat

-------------------

-------------------------------------------

-Vsat-ve saturation

(V1 - V2)

-Vcc

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.

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

SO VRRfV )1(

1

+= (1-2-3)

and its gain is 1

1RR

Gain f+= (1-2-4)

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

saturation region.

Fig 1-2-4 op-amp transfer characteristics



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



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



-V2

+

U1

uA741Vo

V1

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 double-

ended.

• If the same input is applied to both inputs, the operation is called common mode.

Differential gain, Ad

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

d

Od V

VA = (1-2-11)



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.

2

)( 21 VVVC

+= (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.

C

d

AA

CMRR = (1-2-15)

Ideally AC is zero. Hence, the ideal value of CMRR is ∞.



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

Vo

V1

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

DifferentialAmplifier

Differential Amplifier

Buffer and Level

Translator

Output driver



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.

2

)( −+ += BB

BII

I (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 non-

inverting 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.



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

)2(max

PVSRfπ

= (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

VsVf

102/5.0

max ×=

πµ = 7.96 KHz

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 op-

amp 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.



+uA741

-220k

+Vcc

Vo

-Vcc

220k

+

-Vcc

100

+Vcc100

Vo

100k

-

uA741

1-2-5 EXPERIMENT

Use op-amp dc power supply voltages ±15V wherever not specified

1. Input bias current and input offset current

Fig 1-2-7 Input bias and input offset current

DC voltage at

the non-

inverting

terminal

V+

DC voltage at

the inverting

terminal

V-

KVI B 220

++ =

KVI B 220

−− =

Input bias

current

2)( −+ +

= BBB

III

Input offset

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

Fig 1-2-8 Input offset voltage



-

20KHz

+

-Vcc

+Vcc

uA741

1Vpp

Vo

Vout 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

Fig 1-2-9(a) Slew rate and bandwidth

Fig 1-2-9(b )Model graph



-Vcc

Vs +

+Vcc

R2

R1 uA741

-Vo

∆V ∆T SR = ∆V/∆T 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.

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



uA741Vo

Vin

Vo

Vin1

uA741Vin2Vo

VinuA741

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)

Fig (b)

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