INTRODUCTION
An integrated circuit (IC) consists of a single crystal chip of silicon of very small dimensions, containing both active & passive elements.
A number of processes are involved in the manufacture of IC‟s. these include –
• Preparing of wafer,
• Epitaxial growth,
• Diffusion of impurities,
• Ion implantation,
• Oxide growth,
• Photolithography,
• Chemical etching,
• Metallization, etc.
An IC does not have discrete components; instead, all components (both active &
passive) are an integral part of it.
Whole circuit comprising of various elements are properly interconnected, and created on a single chip, and no external wiring is required.
A number of terminals are brought out of the chip, and external connections are made from these terminals.
The main advantages of IC’s are – o Increased reliability,
o Smaller size,
o Increased speed of operation, and
o Reduced cost due to mass production techniques.
Classification of IC’s –
1. Linear IC’s (Analog IC’s) – outputs are proportional to inputs. Both inputs &
outputs can take continues values.
• Applications:- amplifiers, voltage regulators, operational amplifiers, etc.
2. Digital IC’s – input & output can take only two vales: 0 or 1 (or low level & high level).
• Applications:- micro-processors, logic gates, memory chips, counters, clock chips, flip-flops, etc.
Types of IC’s –
i. Monolithic circuits, ii. Thin-film circuits, iii. Thick-film circuits, and iv. Hybrid circuits.
Based on the number of circuits contained in an IC package, integrated circuits are classified as –
I. Small Scale Integration (SSI) – contains less than 30 circuits.
II. Medium Scale Integration (MSI) – contains about 30 to 100 circuits.
III. Large Scale Integration (LSI) – contains about 100 to 1,00,000 circuits.
IV. Very Large Scale Integration (VLSI) – a single IC contains more than 1,00,000 circuits.
To study about an operational amplifier, knowledge of differential amplifier is essential.
A differential amplifier mainly comprises of two transistors with their emitters connected as shown in the figure below.
An operational amplifier is basically a differential amplifier with two inputs – non-inverting & inverting, and one output.
It is used in analog computers to do several mathematical operations like summing, differentiation, integration, inversion, etc. it is also used in control systems.
An operational amplifier is basically a very high gain, direct-coupled amplifier with high input impedance & low output impedance.
The figure below shows the symbol of an Op-Amp.
If a voltage Vi is applied at the inverting input (keeping the non-inverting input at ground) as shown below.
Vi
VO
t
t
Vi VO
Op-amp in inverting mode
The output voltage Vo= -AVi is amplified but is out of phase with respect to the input signal by 1800.
If a voltage Vi is fed at the non-inverting input (keeping the inverting input at ground) as shown below.
Vo
VO
t
Vi
t
Op-Amp in Non-inverting mode
The output voltage Vo= AVi is amplified and in-phase with the input signal.
Note: - Op-Amp is 8 pin IC (named as μA 741) with pin details as shown.
OFFSET NULL NO CONNECTION
INVERTING I/P +VCC
NON-INVERTING I/P OUTPUT
-VEE OFFSET NULL
Pin details of Op-Amp Block Diagram of an Op-AMP
1 8
2 7
μA 741 3 6 4 5 3 6
4 5
An Op-Amp consists of four blocks cascaded as shown above.
Input stage: It consists of a dual input, balanced output differential amplifier. Its function is to amplify the difference between the two input signals. It provides high differential gain, high input impedance and low output impedance.
Intermediate stage: The overall gain requirement of an Op-Amp is very high. Since the input stage alone cannot provide such a high gain. Intermediate stage is used to provide the required additional voltage gain.
It consists of another differential amplifier with dual input, and unbalanced (single ended) output
Buffer and Level shifting stage: As the Op-Amp amplifies D.C signals also, the small D.C. quiescent voltage level of previous stages may get amplified and get applied as the input to the next stage causing distortion the final output.
Hence the level shifting stage is used to bring down the D.C. level to ground potential, when no signal is applied at the input terminals. Buffer is usually an emitter follower used for impedance matching.
Output stage: It consists of a push-pull complementary amplifier which provides large A.C. output voltage swing and high current sourcing and sinking along with low output impedance.
Characteristics of an ideal OP-AMP:-
The voltage gain is infinity.
The input impedance is infinity.
The output impedance is zero.
The band-width is infinity.
When equal voltages are applied at the two input terminals, the output is zero.
There is no change in the characteristic feature, with changes of temperature.
Practical OP-AMPs have –
Large voltage gain.
Very high input impedance.
Very low output impedance.
A small output voltage appears even when equal voltages are applied at the two input terminals.
Some Important Definitions:-
Common mode rejection ratio [CMRR] is defined as the ratio of differential gain of the amplifier to the common-mode gain.
CMMR = |Ad/AC| or CMMR = 20 log |Ad/AC|
Slew rate [SR] is defined as the maximum rate at which an OP-AMP output can change, and it is expressed in terms of Volts/µsec. SR = ΔV0/Δt
The SR represents the ability of an amplifier to handle the varying signals like large step-input signals.
Input bias current [IIB]
A difference amplifier mainly consists of two identical transistors which are direct-coupled. During normal operation, transistors are biased properly using d.c.
voltage sources. Ideally, in an OPAMP, there should be equal d.c. bias currents at both non-inverting and inverting input terminals. But in practice, the transistors may not match perfectly, with the result that the d.c. bias currents at + and – inputs are not exactly equal.
The bias currents are denoted as IIB+
and IIB–
. The average of these two currents is termed as input bias current given as;
IIB = ½ (IIB+ + IIB-)
The DC voltage which makes the output voltage zero, when the other terminal is grounded is called input offset voltage.
The voltage existing at the output, when the inputs are zero, is called output offset voltage. It is usually caused by input bias current & the input offset voltage.
Power supply rejection ratio [PSRR] is defined as the ratio of the change in the input offset voltage due to the change in supply voltage producing it, keeping the other power supply voltage constant. It is also called power supply sensitivity.
PSSR = ΔVios /ΔVCC | VEE constant.
Open Loop Voltage Gain (AV) is the ration of output voltage to input voltage in the absence of feedback. It‟s typical value is AV = 2x105
Input Impedance (Ri) is defined as “The impedance seen by the input (source) applied to one input terminal when the other input terminal is connected to
ground”. Ri ≈ 2MΩ
Output Impedance (RO) is defined as “ The impedance given by the output (load) for a particular applied input”. Ro ≈ 75Ω
Typical values of several parameters for A741 OP-AMP –
• Input offset voltage 1 mV.
• Input bias current 80 nA.
• Slew rate at unity gain 0.5 V/µsec.
• CMRR 90 dB.
Virtual ground concept
We know that, an ideal Op-Amp has perfect balance (i.e., output will be zero when input voltages are equal).
Hence when output voltage Vo = 0, we can say that both the input voltages are equal ie V1 = V2.
V1
Ri VO
V2
Since the input impedances of an ideal Op-Amp is infinite (Ri = ∞). There is no current flow between the two terminals.
Hence when one terminal (say V2) is connected to ground (i.e., V2 = 0) as shown.
VCC
V1 =V2 =0
Ri VO
V2=0
VEE
Then because of virtual ground V1 will also be zero.
Example: Assuming that the input impedance of an OPAMP to be very large and output impedance to be very small, the equivalent circuit of the OPAMP is shown in the following figure.
From the figure; VO = – A Vi where, A is the gain of OPAMP (open loop gain) Also, overall gain = VO/V1 where, overall gain is the gain of the circuit (closed loop gain) In practice, the voltage gain A is very high, say A = 10,000
Let VO = – 5V. ( – sign is used since output is out of phase with the input) Therefore, Vi = – VO/A = 5/10,000 = 0.5 mV.
Now, let the overall gain VO/V1 be unity. (This can be set by adjusting the values of resistances R1 and Rf).
Then, V1 = VO = 5V.
Now, it can be seen that the voltage Vi is quite small, as compared to all other voltages. If the assumption is made that Vi = 0, it means that there is a short-circuit at the input terminals of the OPAMP. But, since the input impedance is infinite, there can be no flow of current through the short. Hence, the short is not true short-circuit, but it is only a virtual short-circuit or virtual ground. As a result, the current I flowing through R1 also flows through Rf.
Applications of Op-Amp – An OPAMP can be used as 1. Inverting Amplifier
2. Non-Inverting Amplifier 3. Voltage follower
4. Adder ( Summer) 5. Integrator
6. Differentiator
Inverting Amplifier
An inverting amplifier is one whose output is amplified and is out of phase by 1800 with respect to the input.
The point “G” is called virtual ground and is equal to zero.
By KCL we have
is the gain of the amplifier and negative sign indicates that the output is inverted with respect to the input.
VO
Vi
t t
Non- Inverting Amplifier
is the gain of the amplifier and + sign indicates that the output is in-phase with the input.
Voltage follower
A voltage follower is one whose output is equal to the input.
VO
Vi VO
Vi
t t
The voltage follower configuration shown above is obtained by short circuiting
“Rf” and open circuiting “R1” connected in the usual non-inverting amplifier.
Thus all the output is fed back to the inverting input of the op-Amp.
Consider the equation for the output of non-inverting amplifier
i f V R V R
1
0 1
When Rf = 0 short circuiting R1= ∞ open circuiting
Vi
1 0 VO
i
O V
V
Therefore the output voltage will be equal and in-phase with the input voltage.
Thus voltage follower is nothing but a non-inverting amplifier with a voltage gain of unity.
Inverting Adder
Inverting adder is one whose output is the inverted sum of the constituent inputs.
R1
Hence it can be observed that the output is equal to the inverted sum of the inputs.
Integrator
An integrator is one whose output is the integration of the input.
C
Differentiator
A differentiator is one whose output is the differentiation of the input.
R
Non-inverting Adder
Non-inverting adder is one whose output is the sum of the constituent inputs.
Let the voltage at the inverting input terminal be Vm. Because of virtual ground at the input terminals, the voltage at G is also Vm.
Applying KCL at the node G, we have;
The OPAMP along with the resistors R and Rf acts as a non-inverting amplifier.
Hence, closed loop voltage gain =
Therefore,
Substituting for Vm, the expression (1) becomes;
Let R1 = R2 = R3 = R = Rf/2; then we have, VO = V1 + V2 + V3, on simplification.
The output is the sum of the input voltages, without change of sign. Hence, the name non-inverting adder.
Subtractor
An OPAMP can function as a subtractor, giving an output voltage which is the difference of two input voltages. The circuit is mainly a basic differential amplifier in which all resistors are of equal magnitude.
The output of the amplifier can be computed on the basis of the principle of superposition. – The waited sum of the output is equal to the sum of the outputs when separate inputs are considered.
Consider the following subtractor figure.
V1 and V2 are the input voltages at the non-inverting and inverting terminals respectively. R is the resistor in the feedback path.
Case(1): V01 denote the output with V1 applied and V2 set equal to zero, as shown below.
Let the potential at node M be VM. We have;
The circuit is non-inverting amplifier with an input V1/2 at the non-inverting input terminal, and the inverting terminal is grounded through resistance R. Hence, the outpot may be directly obtained as;
Case(2): Let V02 denote the output with V2 applied and V1 set equal to zero, as shown below.
This circuit is basically an inverting amplifier whose output is given as;
Hence, when both inputs V1 and V2 are applied, we have the output given by the principle of superposition as;
V0 = V1 – V2 Hence, the circuit is subtractor.
Problems
1. For an inverting amplifier Ri=100KΩ and Rf=600KΩ. What is the output voltage for an input of -3V?
Soln:
2. Design an inverting amplifier for output voltage of -10V and an input voltage of 1V.
Rf
Since the gain is positive:
Choose a non-inverting amplifier
Then we have,
6. Design an Adder using Op-Amp to give the output voltage VO= amplifier should be twice the negative sum of the inputs.
Soln:
K integrator for which R=100KΩ and C=1μF. Find the output voltage.
Soln: Given: R=100KΩ, C=1μF, Vm =5mV, F=1KHz, V0 =?
9. The input to a differentiator is a sindusoidal voltage of peak value 5mV and frequency 2KHz . Find the output if R = 100KΩ and C=1μF.