BEEE 1st Year (Unit 3)
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Unit 3 PPT Notes
Course Syllabus and Suggestive Readings
Unit.3: Operational Amplifiers
Op-amp and its characteristics: Input Impedance, Output Impedance, Gain, Bandwidth, Open loop & closed loop configurations, Operational Amplifiers Basic op-amp circuits: Inverting & Non-inverting voltage amplifiers, adder, subtractor, comparator.
CHAPTER 3 Operational Amplifiers Notes
Lecture Topic 3.1.1
What are Operational Amplifiers?
Operational amplifiers are the basic building blocks of analog electric circuits. They are linear devices with all properties of a DC amplifier. We can use external resistors or capacitors to the Op Amp is many different ways to make them different forms of amplifies such as Inverting amplifier, Non inverting amplifier, Voltage follower, Comparator, Differential amplifier, Summing amplifier, Integrator etc.
- Op-amp amplifies the difference between two signal and diminish common signal. Normally opamps have differential input and a single-ended output.
- In other words, one input produces an inverted output signal and other input produces a non-inverted output.
- Inverted output means that output voltage is 180deg out of phase of input voltage.
- Non-inverted means that output voltage is in same phase with input voltage.
- These are fundamentally voltage amplifying devices used with external feedback components like resistors or capacitors.
- An op amp is a three terminal device, with one terminal called the inverting input, other the non-inverting input and the last one is the output
OPAMP Pin Configuration: Now we will see the pin diagram of operational amplifier.
Description of pin:
In a typical Op Amp there will be 8 pins. These are
Pin1 – Offset Null (Op amps have terminals which are called Offset Null Terminals This means it amplifies the difference in voltage between the two input pins. Because of this fact, its output should be 0V when there is no difference between its inputs, in other words, when its inputs are at equal voltages.)
Pin2 – Inverting input INV
Pin3 – Non inverting input Non-INV
Pin4 – Ground- Negative supply
Pin5 – Offset Null
Pin6 – Output
Pin7 – Positive supply
Pin8 – Strobe or not connected
Equivalent Circuit of OPAMP:
Equivalent Circuit of Practical Op Amp – The circuit which represents op-amp parameters in terms of physical components, for the analysis purpose is called equivalent circuit of an op-amp. The equivalent circuit of an op-amp
(Operational Amplifiers Notes)
The circuit shows the op-amp parameters like input resistance, output resistance, the open loop voltage gain in terms of circuit components like Rin R0 etc. The op-amp amplifies the difference between the two input.
Note: Rin and Ro may be taken as Zin and Zo and AOL as A(V2-V1).
Operational Amplifiers Notes
Lecture Topic 3.1.2
Characteristics of OPAMP (Operational Amplifiers)
Open Loop Gain, (Avo): The main function of an operational amplifier is to amplify the input signal and the more open loop gain it has the better. Open-loop gain is the gain of the op-amp without positive or negative feedback and for such an amplifier the gain will be infinite but typical real values range from about 20,000 to 200,000.
Input impedance, (ZIN): Input impedance is the ratio of input voltage to input current and is assumed to be infinite to prevent any current flowing from the source supply into the amplifiers input circuitry ( IIN = 0 ). Real op-amps have input leakage currents from a few pico-amps to a few milli-amps.
Output impedance, (ZOUT): The output impedance of the ideal operational amplifier is assumed to be zero acting as a perfect internal voltage source with no internal resistance so that it can supply as much current as necessary to the load. This internal resistance is effectively in series with the load thereby reducing the output voltage available to the load. Real op-amps have output impedances in the 100-20kΩ range.
Bandwidth, (BW): An ideal operational amplifier has an infinite frequency response and can amplify any frequency signal from DC to the highest AC frequencies so it is therefore assumed to have an infinite bandwidth. With real op-amps, the bandwidth is limited by the Gain-Bandwidth product (GB), which is equal to the frequency where the amplifiers gain becomes unity.
Offset Voltage, (VIO): The amplifiers output will be zero when the voltage difference between the inverting and the non-inverting inputs is zero, the same or when both inputs are grounded. Real op-amps have some amount of output offset voltage.
Common mode rejection ratio: It is defined as the ratio of differential gain to the common mode gain. Ideally opamp has infinite common mode rejection ratio.
CMRR: Ad/Acm
Slew rate: It is defined as the rate at which how opamp is able to achieve its final value. It is measured in volt/µsec. Ideally opamp has infinite slew rate.
An ideal op amp is usually considered to have the following characteristics:
- Infinite open-loop gain G = vout / vin.
- Infinite input impedance Rin, and so zero input current.
- Zero input offset voltage.
- Infinite output voltage range.
- Infinite bandwidth with zero phase shift and infinite slew rate.
- Zero output impedance R.
Operational Amplifiers Notes
Lecture Topic-3.1.3.
Open loop and closed loop configuration of Operational Amplifiers
Open loop configuration:
In the case of amplifiers the term open loop indicates that no connection, exists between input and output terminals of any type.
The OPAMP functions as a high gain amplifier. There are three open loop OPAMP configurations.
a) The Differential Amplifier
b) The Inverting Amplifier
c) The non-inverting amplifier
The Differential Amplifier:
Input signals vin1 and vin2 are applied to the positive and negative input terminals.
The OPAMP amplifies both ac and dc input signals.
The source resistance Rin1 and Rin2 are normally negligible compared to the input resistance Ri. Therefore voltage drop across these resistances can be assumed to be zero.
v1 = vin1 and v2 = vin2.
vo = Ad (vin1 – vin2 )
where, Ad is the open loop gain.
INVERTING AMPLIFIER: If the input is applied to only inverting terminal and non-inverting terminal is grounded then it is called inverting amplifier.
V1 = 0, V2 = Vin,
Vo = Avd = -AVin
NON-INVERTING AMPLIFIER: If the input is applied to only inverting terminal and non-inverting terminal is grounded then it is called inverting amplifier.
Fig.3 Open Loop Non-Inverting OPAMP
V1 = Vin, V2 = 0,
Vo = Avd = AVin
Closed Loop Configuration:
- Inverting Amplifier: Operational amplifiers can have either a closed-loop operation or an open-loop operation. The operation (closed-loop or open-loop) is determined by whether or not feedback is used. Without feedback the operational amplifier has an open-loop operation. This open-loop operation is practical only when the operational amplifier is used as a comparator (a circuit which compares two input signals or compares an input signal to some fixed level of voltage). As an amplifier, the open-loop operation is not practical because the very high gain of the operational amplifier creates poor stability. (Noise and other unwanted signals are amplified so much in open-loop operation that the operational amplifier is usually not used in this way.) Therefore, most operational amplifiers are used with feedback (closed-loop operation).
The figure above shows an operational amplifier in a closed-loop, inverting amplifier configuration. Resistor R2 is used to feed part of the output signal back to the input of the operational amplifier.
At this point it is important to keep in mind the difference between the entire circuit (or operational circuit) and the operational amplifier. The operational amplifier is represented by the triangle-like symbol while the operational circuit includes the resistors and any other components as well as the operational amplifier. In other words, the input to the circuit is shown in the figure above, but the signal at the inverting input of the operational amplifier is determined by the feedback signal as well as by the circuit input signal.
Vout = – Vin . R2/R1
2. Non-inverting Amplifier:
Fig.5 Non-Inverting OPAMP Circuit
The figure above shows a noninverting configuration using an operational amplifier. The input signal (Vin) is applied directly to the noninverting (+) input of the operational amplifier. Feedback is provided by coupling part of the output signal (Vout) back to the inverting (-) input of the operational amplifier. R1 and R2 act as voltage divider that allows only a part of the output signal to be applied as feedback (Vfb).
Vout = Vin.(1 + R2/R1)
Operational Amplifiers Notes
Lecture Topic-3.1.4.
Numerical Problems (Operational Amplifiers)
Q.1 Determine the output voltage of operational amplifier as shown below if supply voltage is 20mV, Rin = 10 kilo ohm and Rf= 100 kilo ohm
Q.2 Find out output voltage if Vin1 = 20 V, Vin2 = 10 V and gain is 10.
Operational Amplifiers Notes
Lecture Topic-3.1.5
Inverting and Non-Inverting Operational Amplifier
- Inverting Amplifier: Operational amplifiers can have either a closed-loop operation or an open-loop operation. The operation (closed-loop or open-loop) is determined by whether or not feedback is used. Without feedback the operational amplifier has an open-loop operation. This open-loop operation is practical only when the operational amplifier is used as a comparator (a circuit which compares two input signals or compares an input signal to some fixed level of voltage). As an amplifier, the open-loop operation is not practical because the very high gain of the operational amplifier creates poor stability. (Noise and other unwanted signals are amplified so much in open-loop operation that the operational amplifier is usually not used in this way.) Therefore, most operational amplifiers are used with feedback (closed-loop operation).
The figure above shows an operational amplifier in a closed-loop, inverting amplifier configuration. Resistor R2 is used to feed part of the output signal back to the input of the operational amplifier.
At this point it is important to keep in mind the difference between the entire circuit (or operational circuit) and the operational amplifier. The operational amplifier is represented by the triangle-like symbol while the operational circuit includes the resistors and any other components as well as the operational amplifier. In other words, the input to the circuit is shown in the figure above, but the signal at the inverting input of the operational amplifier is determined by the feedback signal as well as by the circuit input signal.
Vout = -Vin. R2/R12. Non-inverting Amplifier:
The figure above shows a noninverting configuration using an operational amplifier. The input signal (Vin) is applied directly to the noninverting (+) input of the operational amplifier. Feedback is provided by coupling part of the output signal (Vout) back to the inverting (-) input of the operational amplifier. R1 and R2 act as voltage divider that allows only a part of the output signal to be applied as feedback (Vfdbk).
Vout = Vin [1 +R2/R1]
Operational Amplifiers Notes
Lecture Topic-3.1.6.
Numerical Problem
Q.1 Determine the output voltage of operational amplifier as shown below if supply voltage is 30mV, R1 = 15 ohm and R2 = 25 ohm.
Q.2 Evaluate the out put voltage and gain of op-amp as shown below in which input resistance is 30ohm and feedback resistance is 100ohm with 30mv input supply voltage:
Operational Amplifiers Notes
Lecture Topic-3.1.7
Adder
An adder is an electronic circuit that produces an output, which is equal to the sum of the applied inputs. … An op-amp based adder produces an output equal to the sum of the input voltages applied at its inverting terminal. It is also called as a summing amplifier, since the output is an amplified one.
n this simple summing amplifier circuit, the output voltage, ( Vout ) now becomes proportional to the sum of the input voltages, V1, V2, V3, etc. Then we can modify the original equation for the inverting amplifier to take account of these new inputs thus:
Operational Amplifiers Notes
Lecture Topic-3.1.8.
OPAMP As Subtractor
It is also called difference amplifier. A basic differential amplifier can be used as a subtractor as shown in the above figure. If all resistors are equal in value, then the output voltage can be derived by using superposition principle.
Similar to the summer circuit, the subtraction of 2 input voltages is possible with the help of op-amp circuit called subtractor or difference amplifier circuit.
To find the relation between the inputs and output let us use Superposition principle.
Let Vo1 be the output, with input V1 acting, assuming V2 to be zero. And Vo2 be the output, with input V2 acting, assuming V1 to be zero.
With V2 zero, the circuit acts as an inverting amplifier. Hence we can write
Vo1 = -Vin. Rf/R1
Similarly if V1 is kept as zero then
Vo2 = [1+Rf/R1] Vin
If Rf>>R1 then 1 can be neglected.
Hence output voltage
Vo = [V2- V1] (Rf/R1)
Operational Amplifiers Notes
Lecture Topic-3.1.9.
Numerical Problem
Q.2 Determine total output voltage of summing amplifier if v1 = 2V and v2 = 5V.
Q.3 Determine total output voltage of subtractor if v1 = 10V and v2 = 20V.
Operational Amplifiers Notes
Lecture Topic-3.1.10
Comparator
A comparator is an electronic circuit, which compares the two inputs that are applied to it and produces an output. The output value of the comparator indicates which of the inputs is greater or lesser. Please note that comparator falls under non-linear applications of ICs.
An op-amp consists of two input terminals and hence an op-amp based comparator compares the two inputs that are applied to it and produces the result of comparison as the output. This chapter discusses about op-amp based comparators.
Types of Comparators
Comparators are of two types : Inverting and Non-inverting. This section discusses about these two types in detail.
Inverting Comparator
An inverting comparator is an op-amp based comparator for which a reference voltage is applied to its non-inverting terminal and the input voltage is applied to its inverting terminal. This comparator is called as inverting comparator because the input voltage, which has to be compared is applied to the inverting terminal of op-amp.
The following figure shows the input and output waveforms of an inverting comparator, when the reference voltage is zero volts.
Non-Inverting Comparator
A non-inverting comparator is an op-amp based comparator for which a reference voltage is applied to its inverting terminal and the input voltage is applied to its non-inverting terminal. This op-amp based comparator is called as non-inverting comparator because the input voltage, which has to be compared is applied to the non-inverting terminal of the op-amp.
These are Operational Amplifiers Notes
