 # Lecture II: Linear Applications of Opamp

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Lecture II: Linear Applications of Opamp
Engr. Tayab Din Memon, Lecturer, Dept of Electronic Engineering, MUET, Jamshoro.

Objectives To introduce the differential amplifier And its typical circuit. To introduce the CMRR. Reasons due to which CMRR is affected Instrumentation Amplifier (IA) Applications of Instrumentation Amplifier Instrumentation Monolithic IC Packages Different Aspects of In-Amplifier AD 620 In-Amplifier Summing Amplifier Lab-Session II Conclusion

Differential Amplifier?
A differential amplifier circuit is commonly used to amplify or buffer differential signals whilst rejecting common mode signals. A differential signal is presented across two terminals; the voltage on one terminal rises as the voltage on the other terminal falls (relative to earth). A common mode signal is one where the voltages on both terminals rise and fall together.

Typical Differential Amplifier
Differential amplifiers also allow one signal to be subtracted from another as shown in figure. It can be simply called a subtractor also. Figure 1: Differential Amplifier.

How Common Mode signals are Generated?
According to the ideal performance equation of the circuit in Figure 1, the output is zero if the two input signals v1 and v2 are equal. The ideal circuit has an infinite CMRR – not the case with practical circuits. In a practical circuit any mismatch in the resistor ratio values connected to the op-amp input terminals causes a common mode signal (e1 e2 ecm) to inject a differential signal to the amplifier. This differential signal is amplified to produce a non-zero output signal. CMRR is thus degraded unless the resistor values are exactly matched.

Common Mode Rejection Ratio (CMRR)
Common-mode Rejection (CMR), the property of canceling out any signals that are common (the same potential on both inputs), while amplifying any signals that are differential (a potential difference between the inputs), is the most important function of a differential and instrumentation amplifier.

Common Mode Gain (ACM) Common-mode gain (ACM), the ratio of change in output voltage to change in common-mode input voltage, is related to common-mode rejection. It is the net gain (or attenuation) from input to output for voltage common to both inputs. For example, differential amplifier with a common-mode gain of 1/1000 and a 10V common-mode voltage at its inputs will exhibit a 10mV output change. The differential gain is the gain between input and output for voltages applied differentially (or across) the two inputs. The common-mode rejection ratio (CMRR) is simply the ratio of the differential gain AD, to the common-mode gain.

Resistor Mismatch effect upon CMRR
Ideally the CMRR of the differential amplifier should be infinite. But practically it is not possible because resistance change at the input terminals of Opamp. In the circuit of Figure 1, CMRR depends upon both resistor matching and upon the CMRR of the op-amp. This resistance change is undesirable for CMRR. In order to overcome the resistance change due to different reasons like temperature variation etc, input impedance should be increased. But there are side effects of increasing the Differential and Common mode Input Impedance. At unity gain, typical dc values of CMR are 70dB to more than 100dB, with CMR usually improving at higher gains.

Side Effects of Input Impedance Change
These side affects are: One effect is stray capacitance that causes degradation in CMRR at the higher frequencies. Another effect is to give an increased offset error because of op-amp bias current. The single differential amplifier has limitation in its performance. To improve its performance two (2) or more opamps configuration should be used.

Instrumentation Amplifier
An Instrumentation Amplifier is a closed-loop gain block that has a differential input and an output that is single ended with respect to a reference terminal. Most common, the impedance of the two input terminals are balanced and have high values, typically 109Ω, or greater. The input bias currents should also be low, typically 1nA to 50nA. As with op amps, output impedance is very low, nominally only a few milliohms, at low frequency.

Typical configuration of In-Amplifier

Different Monolithic IC Packages of In-Amplifier.

Selection of Differential and In-Amp
A difference amplifier is basically an op amp subtractor, typically using large value input resistors. In general, difference amplifiers should be used in applications where signal to noise ratio, CMRR and Accuracy are not so important. In contrast, an instrumentation amplifier is most commonly an op amp subtractor with two input buffer amplifiers. In-amps are needed in applications where the highest accuracy, best signal-to-noise ratio, and lowest input bias current are essential.

Applications of In-Amplifier
Data Acquisition From Pressure, Temperature transducers in noisy environment. Strain Gauges and RTD with Wheat stone bridge. Medical Instrumentation ECG, EEG monitors, blood pressure monitors and etc. Audio applications As microphone pre-amplifier to extract weak signal from a noisy environment and to minimize offset and noise due to ground loops.

Applications of In-Amplifier Cont….
High Speed Signal Conditioning Because the speed and accuracy of modern video data acquisition systems have improved, there is now a growing need for high bandwidth instrumentation amplifiers, particularly in the field of CCD imaging equipment. Video Applications High speed in-amps may be used in many video and cable RF systems to amplify or process high frequency signals. Power Control Applications In-amplifier can also be used for motor monitoring (to monitor and control motor speed, torque, etc), by measuring voltages, currents, and phase relationships of a 3-phase ac-phasor motor.

Applications of Instrumentation Amplifier

Instrumentation Amplifier Applications
Figure: 4mA to 20mA Receiver Circuit

External View of In-Amplifier

An Instrumentation Amplifier with unity gain.

3-Opamp Instrumentation Amplifier with variable gain.

Gain of Instrumentation Amplifier
If R1=R2=R3=R4=R5=R6=R then gain of Instrumentation Amplifier is known as: Av= (1+2R/RG) (V2-V1)

Monolithic In-Amplifier
For many years, the AD620 has been the industry-standard, high performance, low cost in-amp. The AD620 is a complete monolithic instrumentation amplifier offered in both 8-lead DIP SOIC packages. The user can program any desired gain from 1 to 1000 using a single external resistor. By design, the required resistor values fo gain of 10 and 100 are standard 1% metal film resistor values.

Summing Amplifier Summing Amplifier is an amplifier that adds different inputs provided at inverting or non-inverting input terminals. It is highly used when we need to add different inputs. Its applications are in Analog to digital converter.

Typical Circuit of Summing Amplifier

LAB SESSION II

Objectives Analysis of Differential Amplifier. Analysis of Instrumentation Amplifier Analysis of Adder Circuit.

Output Observation of Instrumentation Amplifier when both inputs are same.

Instrumentation Amplifier Output when there are different inputs i. e
Instrumentation Amplifier Output when there are different inputs i.e. 2V=V2 and 1V=V1