Op-amapplication Sir Manan

Operational Amplifier

Op-Amp Buffer It provides electrical impedance transformation from one circuit to another. Typically a buffer amplifier is used to transfer a voltage from a first circuit, having a high output impedance level, to a second circuit with a low input impedance level. The interposed buffer amplifier prevents the second circuit from loading the first circuit unacceptably and interfering with its desired operation. If the voltage is transferred unchanged (the voltage gain is 1), the amplifier is a unity gain buffer; also known as a voltage follower. Although the voltage gain of a buffer amplifier may be (approximately) unity, it usually provides considerable current gain and thus power gain. However, it is commonplace to say that it has a gain of 1 (or the equivalent 0 dB), referring to the voltage gain.

Comparator a comparator is a device which compares two voltages or currents and switches its output to indicate which is larger. More generally, the term is also used to refer to a device that compares two items of data. A standard op-amp without negative feedback can be used as a comparator, as indicated in the following diagram When the non-inverting input (V+) is at a higher voltage than the inverting input (V-), the high gain of the op-amp causes it to output the most positive voltage it can. When the non-inverting input (V+) drops below the inverting input (V-), the op-amp outputs the most negative voltage it can. Since the output voltage is limited by the supply voltage, for an op-amp that uses a balanced, split supply, (powered by ± VS) this action can be written: Vout = VS sgn(V+ − V−) where sgn(x) is the signum function. Generally, the positive and negative supplies VS will not match absolute value: Vout <= VS+ when (V+ > V-) else VS- when (V+ < V-). Equality of input values is very difficult to achieve in practice. The speed at which the change in output results from a change in input (often called the slew rate in operational amplifiers) is typically in the order of 10ns to 100ns, but can be as slow as a few tens of μs. A dedicated voltage comparator chip, such as the LM339, is designed to interface directly to digital logic (for example TTL or CMOS). The output is a binary state, and it is often used to interface real world signals to digital circuitry (see analog to digital converter). The LM339

Comparator The LM339 accomplishes this with an open collector output. When the inverting input is higher, the output of the comparator is connected to the negative power supply. When the noninverting input is higher, the output is floating (has a very high impedance to ground). With a pull-up resistor and a 0 to +5V power supply, the output takes on the voltages 0 or +5 and can be interfaced to TTL logic: Vout <= Vcc when (V+ > V-) else 0. A dedicated voltage comparator will generally be faster than a general-purpose op-amp pressed into service as a comparator. A dedicated voltage comparator may also contain additional features such as an accurate, internal voltage reference and adjustable hysteresis. It is incorrect to consider a comparator as a device with a differential (bipolar) input and a logic (0/Vcc) output as the inputs of real comparators are not isolated. This means that not only their difference affects the output but also their voltages must not exceed the power voltage range: VS- ≤ V+,V- ≤ VS+. In the case of TTL/CMOS logic output comparators negative inputs are not allowed: 0 ≤ V+,V- ≤ Vcc. When comparing a noisy signal to a threshold, the comparator may switch rapidly from state to state as the signal crosses the threshold. If this is unwanted, a Schmitt trigger can be used to provide hysteresis and a cleaner output signal.

DIFRENTIAL AMP

A differential amplifier is a type of an electronic amplifier that multiplies the difference between two inputs by some constant factor (the differential gain). A differential amplifier is the input stage of operational amplifiers, or op-amps, and Vout emitter coupled logic gates. Given two inputs and , a practical differential amplifier gives an output Vout: where Ad is the differential-mode gain and Ac is the common-mode gain. The common-mode rejection ratio is usually defined as the ratio between differentialmode gain and common-mode gain: From the above equation, we can see that as Ac approaches zero, CMRR approaches infinity. The higher the resistance of the current source, Re, the lower Ac is, and the better the CMRR. Thus, for a perfectly symmetrical differential amplifier with Ac = 0, the output voltage is given by,

DIFRENTIAL AMP Note that a differential amplifier is a more general form of amplifier than one with a single input; by grounding one input of a differential amplifier, a single-ended amplifier results. Differential amplifiers are found in many systems that utilise negative feedback, where one input is used for the input signal, the other for the feedback signal. A common application is for the control of motors or servos, as well as for signal amplification applications. In discrete electronics, a common arrangement for implementing a differential amplifier is the long-tailed pair, which is also usually found as the differential element in most op-amp integrated circuits

EQUIVALENT CIRCUIT OF AN OP-AMP

EQUIVALENT CIRCUIT OF AN OP-AMP

SOME BASIC PARAMETERS (Fairchild specs. for 741)

NATIONAL SEMICONDUCTORS SPECS. FOR LM741C

EXPLAINING PARAMETERS

OP-AMP WITH –VE FEED BACK 

INPUT OFFSET CURRENT: The algebric difference between the currents of the two inputs of op-amp. For 741c it is 6nA max. For better performance this parameter should be as low as possible.



INPUT OFFSET VOLTAGE: The voltage that must be applied at both i/ps to null the o/p. For 741c it is normally 6mV. For better performance this parameter should be as low as possible.



INPUT BIAS CURRENT: Average of the two i/p currents. For 741 precision ic it is approx (+ve,-ve)7nA.

CONTINUED 

DIFFERENTIAL INPUT RESISTANCE: It is the other name of input impedance that could be measured by connecting one terminal of the Ohmmeter to the any input end and the other to the ground. For 741c it is 2 M-ohm.



For 741c the input capacitance is 1.4pF



OUTPUT OFFSET VOLTAGE: The voltage in the absence of the input.

CONTINUED OFFSET NULL SETTING: Let a 10 KOhm potentiometer is connected between terminal 1 and 5 with moving end connected at –Vee then offset output voltage can be adjusted to zero if potentiometer is varied. COMMON MODE VOLTAGE: When the same voltage is applied to both inputs it is called Vcm. For 741C it is (+ve,-ve) 13V. Hence exceeding these range means improper function of performance. 

CONTINUED 

COMMON MODE REJECTION RATIO:

Ratio of the differential voltage gain to the common voltage gain. It is specified as 90 dB for 741C. CMRR=

Ad /Acm

/

Acm=Vocm Vcm Or the ratio of output common mode voltage to the Input common mode voltage. Note CMMR is measured in decibels & formula for measurement is CMMR=20log10 [AOL/Acm]

CMMR





CMRR of an amplifier (or other device) measures the tendency of the device to reject input signals common to both input leads. A high CMRR is important in applications where the signal of interest is represented by a small voltage fluctuation superimposed on a (possibly large) voltage offset, or when relevant information is contained in the voltage difference between two signals. (An example is audio transmission over balanced lines.) The CMRR, measured in positive decibels, is defined by the following equation:

assumed that the amplifier output Vo can be modeled as

CONTINUED Opamp in common mode connected to source.

CONTINUED 

SUPPLY VOLTAGE REJECTION RATIO: Change in op-amp input offset voltage caused by variation in supply voltage is called SVRR.

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VOLTAGE GAIN: It is actually large signal voltage gain because it amplifies the difference of the inputs to a very large output.

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OUTPUT RESISTANCE: It can be measured by connection o/p and ground.It is very very small for better performance.

CONTINUED  OUTPUT SHORT CIRCUIT CURRENT:

Current flows when output is grounded which could harm opamp. It is normally 2.8mA for 741C.  SLEW RATE: Rate of change of output with the change in input.

IDEAL OP-AMP

IDEAL OP-AMP CHARACHTERISTICS

IDEAL VOLTAGE TRANSFER CURVE (output offset voltage assumed zero)

Differential Amplifiers

Differential Amplifiers

“Amplifies the difference between two input signals “

Figure 1: Differential amplifier.

 2-I/Ps (inverting & non-inverting).  It is combination of inverting & non-

inverting amplifiers.  open-loop voltage gain of operational amplifiers is too great to be used without feedback.  A practical difference Amp must have –ve feedback.

With V2 = 0, Vo1 = - R2/R1 * V1 With V1 = 0, Vr4 = R4/(R3+R4)*V2 & Vo2 = (R1+R2)/R1 * Vr4 = (R1+R2)/R1) * R4/(R3+R4) *V2

With R3 = R1 & R4 = R2, Vo2 = R2/R1 * V2 With both signals present, Vo = Vo2 + Vo1 = R2/R1 * V2 – R2/R1 * V1 = R2/R1 * (V2-V1) R2 = R1, Vo = V2-V1 R2 > R1, O/P can be made amplified version of the I/P difference.

INPUT RESISTANCES  The I/P resistance for voltage V1 is R1, as

in case of inverting amplifier.  At non-inverting I/P terminal, for voltage V2, I/P resistance is R2+R4.  Why R1=R3 & R2=R4?  Two terms to be considered are:  1-Differential I/P resistance, Ridif.  2-Common mode I/P resistance, Ricm

Ridif: The resistance offered to a signal source which is connected directly across the I/P terminals. It is the sum of two I/P resistances. Ridif = R1+R3+R4 Ricm: The resistance offered to a signal source which is connected b/w ground & both I/P terminals, ie, the parallel combination of the two I/P resistances. Ricm = R1 || (R3+R4)

INVERTING AND NON INVERTING OPERATIONAL AMPLIFIER

INVERTING AND NON INVERTING OPERATIONAL AMPLIFIER

Characteristics  The main characteristics that make

differential amplifiers so useful: 1- Their large gain 2- Ability to reject noise, 3- Fact that they amplify the difference between two signals.

USES  Fundamental configuration in electronics.  Every op-amp has a differential amplifier as

its core.  Backbone of many communication circuits such as mixers and modulators.  instrumentation amplifier.

INVERTING AMPLIFIER  If the voltage going into the 741 chip is

positive, it is negative when it comes out of the 741. In other words it reverses polarity (inverts polarity). Two resistors are needed to make the 741 work as an amplifier, R1 and R2.  Hence the inverting amplifier produces phase shift of 180 degree at the output as was shown in fig.

INVERTING AMPLIFIER CIRCUIT

CONTINUED NON-INVERTING AMPLIFIER: If the voltage going into the 741 chip is positive, it is positive when it comes out of the 741. In other words it retains its polarity (symmetrical polarity). Two resistors are needed to make the 741 work as an amplifier, R1 and R2. Hence it produces an output of 0 degree phagse shift or no phase shift.

NON INVERTING AMPLIFIER

CALCULATE THE GAIN INVERTING AMPLIFIER GAIN (AV) = -R2 / R1 Example : if R2 is 100 kilo-ohm and R1 is 10 kilo-ohm the gain would be : -100 / 10 = -10 (Gain AV) If the input voltage is 0.5v the output voltage would be : 0.5v X -10 = -5v

CONTINUED NON-INVERTING AMPLIFIER GAIN (AV) = 1+(R2 / R1) Example : if R2 is 1000 kilo-ohm and R1 is 100 kilo-ohm the gain would be : 1+ (1000/100) = 1 + 10 OR GAIN (AV) = 11 If the input voltage is 0.5v the output voltage would be : 0.5 X 11 = 5.5v

OFFSET NULL ADJUSTMENT

VOLTAGE FOLLOWER 

The voltage follower with an ideal op amp gives simply but this turns out to be a very useful service, because the input impedance of the op amp is very high, giving effective isolation of the output from the signal source. You draw very little power from the signal source, avoiding "loading" effects. This circuit is a useful first stage. The voltage follower is often used for the construction of buffers for logic circuits.

Frequency Response 

Operational amplifiers have property called "Slew Rate". This means that if the input waveform were to be a square-wave then the output would not change instantaneously. It would take a finite time for the output to change state.

Non-linear configurations

Non-linear configurations

i) Precision rectifier 

Behaves like an ideal diode for the load, which is here represented by a generic resistor RL.

ii) Logarithmic

iii) Exponential

ACTIVE FILTERS

ACTIVE FILTERS

ACTIVE FILTERS  Active filters are implemented using a

combination of passive and active (amplifying) components. Operational amplifiers are frequently used in active filter designs. These can have high Q, and achieve resonance without the use of inductors. However, their upper frequency limit is limited by the bandwidth of the amplifiers used.

FILTERS BY TRANSFER FUNCTION  Low-pass filter  High-pass filter  Band-pass filter  Band-stop filter

First Order Low Pass Filter with Op Amp

RESPONSE CURVE

First Order High Pass Filter with Op Amp

RESPONSE CURVE

Band Pass Filter with Op Amp

RESPONSE CURVE

COMPARATOR

OP AMP AS COMPARATOR

OP-AMP

COMPARATOR AS SCHMIT TRIGGER

OP-AMP COMPARATOR AS SCHMIT TRIGGER

OP-AMP

COMPARATOR AS SCHMIT TRIGGER

LIGHT ACTIVATED ALTERATOR The buzzer emits a tone when light falls on the light dependent resistor. Resistor 2 controls the sensitivity of the circuit. The 741 is working as a comparator and the piezo buzzer sounds when the output form the 741 goes ‘low’ or in other words, changes from a positive to a negative.

LIGHT ACTIVATED ALTERATOR CIRCUIT

CURRENT TO VOLTAGE OP AMP 

A circuit for converting small current signals (>0.01 micro amps) to a more easily measured proportional voltage.

By the current rule:

so the output voltage is given by the expression above. Application to photo-detector

INTEGRATOR

DIFFERENTIATOR

Audio Amplifiers  The Op-Amp makes an ideal audio

amplifier with very few external components. With an output current of up to 20mA then it can also drive headphones and even high-impedance speakers, e.g. 50 - 80 ohms. To use the Op-Amp for linear (undistorted) audio applications we always need to use negative feedback. It is the negative feedback that "tames the wild beast".

Simple Audio Amplifier

Practicle Audio Amplifier  The reactance of the capacitors should be at

least 0.2 x the input or output impedances at the lowest operating frequency

Audio Power Amplifiers  Power = Volts X Amperes. So to get an AF

power amplifier we must have both volts and current. We have already seen how to build a voltage amplifier, now let us give it a little current capability. We can do this by simply adding a pair of power transistors, one to give positive current, the other to give negative current.

Audio Power Amplifiers Circuit

Cross Over Distortion  As the transistor in the output of opamp

are not biased over Vbe=0.7volts hence distotion occurs, known as cross over distortion

Limiting Of Cross Over Distortion 

Transistors must be biased to overcome this so

DIGITAL TO ANALOG CONVERTIONS 

Introduction



Connecting digital circuitry to sensor devices is simple if the sensor devices are inherently digital themselves. Switches, relays, and encoders are easily interfaced with gate circuits due to the on/off nature of their signals.

 Analog devices are involved, interfacing

becomes much more complex. What is needed is a way to electronically translate analog signals into digital (binary) quantities, and visa-versa. An analog-todigital converter, or ADC, performs the former task while a digital-to-analog converter, or DAC, performs the latter.

Digital To Analog Convertor

The R/2nR DAC  This DAC circuit, otherwise known as the

binary-weighted-input DAC, is a variation on the inverting summer op-amp circuit. If you recall, the classic inverting summer circuit is an operational amplifier using negative feedback for controlled gain, with several voltage inputs and one voltage output. The output voltage is the inverted (opposite polarity) sum of all input voltages:

The R/2nR DAC Ckt

The R/2R DAC  An alternative to the binary-weighted-input

DAC is the so-called R/2R DAC, which uses fewer unique resistor values. A disadvantage of the former DAC design was its requirement of several different precise input resistor values: one unique value per binary input bit. Manufacture may be simplified if there are fewer different resistor values to purchase, stock, and sort prior to assembly.



Of course, we could take our last DAC circuit and modify it to use a single input resistance value, by connecting multiple resistors together in series:

oscillators

"Amplifiers oscillate and oscillators amplify"

PRINCIPLE OF OSCILLATION

FEED BACK OSCILLATOR PRINCIPLE : 

The key to oscillator operation is

positive feedback.

The circuit must have regenerative feedback; that is, feedback that results in a combined 360°(or 0°) voltage phase shift around the circuit loop.  The circuit must receive some trigger signal to start the oscillations. 

Regenerative feedback.

BARKHAUSEN CRITETERION 

In order for an oscillator to work properly, the following relationship must be met:



If this criterion is not met, one of the following occurs:

1.

If , the oscillations die out after a few cycles. If , the oscillator drives itself into saturation and cutoff clipping.

,

2.

EFFECT OF OP-AMP ON OSCILLATOR

TYPES OF OSCILLATORS  RC Oscillators  LC Oscillators  Crystal Oscillators  Integrated circuit Oscillators

RC OSCILLATORS 

PHASE SHIFT OSCILLATOR

RC OSCILLATORS 

WIEN BRIDGE OSCILLATOR

RC OSCILLATORS 

THE twin-T OSCILLATOR

LC OSCILLATORS 

THE COLPITTS OSCILLATOR

LC OSCILLATORS 

THE ARMSTRONG OSCILLATOR

THANKS

THANKS FOR YOUR PATIENCE

Current Mirroring

A current mirror is a circuit designed to copy a current through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. The current being 'copied' can be, and sometimes is, a varying signal current. Conceptually, an ideal current mirror is simply an ideal current amplifier with unity current gain. The current through R1 is given by: IR1 = IC1 + IB1 + IB2 Where IC1 is the collector current of Q1, IB1 is the base current of Q1, IB2 is the base Current of Q2 The collector current of Q1 is given by: IC1 = β0IB1 Where β0 is the DC current gain of Q1. If Q1 and Q2 are perfectly matched, β of Q2 will be:

where VA is the Early Voltage. Because VBE1 = VBE2 and Q1 and Q2 are matched, IB1 = IB2. After substituting and rewriting, the collector current in Q2 is given by: If β0 > > 1, then

Typical values of β will yield a current match of 1% or better

Current Mirroring (Th) Transistor Q1 is connected such that it behaves as a forward-biased diode. The constant current through it (due to R1 and Vs) is determined mainly by the series resistance R1 as long as Vs is significantly larger than 0.7V, the typical forward VBE voltage for a silicon BJT. It is important to have Q1 in the circuit instead of a regular diode because, assuming the two transistors are closely matched, the base current for each transistor should be nearly identical since VBE for each transistor is identical. With nearly identical base currents, the matched transistors should then have nearly identical collector currents as long as VCE2 is not significantly larger than VBE. If VCE2 is much larger than VBE, the collector current in Q2 will be somewhat larger than for Q1 due to the Early effect and further, Q2 may get substantially hotter that Q1 due to the associated higher power dissipation. When this occurs, the transistors will no longer be matched. To maintain matching, the temperature of the transistors must be nearly the same. In integrated circuits and transistor arrays where both transistors are on the same die, this is easy to achieve. But if the two transistors are widely separated, the precision of the current mirror will not be stable. Additional matched transistors can be connected to the same base and will supply the same collector current. In other words, the right half of the circuit can be duplicated several times with differing values of R2 on each. Note, however, that each additional right-half transistor "steals" a bit of collector current from Q1 due to the non-zero base currents of the right-half transistors. This will result in a small reduction in the programmed current.)