OPERATIONAL AMPLIFIER (OP-AMP)
INTRODUCTION:-
• An electrical amplifier is abbreviated as op-amp.
• It is basically a multistage, very high gain , direct coupled, negative feedback amplifier that provides a stabilize v/g gain.
• It has high i/p impedance ( less than 100 Ohms) and has capability of amplifier signal having frequency raging from zero Hz to 1MHz. ie op-amp can be used to amplify dc as well as ac input signal.
• An op-amp named as it was designed to perform mathematical operations such as summation subtraction, multiplication, differentiation and integration etc.
• The other uses of op-amp are signal changing, scale changing, phase shifting, v/g regulation, along cputer operations, in instrumentation and control systems, oscillator , pulse generators square wave generators, triangular- wave genarator corporators along to digital and digital to a long converters (ADC & DAC); v/g to - current converters, current- to- v/g converters, sample & Hold ckt etc.
• The op- Amp manufactured with integrated transistor, diodes, resistor & capacitors.
• The op-amp has the advantages short is low cost small size hai reliability temperature stability and low value of affset v/g and current.
• The op-amp was first built with vacuum tube.
• An op-amp referred to as the IC741, the various manufacturer has their own designation.
Manufacturer IC designation
National semiconductor--> LM741
Motorola. --> MC1741
RCA. --> CA3741
Texas instrument. --> SN52741
Signetics. --> N5741
# the last three digits in each manufacturers
desiguation are 741 indicating that all these op- Amp have same specification and behaves the same way.
BLOCK DIAGRAM OF A OPERATIONAL AMPLIFIER:-
• An op-amp is multistage amplifier and consists of some basic building blocks are as shown in figure. a.
• It consists of a four stage direct- coupled amplifier.
• The first stage is double-ended high gain (60dB) differential amplifier i.e dual i/p balanced o/p differential amplifier with a constant current source.
• In this stage would be a negligible effect on the o/p of any short coming in the following stage.
• This stage determines the I/p resistance of the op-amp.
• o/p of this stage is taken b/w collectors of two emitter based ckt so that o/p remains balanced and the dc v/g at o/p in quiescent condition maintains zero level.
•The second stage is called the intermediate stage is usually another differential amplifier which is driven by the o/p of first stage.
• In most amplifier the intermediate stage is dual i/p unbalanced (single ended) o/p differential amplifier in order to increase the gain.
• In this amplifier o/p is measured at the collector of only one of the two transistor w.r.t ground.
• In the quiescent condition same dc v/g exist at the o/p terminal and there is no other collector v/g at o/p to balance or nullify this o/p dc v/g.
• This type of amplifier is called unbalanced o/p type differential amplifier.
• The unbalanced dc v/g present at the o/p acts as an error v/g in the desired o/p signal.
1. The third stage in known as level shifting stage is usually and either blower ckt in order to shift the DC level at the output of the the intermediate stage downward 20 volt with respect to groundground.
• the signal is developed in the intermediate stage due to direct coupling and gets amplified in the stage.
• this increase in DC level tends to shift the operating point of the succeeding stages which also limits the output voltage swing for may distort the o/p signal.
• to overcome these problems used to to a level shifter ckt because necessary to bring this dc level to zero volt (ground potential)
2. The final stage is called output stage use boli push-pull complimentary amplifier.
The stage increase the output voltage swing and the current supply capability of the amplifier The output of the op-amp is the voltage measured at output of this final stage push-pull amplifier w.r.t. ground. figure.b.
figure.b.
--> A is the v/g (voltage) gain.
--> V1 is non inverting I/p.
--> V2 is inverting I/p.
--> The differential I/p is VD= v1-V2.
--> The o/p v/g is, volt= AVin= A(V1-V2)
# The v/g V1, V2 and volt are node v/g and they are always measured w.r.t. ground. figure.c.
# The i/p are marked with plus (+) and minus (-) to indicate noninverting & inverting I/p respectively.
# The plus & minus signs never means that the v/g V1 and V1 are positive & negative respt.
--> op-amp have five besic terminals namely two i/p terminals { Non- inverting i/p & inverting i/p terminals}, one o/p terminals and two
Power supply terminals { -ve bias supply terminals & +ve bias supply terminals} figure.d.
--> This figure is useful in analyzing the basic operating principal of op-amp and in observing the effects of feedback arrangments.
--> The v/g source A V1 is an equivalent thevenin v/g of the ckt skown in figure.
Volt= Avd= A( V1-V2)
Where A is large signal v/g gain
Bd= differential i/p (input)
V1 & V2 are the the i/g voltage at indivesting terminals w.r.t ground.
--> The equation shown that o/p(output) voltage is directly proportional to the algebraic difference of the two i/p voltage which means that the op-amp amplifier the difference b/w the i/p voltage.
--> The polarity of the o/p voltage depends on the polarity of the difference voltage VD.
Open- loop op-amp configuration:-->
--> Open means that there is no connection belong input and output terminals are the direct or via another network. i.e. the output signal not feedback in any from as part of the input signal and the loop that would format with feedback is open.
--> An op-amp in open loop configuration acts as high gain amplifier.
--> There are there open loop op-amp configuration.
1. Differential amplifier
2. Inverting amplifier
3. Non-inveting amplifier
1. Differential amplifier:-->
--> In this ckt, input are applied to both the inverting and non inverting terminals.
--> In this configuration, the difference b/w two i/p signals is amplified so the configuration called differential. figure.e..
Input signals Vin1× Vin2 main bhi are either AC or DC voltage as op-amp is capable of amplifier both dc and ac i/p voltage.
--> Source resistance Rin1 & Rin2 are usually negligible small as compared to input resistance of op-amp (Rin) , neglecting voltage drop across source resistance, V1= Vin1 & V2= Vin2
and output voltage is given as volt = A( V1-V2) = A (Vin1-Vin2)
Where A is open loop gain
--> the output voltage is 8 times the difference between two input voltages and the polarity of the output voltage depends upon the polarity of the input difference voltage ( Vin2-Vin2)
2.Inverting amplifier:-->
--> in investing configuration the input signal is applied to the inverting minus (-) input terminals and noninverting plus (+) i/p input terminals is grounded. figure.f.
figure.f.
--> since V1= 0 and V2= Vin
--> o/p voltage, voult= AVd= A(V1-V2)= - A Vin.
Thus negative signal indicates the output voltage is out of phase with respect to voltage by 180 degree. Thus output voltage is a times the input voltage and is of opposite polarity.
3. Non inverting amplifier:-->
--> In this configuration, the i/p (input) signal is applied to the moninverting plus (+) input terminals and inverting minus (-) i/p input terminals is grounded.figure.g.
figure.g.
--> Since V1= Qin and V2 =0,
the output voltage voult= AVd= A(V1-V2)= AVin.
--> The output vout is A times voltage Vin and is of the same polarity (i.e voult and Vin are in phase.)
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