Basics of Operational Amplifier (EEE Department @ LAQSHYA College)

Basics of Operational Amplifier

Operational amplifier is so named as it is used to perform mathematical operations such as addition, subtraction, multiplication, differentiation, integration and many more. As op-amp has wide range of applications, some of its various applications are in industrial, communication, computer, control, and medical applications in additional with them in military applications too.

            An integrated circuit manufacturing industries incorporates integrated transistors, diodes, resistors and capacitors within op-amp ICs. So it is an extremely versatile device which has countless applications in many more areas.

            Since the op-amp is an integrated device, we don’t find any discrete components like active components such as transistors and diodes and passive components like R, L & C. consequently, it offers small size, low cost, high reliability, more temperature stability and low power consumption.

Block Diagram of A typical Op-Amplifier:

1à Non inverting input terminal

2à Inverting input terminal

3àDual input balanced output differential amplifier

4à Dual input unbalanced output differential amplifier

5à Emitter follower with constant current source. It is used to shift the DC level to ground in order to keep Q-point stable and also to limits the output voltage swing.

6à Complementary symmetry push-pull amplifier.

Schematic Symbol of an Op-Amplifier

Five basic terminals of op-amp :

                                Pin2àNon inverting terminal

                                Pin3à Inverting terminal

                               Pin7&pin4à Power supplies

                                Pin6à Output terminal

Equivalent Circuit of an Op-Amplifier:

V_dà Differential input voltage

R_inà Input resistance of an op-amp

R_outà Output resistance of an op-amp

A_vd & R_outà  Thevenin’s voltage source and Thevenin’s resistance respectively looking back into the output terminals of an op-amp

Note: An electrical equivalent circuit is used to analyze basic operating principles of op-amp and in observing the effects of feedback.

Ideal Op-Amp Characteristics :
  _a) R_i=∞     
   b) R_o=0 

              c) A_0L=∞         

  d) BW=∞       

  e) Zero offset voltages           

  f) CMRR=∞

R_i=∞:

     Since input resistance is infinite, Ib1&Ib2 bias currents are ideally zero and practically very small. Due to R_i is very large loading effect is avoided.

R₀=0:

     Since output resistance is zero the voltage across output terminals is independent of current flowing through the load. If R_o=0 , it is used to drive infinite number of sources.

A_0L=∞:

     It implies there is a finite amount of output voltage for the zero differential input voltages.

BW=∞:

     It shows, op-amp is used for both DC&AC where the frequency ranges from 0 HZ to high frequency.

Zero offset :

     It means for V₁=V₂=0 the V_o  must be zero.

CMRR(Common Mode Rejection Ratio):

     For an ideal op-amp,                                  CMRR=ρ=A_{d /} A_c=∞  

      A_dàdifferential mode gain                        A_càcommon mode gain

DC Characteristics of Op-Amp:

a)      V_ios: The spurious i/p voltage causes to get small mv of output even in the presence of both the inputs are grounded. For an ideal op=amp it should be zero.

b)     I_ios:  The algebraic difference between the two bias currents is called input offset currents.

                        I_ios=|I_b1-I_b2|

c)      I/P Bias Current: The average sum of two bias currents flowing into an op-amp for the two bases of the transistors is called as input bias current.

                                      I_b=(I_b1+I_b2)/2

d)     Thermal Drift: The effect of variation in temperature causes changes in V_ios, I_ios & I_b is referred as thermal drift.

AC Characteristics :

a)      Gain Bandwidth Product: The range of operating frequencies of an op-amp at its unity gain is called gain bandwidth product. It also describes frequency response where variation in magnitude and phase of the gain due to change in frequency.

b)     Slew Rate: The maximum rate of change of output voltage is known as slew Rate.

SR=dVo/dt  | _max

                 SR= dVo/dt | _max=I_max/C

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Article By

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Mr.K.Pithamber                

Assistant Professor

CSE Department

LAQSHYA Institute of Technology & Sciences              

Slicers and its Working (ECE Department @ LAQSHYA)

Slicers

Clipping circuits are also referred to as voltage (or current) limiters, amplitude Selectors or slicers. They are used to select for transmission that part of an arbitrary waveform which lies above or below some particular reference voltage level.

A clipping circuit comprises of linear elements like resistors and non-linear elements like junction diodes or transistors, but it does not contain energy storage elements like capacitors.

A clipper circuit can remove certain portion of an arbitrary wave form near the positive or negative peaks. Clipping may be achieved either at one level or at two levels.

fig : Clippers Classification

In shunt clippers, the diode is connected in series with reference voltage across the output terminals. In series clippers, the diode forms a series path connecting the input and output terminals.

·         The analysis of any clipper circuit has the following three stages.

    (i) Study of working of the diode.

    (ii) Formulation of the transfer characteristic equations and

    (iii) Plotting of the transfer characteristic curves

Now we discuss about the principle and working of CLIPPING ABOVE THE REFERENCE LEVEL circuit. The shunt clipper (CLIPPING ABOVE THE REFERENCE LEVEL) circuit diagram is as shown below.

Working :  It is seen that, for V_i<V_R+V_γ, the diode D is OFF  because it is reverse biased and hence  it does not conduct therefore no current flows, hence there is no voltage drop across R.

                                            V_O=V_i         for         V_i< V_R+V_γ.

 For V_i>V_R+V_γ, the diode D is ON because it is forward biased and the potential barrier is overcome hence it conducts, therefore the output voltage is equal to the reference voltage

                                                                V_O=V_R                              for           V_i> V_R+V_γ.

From the above discussion we can conclude that the transfer characteristic equations are

                                                                V_o =V_i           for   V_i<V_R+V_γ and,

                                                                V_o =V_R+V_γ    for    V_i>V_R+V_γ,

 Transfer Characteristic Curve: It is graph between input and output voltages. When Vo =V_i=> Vo/V_i= 1 i.e. slope=1 and When Vo=V_R+V_γ i.e. Vo is constant since both V_R and V_γ are of fixed magnitude hence Slope=0                     When D is OFF, V_O=V_i, there is no clipping action and the input signal is transmitted without any alteration of the wave shape.

    However, when D is ON, it is seen that Vo is constant whatever the instantaneous magnitude of Vi. Hence there is clipping action. The portion of the input signal greater than (V_R+V_γ) is not transmitted. This is clearly shown in the following fig.

Fig: Transfer Characteristic curve for clipping above the reference level.

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Article By

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Mr.A.Ravi Shankar                        

Assistant Professor

ECE Departement

LAQSHYA Institute of Technology & Sciences              

Time Base Generator (ECE Department @ LAQSHYA)

Time Base Generator

A time base generator is an electronic circuit which generates voltage or current that varies linearly with time. Ideally the output waveform of time base generator should be a ramp. The most popular and important application of such a ramp voltage is in a cathode ray oscilloscope (CRO), for deflecting the electron beam horizontally across the screen we need sweep voltages or ramp voltages. This sweep signal is either voltage or current.

To generate these signals we go for linear time base generators. It generates a voltage or current which varies linearly with time.

There are two kinds of sweep generators, based on its output

 Voltage time base generator.

 Current time base generator. 

        Time base generator also finds application in radar, television, time modulation and precise time measurements  etc.

Time –base wave forms are generated by the following methods

o    Exponent charging

o    Constant current charging

o    Miller circuit

o    Boot strap sweep circuit

Now we will discuss about the principle and working of Bootstrap sweep circuit

Bootstrap sweep generator

Principle: This constant current flowing through the capacitor develops ramp voltage across it.

Operation:The circuit of a transistor bootstrap ramp generator is shown in figure. The ramp is generated across capacitor C1, which is charged via resistance R1. The discharge transistor Q1 holds the capacitor voltage V1 down to V_{CE (sat)} until a negative input pulse is applied. Transistor Q2 is an emitter follower that provides a low output impedance emitter through Re is connected to a negative supply level, rather than to ground. This is to ensure that Q2 remains conducting when its base voltage V1 is close to ground. Capacitor C3, known as the bootstrapping capacitor, has a much higher capacitance than C1. The function of C3, as will be shown, is to maintain a constant voltage across R1 and thus maintain the charging current constant.

The input Vi is a pulse voltage, when the input signal Vi is positive, the transistor Q1 becomes ON i.e. goes into saturation. Potential of point V_A= V_CE(sat)=0.3v

Output voltage VO=V_A-V_BE (Q2) (in active region)

                              = 0.3-0.6= -0.3

The emitter of Q2 is coupled to the collector of Q1 through the capacitor C3.         

Hence point B becomes negative, hence Diode D readily conducts, with the result that potential at V_B =Vcc

    When the input Vi goes negative, Q1 becomes OFF.C1 startscharging via R1.voltage V1 now increases, and the emitter voltage Vo of Q2 also increases. As Vo increases, the lower terminal of C3 is pulled up. Because C3 has a high capacitance, it retains its charge and as Vo increases, the voltage at the upper terminal of C3 also increases .the result is that the potential of B also rises by the same amount.

Thus V_B rises from Vcc to Vcc +V_A =>    V_B=Vcc +V_A

Let I denote the current through R1

i.e   I =(V_B-V_A)/R1 =Vcc/R1

Since V_B=Vcc +V_A

Since both Vcc and R1 are of fixed magnitude, the ratio (Vcc/R1) is constant. Hence current I is of constant magnitude.

Since the collector current of Q1 is (Ic1=0) zero, I=I₁+I_B2

I_B2    is the base current of Q2.since Q2 is an emitter follower, its input impedance is very, very high and hence I_B2 is practically zero therefore I1=I, a constant current As the current flows through the capacitor C1,a ramp voltage develops across it.

For an emitter follows, voltage gain is almost unity. Therefore, the output Voltage Vo is also a ramp voltage. Thus the bootstrap circuit generates a ramp voltage.
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Article Presented By 

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A.Ravishankar
Assistant Professor
ECE Department
LAQSHYA Institute of Technology & Sciences

Large Signal Amplifiers (Part-1) (ECE Department @ LAQSHYA)

The main aim of large signal amplifier also called Power Amplifier is to produce large amount of power to   load. It takes power from the DC power supply connected to the output circuit and converts it into AC output power. This type of AC output power is controlled by the input signal.

 Based on the conduction angle of a transistor, large signal amplifiers can be classified into four types namely class A, class B, class AB, and class C amplifiers.

Class A power amplifier is one in which the Quiescent point and input signal are selected such that output signal is obtained for a full input cycle. It conducts 0 to 360 degrees of input signal. The Q point is located at the centre between cut off and saturation points.

Class B power amplifier is one in which the output signal is obtained for half cycle of AC input signal. It conducts 0 to 180 degrees of input signal. The Q point is located at the cutoff point.

Class AB power amplifier is one in which the output signal is obtained for more than 180 degrees but less than 360 degrees of input signal. The Q point is located just above the x-axis but below the midpoint on a load line.

Class C power amplifier is one in which the output signal is obtained for less than a half cycle of input signal. The Q point is located just below the X- axis.

Class A power amplifier:

Class A power amplifiers can be classified in to two types namely 

1) Series fed class A power amplifier     

2) Transformer coupled class A power amplifier

Following figure (a) represents series fed class A power amplifier,

AC output power = Pac =vce * ic

But vce = Vm/√ 2 = (Vmax-Vmin)/2√2 =Vcc/2√2

Ic=Im/√2 = Imax – Imin/2√2 = Vcc/2√2 Rc

Pac = Vcc/2√2 * Vcc/2√2 Rc = Vcc 2/8 Rc

DC input power = Pdc = Vcc * IcQ

But IcQ =I max – I min/2 =Vcc/2Rc

Pdc=Vcc2/2Rc

Efficiency =η= Pac/Pdc *100 = (Vcc/8Rc) (2Rc/Vcc2)100 =25%

Thus the maximum efficiency in series fed class A power amplifier is 25%

        

Figure (a): Series Fed Class A power Amplifier

In order to increase the efficiency, transformer coupled class A power amplifier is used. For getting maximum AC output power, the primary winding of a transform is having high impedance and low resistance value but the secondary winding is having low impedance. Following figure (b) represents transformer coupled class A power amplifier.

AC output power= pac=vce*ic

But vce= (Vmax-Vmin)/2√2=2Vcc/2√2=vcc/√2

Ic= (Imax-Imin)/2√2=2IcQ/2√2=IcQ/√2

Pac=Vcc*IcQ/2

DC input power=Pdc=Vcc*IcQ

Efficiency=η= (Vcc*IcQ/2) (1/Vcc*IcQ)*100=50%

Thus the maximum efficiency in Transformer coupled class A power amplifier is 50%. But practically, the efficiency is always less than 50%.

Figure (b): Transformer Coupled Class A power Amplifier

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Article Presented By 

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M. Srinivasulu

 Professor 

HOD : ECE Department

LAQSHYA Institute of Technology & Sciences

Multi Rate Sigal Processing (ECE Department @ LAQSHYA)

Multi Rate Sigal Processing

Have you ever heard about conversion of audio tape to CD (compact disc)? we know that both are storage devices, though both are for storage purpose it needs a conversion process because there is change in their sampling rates. Sampling rate of CD is 44.1kHz, audio tape is 48kHz.

Normally TV broadcasting can be done in any one of the following schemes.

1)      PAL (Phase Alternating Line) European standard.               

2)       NTSC (National Television System Committee)American standard.
TV receiver set made to receive programs transmitted on PAL scheme butcannot receive NTSC programs and vice-versa. That is one who want to watch an European program in America on TV it is not possible, because Europe and America adapt different schemes of transmission. To avoid this TV receiver sets provided with sampling rate converter so that one can watch programs comfortably.
From the above example we can know that we need signal processing system that can process varying sampling rates such systems are known as ‘MULTIRATE SIGNAL PROCESSING SYSTEMS’.
The process of converting a signal in one sampling rate to another sampling rate is known as ‘SAMPLING RATE CONVERSION’. 
This type of conversion needs in many applications. Some are
1) High quality data acquisition and storage systems.
2) In trans multiplexers.
3) In speech processing to reduce the storage space or the transmitting rate of the speech data.
4) Narrow band filtering for fetal ECG and EEG.etc.
The sampling rate conversion can be done in two ways.
1)      D/A conversion and resampling at required rate.
2)      Sampling rate conversion in digital domain( Multi rate sampling )
Broadly sampling rate conversion can be of two types
1)      Down sampling                              
 2) Up sampling
   Down sampling : In this, sampling rate decreases by a factor M. If x(n) is a discrete time signal then it’s down sampled version that can be denoted by x(Mn). Down sampled signal by a factor ‘M ‘can be formed by taking every M^thvalue of x(n). In this sampling rate of the resultant signal decreases.
Up sampling : In this,sampling increased by a factor L. If x(n) is a discrete time signal then it’s up sampled version can be denoted by x(n/L). 
Up sampled signal by a factor ‘L’ can be formed by placing (L-1) zeros between each pair of samples of x(n). In this sampling rate of the resultant signal increases
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Article Presented By 

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P.Koteswar Rao
Associate Professor
ECE Department
LAQSHYA Institute of Technology & Sciences