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OP-AMP BASIC

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OP-AMP BASIC Powered By Docstoc
					ECE, ME – I SEM


   BASIC
ELECTRONICS
OPERATIONAL AMPLIFIER


           -
           +
               Contents
          A)Basics of Op-amp
         B)Ideal Characteristics
            c)Applilcations
Operational Amplifier
   It refers to a circuit that is composed of transistors,
    resistors, and capacitors, and has a very large Thévenin
    input resistance and very small Thévenin output
    resistance.
   It behaves, over at least a portion of its operating range, as
    a high-gain linear voltage amplifier.
   It is an integrated circuit component, and has two signal
    input terminals and one signal output terminal.
   As an integrated circuit, the op-amps needs DC power
    supplies for its operating.
   Using the op-amps we can construct many useful complex
    circuits. Therefore the op-amps are the versatile
    components for constructing circuits
Typical Op Amp IC Packages
Op Amp Pin-out connections
Dual-in-Line Plastic Package
   Top view
Op Amp Pin-outs Metal Can
Two matched transistors form a differential
amplifier
Transisterized
Differential Amplifier

 Double ended input - double ended output diff.
  amp:
 vout=A (v1-v2)




                         vout
            v1
                         - +             v2
Op Amp stages with pin-outs of IC741



                             7
       2



                                       6



       3
                             4
Ideal Operational Amplifier

 An ideal Operational Amplifier is basically a
  3-terminal device that consists of two high
  impedance inputs, one an Inverting input
  marked with a negative sign, ("-") and the other a
  Non-inverting input marked with a positive
  plus sign ("+").
 The amplified output signal of an Operational
  Amplifier is the difference between the two
  signals being applied to the two inputs. In other
  words the output signal is a differential signal
  between the two inputs and the input stage of an
  Operational Amplifier is in fact a differential
  amplifier as shown below.
Equivalent Circuit for Ideal Operational
Amplifiers
The transfer characteristic of an op-
amp is shown:
Explanation
  It includes Three regions

1. Negative saturation region, the output voltage is fixed to -
Vcc (<0):


2. Positive saturation region, the output voltage is fixed to
+Vcc (>0)


3. Linear region, the output voltage is determined linearly by
input difference.


where, A is called the open-loop voltage gain of an op-amp,
which is usually excess of 10,000.
IDEALIZED CHARACTERISTIC
a) Voltage Gain,
   (A)Infinite - The main function of an operational amplifier is to
   amplify the input signal and the more open loop gain it has the
   better, so for an ideal amplifier the gain will be infinite.

b)   Input impedance,(Zin)Infinite - Input impedance is assumed to
     be infinite to prevent any current flowing from the source supply
     into the amplifiers input circuitry.

c)    Output impedance, (Zout)Zero - The output impedance of the
     ideal operational amplifier is assumed to be zero so that it can
     supply as much current as necessary to the load.

d)   Bandwidth, (BW)Infinite - An ideal operational amplifier has an
     infinite Frequency Response and can amplify any frequency
     signal so it is assumed to have an infinite bandwidth.

e)    Offset Voltage, (Vio)Zero - The amplifiers output will be zero
     when the voltage difference between the inverting and non-
     inverting inputs is zero
Points to Remember
   However, real Operational Amplifiers such as
    the commonly available uA741, for example do
    not have infinite gain or bandwidth but have a
    typical "Open Loop Gain" which is defined as the
    amplifiers output amplification without any
    external feedback signals connected to it and for
    a typical operational amplifier is about 100dB at
    DC (zero Hz). This output gain decreases linearly
    with frequency down to "Unity Gain" or 1, at
    about 1MHz and this is shown in the following
    open loop gain response curve.
Op amp Characteristics
    Common Mode Rejection Ratio
    (CMRR)
 It is the ability of an op amp to reject the signal
  which is present at its both inputs
  simultaneously i.e. the common mode signal
 CMRR = Ad / ACM, where ACM is common mode
  voltage gain defined by Vout / VCM, Ad is differential gain
    of Op-Amp.
 Ideally CMRR is infinite
 For IC 741 it is 90 dB
Opamp Slew Rate


  •Effect of slew-rate
  limits output for
  sinusoidal waveforms.
  •Slew rate is caused by the finite
  response time of the circuit elements
  of an op amp
                            i   m ax
  It limits the highest possible
  frequency of operation

    dvi
         Vi cost  Vi                      dv0
    dt                                    SR 
                                               dt
  •Limited BW,                                       m ax
                                                            17
Offset Voltage
Input Offset Voltage
 Small voltage needed to be applied between INV
  and NI terminals to get zero output voltage
 Ideally it should be 0 V (CM operation)

Output Offset Voltage
   Under common mode operation, output voltage
    should be zero, but due to mismatch is devices it
    is non-zero (Can be corrected by applying voltage
    between Pins 1 and 5 of IC 741)
Characteristics (Cont’d)
 Power-supply rejection ratio.
  The power-supply rejection ratio PSRR is the
   ratio of the change in input offset voltage to the
  corresponding change in one power-supply, with
  all remaining power voltages held constant. The
  PSRR is also called "power supply insensitivity".
  Typical values are in:V / V or mV/V
 Input Bias Current:
  The average of the currents into the two input
  terminals with the output at zero volts.
 Input Offset Current :
  The difference between the currents into the two
  input terminals with the output held at zero
Open Loop Gain

 AOL is ideally infinite
 In case of typical practical op amp like IC 741 it
  is of the order of 105 i.e. about 100 dB
Input Resistance
   RI is ideally infinite
   In case of IC 741 it is 2 MΩ
Output Resistance
 RO is ideally zero
 In case of IC 741 it is 75 Ω
Open-loop Frequency Response Curve
 Product of the gain against frequency is constant
  at any point along the curve. Also that the unity
  gain (0dB) frequency also determines the gain of
  the amplifier at any point along the curve. This
  constant is generally known as the Gain
  Bandwidth Product or GBP.
 GBP = Gain x Bandwidth or A x BW.
Example
 For example, from the graph above the gain of
  the amplifier at 100kHz = 20dB or 10, then the
  GBP = 100,000Hz x 10 = 1,000,000.
 Similarly, a gain at 1kHz = 60dB or 1000,
  therefore the
   GBP = 1,000 x 1,000 = 1,000,000. The same!.
An Operational Amplifiers Bandwidth
   The operational amplifiers bandwidth is the
    frequency range over which the voltage gain of
    the amplifier is above 70.7% or -3dB (where 0dB
    is the maximum) of its maximum output value as
    shown below. Here we have used the 40dB line as
    an example. The -3dB or 70.7% of Vmax down
    point from the frequency response curve is given
    as 37dB. Taking a line across until it intersects
    with the main GBP curve gives us a frequency
    point just above the 10kHz line at about 12 to
    15kHz. We can now calculate this more
    accurately as we already know the GBP of the
    amplifier, in this particular case 1MHz
Key Points
   Open loop gain of op amp is defined as:
           AOL = Vo / VD
   where VD = VNI – VINV
 Open loop gain of op amp is very high
  (ideally infinite).
 Any small difference between VNI and VINV
  results into saturation of output voltage ±VSAT
  i.e. for VNI < VINV output is – VSAT and
       for VNI > VINV output is + VSAT
 Value of VSAT is limited by the supply voltage of
  op amp
Important features of Op Amp
1.   High open loop gain (ideally infinite) which
     implies that even the smallest difference
     between the two inputs results into saturated
     output voltage
2.   High input impedance (ideally infinite) implies
     that there is no current flowing into the input of
     an op amp
Virtual Ground
a) These two conditions give rise to VIRTUAL
  GROUND, where the voltages at both the inputs
  are maintained at exactly same level.
b) To achieve this condition, a feedback circuit
  between the output and the inverting input
  terminal of the op amp is necessary.
c) This results into many applications of op amp,
  which qualify it to be OPERATIONAL: adder,
  subtractor, multiplier, divider etc.
Opamp can be used in two configuration
A) Open Loop configuration:
   a) There is no connection from output to input.
   b) When connected in open-loop configuration,
   the op-amp functions as a high gain amplifier.
   These are three open loop op-amp
   configurations.
 Differential Amplifier
 Inverting Amplifier
 Non-Inverting Amplifier
  Note: These configurations are classed according
   to the number of inputs used and the terminal
   to which the input is applied when a single
   input is used.
Disadvantages of open loop
configurations
   The open loop of the op-amp is very high. Therefore only
    the smaller signals having low frequency may be
    amplified accurately without distortion.
   Open loop Voltage gain of the op-amp is not a constant
    voltage gain varies with changes in temperature and
    power supply as well as mass production techniques. This
    makes op-amp unsuitable for many linear applications
   Bandwidth of most open loop op-amps is negligibly small
    or almost zero therefore op-amp is impractical in ac
    applications.
    Note: For these reasons, the open loop op-amp is
    generally not used in linear applications. In certain
    applications, the open loop op-amp is purposely used as a
    non -linear device; that is a square wave output is
    obtained by applying a relatively large input signal
Closed Loop Configuration
   There is connection from output to input
   The use of feedback i.e. an output signal is fed back to the
    input either directly or via another network. If the signal
    fed back is of opposite polarity or out of phase of 180
    degree. with respect to the input signal, the feedback is
    called negative feedback. Negative feedback is also known
    as degenerative feedback.
   When used in amplifiers, negative feedback stabilizes the
    gain, increases the bandwidth and changes the input and
    output resistance. Other benefits include a decrease in
    harmonic or non-linear distortion and reduction in the
    effect of input offset voltage at the output. It is also reduces
    the effect of variations in the temperature and supply
    voltages on the output of the op-amp.
These are three open loop op-amp
configurations
 a) Differential Amplifier
 b) Inverting Amplifier
 c) Non-Inverting Amplifier
Inverting Amplifier
Multiplier/Divider circuit, whose gain is decided by
 the values of resistors

              Rin          Rf


      Vin
                           +
                                         Vout
Inverting Amplifier Analysis

 Due to virtual ground:
   Iin= Vin / Rin= If = – ( Vout / Rf )
 Vout= – (Rf / Rin ) Vin

 Rf > Rin →multiplier Rf < Rin→ divider

             Iin R       If    Rf
                  in



       Vin
                               +
                                     Vout
Non-inverting Amplifier
 Due to virtual ground:
 VINV = VNI = Vin
 If = (Vout–Vin) / Rf = Iin= Vin / Rin
 Vout = [1+ (Rf / Rin )] Vin




                              +
                  Vin
                                       If
    Vin                                     Vout
                Iin               Rf
                        Rin
The Difference Amplifier
                                                                 R
                                              Since v-= v+ vo   2 (v  v )
                                                                 R 1 2
                                                                  1
                                              For R2= R1 vo  (v1  v2)
                                          This circuit is also called a
                                           differential amplifier, since
                                           it amplifies the difference
                                           between the input signals.
                                          Rin2 is series combination of
v o  v-  i R  v-  i R
            2 2         1 2                R1 and R2 because i+ is zero.
                              
       R              R R         R  For v2=0, Rin1= R1, as the
 v-  2 ( v  v- )        2  v-  2 v
                       1      
                                           circuit reduces to an
       R 1            
                      
                          R         R 1 inverting amplifier.
        1                 1        1
                    R                     For general case, i1 is a
  Also, v             2 v                 function of both v1 and v2.
              
                 R R       2
                   1 2
Unity Gain Amplifier
(Non-inverting Buffer




                  –
                        Vout=Vin
     Vin          +
Summary
 Op amp can be used for various (mathematical)
  operations like addition, subtraction,
  multiplication, division, differentiation,
  integration etc.
 Infinite open loop gain and infinite input
  resistance give rise to VIRTUAL GROUND
 Op amp is available in a simple to use IC form
  which require dual supply
 Many other applications can be implemented
  using op amp

				
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