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Operational Amplifiers

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					            Operational Amplifiers

What is an Op amp?
  A multistage high-gain amplifier integrated in analysis as a
  separate block.
 The input of an op amp is a differential amplifier therefore
  has 2 inputs.
 The output is singled ended.
 Typically configured for a dual power supply rails (+/_V)
      Op Amps (differential input)

Differential Amplifier considerations :
2 NPN, 2 PNP or 2 FETs (matched pairs)
Inherent symmetry
Current stability
Input impedance
Beta
          Op amps (differential input)

D.C. considerations of Diff Amp:
 Emitter type bias
 Total current ( )
 Input offset current and voltage
  Will both transistors have identical characteristics?
  - Differences in junction characteristics will develop
  variations in base and collector currents. Because the diff
  amp amplifies the difference potential it sees between the
  two inputs so too will these unwanted differences be
  amplified.
        Op Amps (differential input)

The D.C. performance of an op amp can be measured.
 Input offset current (difference between both base
  currents)
 Output offset current )difference between two collector
  currents)
 Output offset voltage (difference between two collector
  voltages)
 The amount of voltage required to offset the base voltage
  difference is called the input offset voltage.
 Ideal op amp will have all three at 0.
IC1   IC2
   Op Amps (differential amplifiers)

A.C. CONSIDERATIONS
Four signal circuit configurations:
1. Differential input, differential output
2. Differential input, single-ended output (most common)
3. Single-ended input, differential output
4. Single-ended input, single-ended output
     Op Amps (differential amplifier)

A.C. CONSIDERATIONS
Assume a single-ended input and single-ended output:
 The voltage output of the Q2 collector is in phase with the
  Q1 input.
 The voltage output of Q1 is out of phase with the Q1 input.


 The voltage seen across the two collectors will be 180
  degrees apart. If we take the differential output we observe
  a doubling of the signal.
Vout 2
         Vout 1
    Op Amps (differential amplifier)

COMMON MODE
In phase signals are common-mode signals
Ideal op amp will suppress common-mode signals entirely
  with a common mode gain of 0
A real op amp will not display perfect common mode
  rejection
Dominant causes of error are:
      1. Resistor tolerance variations.
      2. Transistor parameter deviation..
     Op Amps (differential amplifier)

A.C. CONSIDERATIONS
Assume a single-ended input and a single ended output:
 Voltages at both inputs that are in phase will develop out
  of phase collector outputs that when added will be 0. (see
  diagram)
 In summary, a differential amplifier suppresses in phase
  input signals and elevates out of phase input signals.
                      Op Amps

Common applications:
• Inverting amplifier
• Non inverting amplifier
• Comparators
• Voltage level detectors
• Instrumentation amplifiers.
        +V




             Vout
 Vin-

Vin+




        -V
        +Vcc



 Vin-

               Vout

Vin+


        -VEE
                   Op Amps

Attributes of an ideal op amp:
Infinite gain
Unlimited bandwidth
Infinite input impedance
Very little output impedance
                       Op Amps

Open loop versus closed loop configuration:
=>Open loop has infinite gain (no feedback) therefore it is
  suitable for comparator or voltage detection applications.
  Why? Output can drive to a desired saturation level
  depending on events observed at the input.
=> Closed loop uses negative feedback to control gain at the
  cost of bandwidth.
Inverting Amplifier



      VCC




     -VEE
                 Non Inverting Amplifier



                      VCC




                     -VEE




Virtual ground

				
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