Analog Circuits

1
Analog Circuits

• Computer Hardware Concepts

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Types of Analog Circuits

• Ones using diodes
• We will only cover power diodes
• We will cover other types when we cover sensors
• Ones using transistors
• We will cover the basic amplifier types
• Ones using ICs
• We will only cover OP amps and their applications
• We will cover some other analog devices later

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Overview of Diodes

• For most applications, wewant the diode to act
  likethe ideal diode in the figure
• all current will pass in theforward direction
  (or whenit is has a forward bias)
• no current will pass whenit has a reversed bias
• In reality, there is a voltage component to the
  way a works diodes
• up to some voltage point, less current will flow
  when it has a forward bias
• some current will get through in the reverse bias
  direction
• The base material determines the actual diode
  characteristics
• Germaniums forward bias is more ideal
• Silicons reverse bias is more ideal

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Zener Diodes

• Since you cannot getideal characteristicsfrom a
  diode
• It is possible to purposely change the
  characteristics
• One example of usingthis is the zener diode
• At a set voltage
• the diodes ability to block current in the
  reverse bias direction breaks down
• If you put this diode in a circuit in its reverse
  bias direct
• It can act as a voltage regulator over a give
  current range (A to B in the figure)

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Tunnel Diodes

• Another type of diode, called a tunnel diodes
• can be designed so that over a range of forward
  bias voltages, the pass currentdecreases with
  voltage
• in this region, the diode obeysa reverse ohms
  law, and thus, appears to have negative
  resistance
• This can be use to create a solid state
  oscillators
• which is a very useful thing, but it is beyond
  the scope of this course to go into the details

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Half-wave Rectifier

• One of the most common uses of a diode is in AC
  toDC power converter
• The simplest way to do this is the half-wave
  rectifier
• It uses only one diode to clip off the bottom of
  the AC wave
• By adding a capacitor
• You can get a fairly good DC signal
• But it does waste half of the power of the AC
  input

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Full-wave Rectifier w/Center Tap

• By using a transformer with a center tap and two
  diodes
• You can create a full-wave rectifier
• The center tap creates two half-wave phases
• You are still losing half of the power on each
  phase, but the results are addedto look like
  you are getting the full wave
• Again, a capacitor is used to smooth out the
  results

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Full-wave Bridge Rectifier

• By using a transformer (without a center tap)
  and four diodes
• You can also create a full-wave rectifier
• Here, you do notlose any of the AC input power
• Again, a capacitor is used to smooth out the
  results

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Transistors

• NPN and PNP bipolar transistors work
  inbasically the same way
• A small current passing between the B-E
  connection controls a bigger current passing
  between the C-E connection

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Transistor Bias

• Transistors are basically DC devices
• To use them for AC
• You need to DC bias theB-E and C-E junctions
• The bias circuits can be joined to create a
• Common emitter
• Common collector
• Common base
• The figure shows a common emitter bias using two
  batteries

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Practical Transistor Bias

• Using two batteries is a simple way
  todemonstrate how bias works
• but normally biasingis done with resistors
• One problem with using either resistors or
  batteries is that
• the resistance of the circuit changes with
  temperature
• expensive audio amplifiers will include
  components designed to regulate these temperature
  changes

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Common Emitter NPN Amplifiers

• The simplest method of amplification is
  thecommon emitter
• A low input voltage isamplified to a
  higheroutput voltage
• This results from boththe B-E and C-E currents
  passing through a separate resistance
• NPN amplifiers use a positive bias
• note that you get a 180 degrees phase shift
  between the B-E and C-E voltages
• this can cause problems if multiple stages are
  used

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Common Emitter PNP Amplifiers

• PNP amplifiers use a negative bias
• Just flip both batteriesto get this
• You still get
• a voltage amplifier
• there are still twoseparate resistors
• a 180 degrees phase shift between the B-E and C-E
  voltages

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Common Collector NPN Amplifiers

• In a common collector amplifier, both
  currentpaths share the same resistance load
• This results in a current(not voltage) amplifier
• NPN amplifiers againuse a positive bias
• note that you do not get a 180 degrees phase
  shift between the B-E and C-E voltages
• This can be very useful in some circuits

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Common Collector PNP Amplifiers

• PNP amplifiers again use a negative bias
• Again, just flip both batteries to get this
• You still get
• a current amplifier
• no phase shift between the B-E and C-E currents

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Common Base Amplifiers

• Both NPN and PNP have some odd characteristics
• bias is different of the B-E and C-E voltages
• the voltage amplification can be very high
• But since all of the current inthe circuit must
  travel through the B-E junction and these
  junctions arenot normally rated for a lot of
  current
• the possible applications of a common base setup
  are extremely limited

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The Op Amp

• A number of useful amplifier types can be built
  from the building block we have just studied
• but the details of how theseare built are
  outside the scope of this class
• One important type of amplifier is the Op Amp
• It generates a single output based on the sum of
  an inverted and non-inverted input
• The things you can do with an Op Amp are almost
  limitless

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An Op Amp Adder

• One simple device thatcan be build from an
  OpAmp is an analog adder
• The resistors on each inputline turn the
  voltages into a currents
• The resistor Rf provides a reference voltage
• As we will see
• an adder like this can be added to other Op Amp
  based components to do some interesting things

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An Op Amp Integrator

• Op Amps can also beused to build circuits that
  differentiate andintegrate
• Without going into the gory math
• The resistor and capacitor cause this circuit to
  performan integration of the input
• Swap their location and the circuit would
  differentiate theinput

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An Analog Computer

• Putting all of the Op Amp components together
• We can build an analog computer