Nodal analysis

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Lecture 12 Circuit models for Diodes, Power
supplies
Reading Malvino chapter 3, 4.1-4.4 Next 4.10,
5.1, 5.8 Then transistors (chapter 6 and 14)
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Circuit models

• Now that we have studied the physics underlying
  how a diode works, we are going to hide all of it
  in a circuit model
• Why?
• If we create a circuit model, then we can draw
  and analyze electronic circuits without getting
  lost in the details.

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IV curve for an ideal diode

• The IV curve for a ideal diode is to have zero
  current in the reverse direction, and no
  resistance when forward biased

Current ?
Voltage ?
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Real diode IV curve
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Idealized devices

• We have encountered the idea of ideal devices
  before
• A voltage source is like a battery, but produces
  a perfect voltage regardless of current
• And the ideal
  current
• source, a current
• regardless of
  voltage

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The ideal diode

• We now add another ideal device, the ideal diode.
• A real diode
• drawn as the same symbol
• sometimes in a circle to
• make it clear that it is not
• a ideal diode

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The ideal diode as a switch

• The ideal diode behaves as a switch
• If current is being pushed through in the forward
  direction the switch is closed.
• If a reverse bias voltage is applied, the circuit
  is closed.

Reverse Bias
Forward Bias
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Ideal diode vs real diode IV curve
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Ideal diode vs real diode IV curve
We could improve our model for real diode by not
closing the switch until the voltage gets about
0.7 volts into the forward bias. We can do this
in a circuit by making a circuit model
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The ideal diode

• To make a somewhat better model of a real diode
• We use an ideal diode in series
• with an ideal voltage source

0.7 volts -

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Ideal diode vs real diode IV curve
We could improve our model further by sloping the
IV curve for the region where forward current is
flowing
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Improved diode model

• To make an even better model of a real diode
• We use an ideal diode in series
• with an ideal voltage source and
• a resistor. The resistance needed for
• the model is given by the inverse
• of the slope of the IV curve

R
0.7 volts -

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Key point the model can change

• Which model you use for a device can change
  depending on
• What the mode of operation of the device is
• how accurately you need to model the device
• For example A hand analysis of a power supply
  would probably use an ideal diode, and then break
  the problem into two time periods
• When the diode is forward biased
• When the diode is reverse biased

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Higher accuracy models

• If a diode was to be used at high frequencies
  (hundreds of megahertz or higher) then the model
  would have to account for the movement of charge
  in and out of the depletion zone, a capacitive
  effect.
• It is important to use a model which is accurate
  enough to account for the necessary effects,
  without using so complicated a model that it is
  difficult to understand what is going on!

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Applications

• Applications of diodes include
• Power supply rectifiers
• Demodulators
• Clippers
• Limiters
• Peak detectors
• Voltage references
• Voltage multipliers

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Half-wave rectifier

• A single diode can be used to take an alternating
  current, and allow only the positive voltage
  swing to be applied to the load

R
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An AC input is sinusoidal
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The diode blocks the negative voltages
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Full-wave rectifier

• If we add an additional diode, it does not pass
  current at the same time as the first diode, but
  the load is now disconnected during the negative
  half cycle.
• What if we could flip the connection and use the
  negative half wave?

R
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Full-wave rectifier

• The result is called a full wave rectifier

R
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Full-wave rectified voltage
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Transformers

• In order to use a full wave rectifier, the source
  and the load must be able to float with respect
  to each other
• One way to isolate AC power is to use a
  transformer. A transformer is a couple of coils
  of wire which transfer power by a changing
  magnetic field.
• By having different numbers of windings, or turns
  of wire, a transformer can step up or step down
  an AC voltage.

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Transformers
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• The voltage across the secondary of the
  transformer (the output windings) is
• But this only works for changes in the
  voltageand therefore for AC only

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Filtering

• A transformer and a full wave rectifier will
  produce a voltage which is always positive, but
  varies with time
• In order to power electronic devices, we need to
  smooth out the variations with time.
• Another way to look at this is that we need to
  store energy temporarily while the input voltage
  changes sign.

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Power supply filter capacitor

• If we add a capacitor in parallel with the load,
  it will charge up when power is available from
  the voltage source, and then it will slowly
  discharge through the load when the diodes are
  off.

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Full wave rectified, with filtering
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Ripple
The result is a DC voltage, with some residual
variations at twice the frequency of the AC
power. The variation is called ripple.