Diode Applications

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CHAPTER 3

• Diode Applications

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Shokley Equation
The forward bias current is closely approximated
by
where VT is 25.8mV at room temp T 300K or 27C
k Boltzmans constant 1.38 x 10-23
joules/kelvin T absolute temperature q
electron charge 1.602 x 10-19coulombs ?
constant dependent on structure, between 1 and 2
(if not given assume n 1) IS reverse
saturation current Notice there is a strong
dependence on temperature
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• We can approximate the diode equation for igtgt IS

In reverse bias (when v ltlt 0 by at least VT),
then
In breakdown, reverse current increases rapidly
can be approximated by a vertical line
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Dynamic resistance
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Load Line Analysis

• The load of a circuit determines the point or the
  region of operation of a diode (or device)
• The method A line is drawn on the characteristic
  of the device. The intersection point gives the
  point of operation (called Q-point)

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Load line
Look at the simple diode circuit below. We can
write two equations
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Diode in DC Series Circuit Forward Bias
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Diode in DC Series Circuit Reverse Bias
E VD IDR ?ID 0A VD E VR IDR 0V
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Example1
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Solution
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VDQ0.7 V and IDQ9.25mA
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Example

• Repeat the analysis of previous example using
  the ideal diode model
• VDQ0 V and IDQ10 mA

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VDQ0.7 V IDQ4.6 mA
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VDQ0.7 V IDQ4.6 mA
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PWLDM (Piecewise Linear Diode Model)
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SERIES DIODE CONFIGURATIONS WITH DC INPUTS
Si
8V
2.2 k?
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Example

• Determine ID, VD2 and Vo for the circuit.

Remember, the combination of short circuit in
series with an open circuit always results in
an open circuit and ID0A.
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Example

• Determine I, V1, V2 and Vo

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VoE-VD1-VD212-0.7-0.311V
IDIR11/5.61.96 mA
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Parallel Configurations
Solve this circuit like any Parallel circuit,
knowing VD 0.7V (or up to 0.7V) in forward bias
and as an open in reverse bias. VD1 VD2 Vo
0 .7V VR 9.3V Diodes in parallel are used to
limit current IR E VD 10V -0 .7V
28mA R 0.33k? ID1 ID2
28mA/2 14mA
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Example

• Determine the resistance R for the network when
• I200mA.

Si
Si
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Example

• Determine the currents I1, I2, and ID2 for the
  network

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Diode as logic gate

• OR Gate

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• AND Gate