Introduction to Switches Using Bipolar Transistors

1

• Introduction to Switches Using Bipolar Transistors

Prof. Luddy Harrison CS231

• This is optional material, intended to whet your
  appetite for digital electronics
• The circuits depicted here are incomplete. They
  are intended merely to illustrate some of the
  basic principles of switching using transistors

2
Review of Ohms Law

• Voltage V
• Current I
• Resistance R
• V IR
• I V/R

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Series Resistance
Overall resistance is greater than either R or S.
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Parallel Resistance
Overall resistance is less than both R and S.
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Voltage Divider
The voltage V drops over the two resistors (from
V to ground0).The fraction of the voltage V
that drops at R is R/(RS) and the fraction of
the voltage that drops at S is S/(RS).
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A Bipolar (NPN) Transistor
collector
base
emitter
Iemitter Ibase Icollector
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Inverter Using a Bipolar Transistor
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Inverter (Input Low)
With the base-emitter junction at 0V, the
collector-emitter junction looks, roughly
speaking, like an open circuit.
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Inverter (Input High)
With the base-emitter junction at gt 0.7 volts,
the collector-emitter junction behaves like a low
resistance.
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NOR Gate Using Bipolar Transistors
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Bipolar NOR Gate (all inputs low)
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Bipolar NOR Gate (at least one input high)