Lecture 14 Transistors and Operational Amplifiers

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Lecture 14Transistors and Operational Amplifiers

• ECE 205
• Prof. Ali Keyhani

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Transistor

• A semiconductor device that can be modeled with
  dependent sources
• Transistor types
• Bipolar Junction Transistor (BJT)
• Field Effect Transistor (FET)
• A transistor has different operating modes with
  different i-v characteristics

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Bipolar Junction Transistor (BJT)

• BJTs are made up of alternating layers of n and p
  semiconductor materials joined metallurgically
• Two types of BJTs
• pnp-type Principal conduction by positive holes
• npn-typePrincipal conduction by negative
  electrons

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Bipolar Junction Transistor (BJT)

• BJT is a three terminal device
• Emitter (E)
• Collector (C)
• Base (B)

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Bipolar Junction Transistor (BJT)

• Applying KCL to BJT
• Finding two currents can yield the third one
• Three operating modes of the BJT transistors
• Active mode
• Cutoff mode
• Saturation mode

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Active Mode

• In active mode collector current is controlled by
  base current and base-emitter voltage is
  constant

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Active Mode

• Circuit model in active mode

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Cutoff mode

• The device acts like an open circuit between
  collector and emitter
• Circuit model

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Saturation Mode

• The device acts like a short circuit between the
  collector and emitter
• Circuit model

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Operating Bounds

• Active Mode
• Cutoff Mode
• Saturation Mode

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BJT Common Emitter Circuit

• Assuming the circuit operates in active mode

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Output Response of BJT Circuit
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Operational Amplifier (OP AMP)

• A five terminal device two inputs, one output
  and positive and negative supply voltages
• OP AMPS are high gain dc amplifiers that perform
  mathematical operation
• The positive and negative power supplies
  determine the upper and lower limits on the
  output voltage.

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OP AMP

• OP AMP packaging

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• OP AMP Voltage and current definitions

• The input currents are negligible and ideally
  zero

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Transfer Characteristics

• Provides the relationship between the
  noninverting input vP and inverting input vN and
  the output voltage vO

• Has three Operating modes
• Linear
• Saturation
• - Saturation

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Transfer Characteristics

• Linear Mode
• A is the voltage gain which is usually very
  large
• Saturation Mode
• - Saturation Mode

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Ideal OP AMP Model

• Based on the equation of the OP AMP operating in
  the linear region the dependent source model of
  the OP AMP is derived
• High Input resistance
• Low output resistance
• High voltage gain

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Ideal OP AMP Model

• For an Op AMP operating in linear mode
• In an ideal OP AMP A?8 therefore
• In an ideal OP AMP RI is large hence

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Ideal OP AMP Model
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Noninverting OP AMP

• Since there is no input current voltage
  division can be used to find the voltage at
  inverting input