ENSC 325 Project

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ENSC 325 Project

• LT1259 - Low Cost Dual and Triple 130MHz Current
  Feedback Amplifiers with Shutdown

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CFB vs VFB

• High input slew rate (270V/us)
• Gain-dependent output slew rate (Typ 1600V/us)
• Non-constant GBP
• High BW parameter set by FB resistors and
  internal capacitors
• Requires resistive feedback from Output to
  Inverting Input for stable operation
• Higher VOS than VFB
• Higher Noise Density than VFB

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Typical Application
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Simplified Schematic
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Sub-Circuits

• Enable Circuitry
• High impedance output when circuit disabled
• Establishes bias currents for Input Stage
• Power-efficient design
• Input Stages
• Sense voltage at IN and feedback current at IN-
• Output Stages
• Bulk of the current gain
• Differential ? Single-ended converter
• Output current used in negative feedback

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Sub-Circuit 1 Enable Circuitry

• Matched current mirrors provide bias currents
  which turn on the chip
• VAB determines IREF for mirrors and is controlled
  via the EN pin
• EN Low ? VAB 0.7V IREFR EN
  High ? VAB 0V IREFR
• When the drain-source voltage of the JFET becomes
  zero the current in the mirrors will become zero
  as well. The amplifier is thus disabled.
• EN pin draws typically 60uA when active

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Sub-Circuit 2 Input Stage

• The error current will be proportional to Vin -
  Vin-
• Rin rp(ß1)(rorp) / 2
• Rin- rp(rogm-1) / 2(ß1)

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I-V Conversion

• The error signal (current) propagates into the
  base of the second BJT and is converted to
  voltage.
• Vout / Iin -ßr0

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Output Stage

• Class AB unity gain push-pull amplifier.
• Resistances attached at output node are very
  small and used to compensate for transistor
  mismatches.
• Rout 1/(2gm)