Transducer Bridge Amplifiers Flashcards

Study Unit 7: Linear Opamp Circuits - Transducer Bridge Amplifiers

Lecture Outcomes

• Topics for today’s discussion:
  • Transducer bridge amplifiers
  • Basic transducer bridge and instrumentation amplifier
  • Bridge calibration
  • Single opamp amplifier
  • Bridge linearization

Transducer Bridge Amplifiers

Basic Transducer Bridge and Instrumentation Amplifier (1)

• Circuit Diagram:
  • A Wheatstone bridge configuration is used with a reference voltage VREF and current IREF. The bridge consists of resistors R1, R, and a variable resistor R(1 + δ). Voltages v1 and v2 are measured at the nodes of the bridge.
• Branch Voltages:
  • v1 = \frac{R(1+δ) }{R1+R(1+δ)}VREF = \frac{R}{R1+R} VREF + \frac{δVREF}{2+R1/R+R/R1+(1+R/R1)δ}
  • v2 = \frac{R}{R1+R} VREF
• Observations:
  • When δ = 0, the reference current IREF splits equally into the two branches.
  • When δ \neq 0, the total resistance for each branch differs, leading to:
    • Different branch currents (that are not equal to IREF/2).
    • Different voltage values for v1 and v2 (v1 \neq v2).
  • δ is related to the difference between v1 and v2, however, the relationship is non-linear.
  • Normally, v1 - v2 is very small but can be easily amplified.

Basic Transducer Bridge and Instrumentation Amplifier (2)

• Instrumentation Amplifier Configuration:
  • An instrumentation amplifier (IA) is connected to the bridge to amplify the differential voltage (v1 - v2). The amplifier has a gain of A and uses a gain resistor RG.
• Output Voltage:
  • vout = A (v1 − v2)
• Design Equation:
  • vout = \frac{AVREF δ}{2+R1/R+R/R1+(1+R/R1)δ}
  • When δ << 1: vout \approx \frac{AVREF δ}{2+R1/R+R/R1}
• Simplified Approach (Selecting R = R1):
  • vout = \frac{AVREF}{4} \frac{δ}{1+δ/2}
  • When δ << 1: vout \approx \frac{AVREF}{4} δ

Transducer Bridge Calibration

• Circuit Diagram:
  • A bridge circuit with resistors R1, R2, R3, and R(1 + δ), connected to an instrumentation amplifier. This setup allows for calibration of the transducer bridge.

Single Opamp Bridge Amplifier

• Circuit Diagram:
  • A single opamp configuration is used to amplify the output of the bridge. The circuit includes resistors R1 and R2, the variable resistor R(1 + δ), and a reference voltage VREF.
• Design Equation:
  • vout = \frac{R2}{R} \frac{VREF δ}{R1/R + (1 + R1/R2) (1 + δ)}
  • When δ << 1: vout = \frac{R2}{R} \frac{VREF δ}{1 + R1/R + R1/R2}

Bridge Linearization (1) - Instrumentation Amplifier

• Circuit Diagram:
  • An instrumentation amplifier is used with a modified bridge circuit. The bridge includes resistors R, R(1 + δ), and an opamp (OP) in the feedback loop for linearization.
• Design Equation:
  • vout = \frac{ARVREF}{2R1} δ

Bridge Linearization (2) - Single Opamp

• Circuit Diagram:
  • A single opamp configuration is used for bridge linearization.
• Design Equation:
  • vout = \frac{R2VREF}{R1} δ