Opamp bridge amplifier

Definition

Op amp bridge amplifiers are electronic circuits used in measuring the unknown resistance of a transducer in one branch . Normally all the four branches will have equal resistances so that by Wheatstone bridge principle output voltage will be zero. However if one of the resistance in any one of the branch gets altered due to change in some physical parameter, the output voltage developed will be proportional to (∆R/R)/(1+(∆R/R)) if (∆R/R) << 1 then Vo = A*(∆R/R) from which ∆R can be known.

Circuit operation of Opamp bridge amplifier

The op amp bridge amplifier is shown below 

Bridge amplifier

Bridge amplifier

In the figure shown above the resistance shown as R+ΔR can be any sensor such as platinum resistor,strain gauge,thermistor, sensistor e.t.c . The resistors labeled as R are reference resistors with which the varying resistance can be measured. Since the opamp is in open loop configuration the output of opamp is given as

                                                    Vo = Ad*(V+-V

Where Ad is open loop differential gain of opamp. The current flowing through the input terminals of an op amp will be zero(except for small bias currents) due to infinite input resistance of opamp. The Opamp bridge amplifier can be redrawn as follows

Bridge amplifier simplified circuit

Bridge amplifier simplified circuit

Using the voltage divide rule

The voltage at inverting terminal of amp V = V*(R+ΔR)/(R+R+ΔR)

                                                            V- = V*(R+ΔR)/(2*R+ΔR)

dividing the numerator and denominator by R, we get

                                                           V- = V*((R+ΔR)/R)/((2*R+ΔR)/R)

                                                           V- = V*(1+(ΔR/R))/(2+(ΔR/R))

let ΔR/R = δ, the voltage at inverting terminal of op amp V- = V*(1+δ)/(2+δ)

The voltage at inverting terminal of amp V- = V*R/(R+R)

                                                        V- = V/2

Hence the output of op amp is  Vo = Ad*(V+-V) = Ad*(V/2-V*(1+δ)/(2+δ))

                                                      Vo = Ad*V*(1/2-(1+δ)/(2+δ))

                                                 Vo = Ad*V*(2+δ-2*(1+δ))/(2*(2+δ))

                                                Vo = Ad*V*(2+δ-2-2*δ))/(2*(2+δ))

                                                Vo = Ad*V*(-δ)/(2*(2+δ))   for (∆R/R) = δ << 1

The output voltage of op amp reduces to Vo = Ad*V*(-δ)/4. When all the resistors are matched i.e. δ=0, output voltage goes to zero. 

Applications

Following are some of the applications of bridge amplifiers

  • The very high value, closely matched input resistances characteristic of bridge amplifiers make them ideal for measuring low level voltages and currents—without loading down the signal source.
  • Bridge amplifiers are used in signal conditioning circuits along with instrumentation amplifiers to improve Common Mode Rejection Ratio.
  • Bridge amplifiers are used in Single supply common mode suppression circuits.
  • Common bridge amplifier applications include strain and weight measurement using  load cells and temperature measurement using resistive temperature detectors.

 

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