Voltage Follower and Difference Amplifier Configuration-Operational Amplifiers Types Tutorials Series

1. The op-amp configuration shown above is a voltage-follower often used as a buffer amplifier 

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Related Tutorials

Operational Amplifiers Tutorials Series
Op-amp Integrator Network
Op-amp with T-bridge Feedback Network
Instrumentation and Summation Configuration
Linear Op-amp Operation-Inverting And Non-Inverting Configuration

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–Output is connected directly to negative input (negative feedback)
–Since v+ = v- = vIN, and vOUT = v-, we can see by inspection that the closed-loop gain Ao = 1
–We can obtain the same result by writing

vOUT = A (vIN – vOUT) or

vOUT/vIN = A/(1 + A) = 1 for A >> 1 

2. A typical voltage-follower transfer curve is shown below for the case VPOS = +15V and VNEG = -10V 

–For vIN between –10 and +15 volts, vOUT = vIN
–If vIN exceeds +15V, the output saturates at VPOS
–If vIN < -10V, the output saturates at VNEG

3. Since the input current is zero giving zero input power, the voltage follower can provide a large power gain

Op-amp Difference Amplifier

1. The “difference amplifier” shown above combines both the inverting and non-inverting op-amps into one circuit 

–Using superposition of the results from the two previous cases, we can write
–vOUT = [(R1 + R2)/R1]v1 – (R2/R1)v2
–The gain factors for both inputs are different.

2. We can obtain the same gain factors for both v1 and v2 by using the modified circuit below 

–Here the attenuation network at v1 delivers a reduced input v+ = v1(R2/(R1 + R2))
–Replacing v1 in the expression above by the attenuation factor, gives us

Vout = (R2/R1)(v1 – v2) 

3.The difference amplifier will work properly if the attenuation network resistors (call them R3 & R4) are related to the feedback resistors R1 & R2 by the relation R3/R4 = R1/R2 (i.e. same ratio)