WEEK 15: FINAL CORRECTING DEVICES: AMPLIFIERS

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Sections: Basic | Circuits | Op-Amps

Differential Op-Amps.

Definition. Amplifies the voltage differnce between two input lines.

Differential input current is virtually zero, R₂ and R_D are virtually in series. The voltage-divider equation applies:

V_NI = V₂ x [ R_D ¸ (R₂ + R_D) ] (eq. 1)

Where:
V_NI = Non-inverting voltage
V₂ = Voltage-to-reference at terminal no. 2
R₂ = The resistance at terminal no. 2
R_D = The dumb resistance, near terminal no. 2

R₁ and R_F are also virtually in series. The voltage-divider equation applies:

V_R1 = [V₁ - V_OUT] x [ R₁ ¸ (R₁ + R_F) ] (eq. 2)

Where:
V_R1 = Voltage Drop at R₁
V₁ = Voltage-to-reference at terminal no. 1
R₁ = The resistance at terminal no. 1
R_F = The feedback resistance, near terminal no. 2
V_INV = Voltage at inverting terminal.

But, V_INV = V₁ - V_R1

Thus, V_INV = V₁ - [V₁ - V_OUT] x [ R₁ ¸ (R₁ + R_F) ]

Then, V_INV = V₁ x [ R_F ¸ (R₁ + R_F) ] + V_OUT x [ R₁ ¸ (R₁ + R_F) ]

But, V_INV = V_NI and from (eq. 1), V₂ x [ R_D ¸ (R₂ + R_D) ]

Thus,
V₁ x [R_F ¸ (R₁+R_F)] + V_OUT x [R₁ ¸ (R₁+R_F)] = V₂ x [R_D ¸ (R₂+R_D)]

In most circuits, R₁ = R₂ and R_F = R_D.

Thus, V_OUT x R₁ = (V₂-V₁) x R_F.

Or, V_OUT = (V₂-V₁) x (R_F ¸R₁).

Therefore, the Differential Amplifier, amplifies the difference between two input lines, with the voltage gain dependent on (R_F/R₁).

Operational AmplifiersVoltage-to-Current Op-Amps

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