Operational Amplifiers

Operational Amplifiers Section Summary

The following is a summary of the different types of Operational Amplifiers and their configurations discussed in this tutorial section.

• The Operational Amplifier, or Op-amp as it is most commonly called, is an ideal amplifier with infinite Gain and Bandwidth when used in the Open-loop mode with typical d.c. gains of 100,000, or 100dB.
• The basic construction is of a 3-terminal device, 2-inputs and 1-output.
• An Operational Amplifier operates from a dual positive (+V) and an corresponding negative (-V) supply but they can also operate from a single DC supply voltage.
• It has Infinite Input impedance, (Z∞) resulting in "No current flowing into either of its two inputs" and zero input offset voltage "V1 = V2".
• It also has Zero Output impedance, (Z=0).
• Op-amps sense the difference between the voltage signals applied to the two input terminals and then multiply it by some pre-determined Gain, (A).
• This Gain, (A) is often referred to as the amplifiers "Open-loop Gain".
• Op-amps can be connected into two basic circuits, Inverting and Non-inverting.

The Two Basic Operational Amplifier Circuits

• The Open-loop gain called the Gain Bandwidth Product, or (GBP) can be very high and is a measure of how good an amplifier is.
• Very high GBP makes an Operational Amplifier circuit unstable as a microvolt input signal causes the output voltage to go into Saturation.
• By the use of suitable Feedback Resistor, (Rf) the overall gain of the amplifier can be accurately controlled.

Gain Bandwidth Product

• For Negative feedback, where the fed-back voltage is in "Anti-phase" to the input the overall gain of the amplifier is reduced.
• For Positive feedback, where the fed-back voltage is in "Phase" with the input the overall gain of the amplifier is increased.
• By connecting the output directly back to the negative input terminal, 100% feedback is achieved resulting in a Voltage Follower (buffer) circuit with a constant gain of 1 (Unity).
• Changing the fixed feedback resistor (Rf) for a Potentiometer, the circuit will have Adjustable Gain.
• The Differential Amplifier produces an output that is proportional to the difference between the 2 input voltages.

Differential and Summing Operational Amplifier Circuits

• Adding more input resistor to either the inverting or non-inverting inputs Voltage Adders or Summers can be made.
• Voltage follower op-amps can be added to the inputs of Differential amplifiers to produce high impedance Instrumentation amplifiers.
• The Integrator Amplifier produces an output that is the mathematical operation of integration.
• The Differentiator Amplifier produces an output that is the mathematical operation of differentiation.
• Both the Integrator and Differentiator Amplifiers have a Resistor and Capacitor connected across the op-amp and are affected by its RC time constant.
• In their basic form, Differentiator Amplifiers suffer from Instability and Noise but additional components can be added to reduce the overall Closed-loop gain.

Differentiator and Integrator Operational Amplifier Circuits