We can conclude our section and look at Operational Amplifiers with the following summary of the different types of Op-amp circuits and their different configurations discussed throughout this op-amp tutorial section.
Operational Amplifier General Conditions
- • The Operational Amplifier, or Op-amp as it is most commonly called, can be an ideal amplifier with infinite Gain and Bandwidth when used in the Open-loop mode with typical DC gains of well over 100,000 or 100dB.
- • The basic Op-amp construction is of a 3-terminal device, with 2-inputs and 1-output, (excluding power connections).
- • An Operational Amplifier operates from either a dual positive ( +V ) and an corresponding negative ( -V ) supply, or they can operate from a single DC supply voltage.
- • The two main laws associated with the operational amplifier are that it has an infinite input impedance, ( Z = ∞ ) resulting in “No current flowing into either of its two inputs” and zero input offset voltage V1 = V2.
- • An operational amplifier also has zero output impedance, ( Z = 0 ).
- • Op-amps sense the difference between the voltage signals applied to their 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”.
- • Closing the open loop by connecting a resistive or reactive component between the output and one input terminal of the op-amp greatly reduces and controls this open-loop gain.
- • Op-amps can be connected into two basic configurations, Inverting and Non-inverting.
The Two Basic Operational Amplifier Circuits
- For negative feedback, were the fed-back voltage is in “anti-phase” to the input the overall gain of the amplifier is reduced.
- For positive feedback, were 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 ( Rƒ ) for a Potentiometer, the circuit will have Adjustable Gain.
Operational Amplifier Gain
- 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 micro volt input signal causes the output voltage to swing into saturation.
- By the use of a suitable feedback resistor, ( Rƒ ) the overall gain of the amplifier can be accurately controlled.
Differential and Summing Amplifiers
- By adding more input resistors 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 Differential Amplifier produces an output that is proportional to the difference between the two input voltages.
Differentiator and Integrator Operational Amplifier Circuits
- 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.
