- Op Amp Definition: An operational amplifier (op amp) is a versatile component in electronic circuits, amplifying direct current from high input impedance to low output impedance.
- Op Amp Characteristics: Key characteristics of op amps include their ability to amplify signals with high gain and efficiency, pivotal for understanding their real-world deviations from ideal models.
- Pin Configuration: The IC 741 op amp is an example with an 8-pin setup including inverting and non-inverting inputs, output, power supply pins, and offset nulls.
- Applications of Op Amps: Op amps are fundamental in creating circuits like comparators, adders, and integrators, showcasing their flexibility in electronic design.
- AC Equivalent Circuit: The AC equivalent circuit of an op amp is essential for analyzing how internal resistance and voltage differences affect the output, offering a practical viewpoint for circuit design.
An Operational amplifier or Op Amp circuit is the IC pin-out, the internal transistor network and the AC model, plus the external wiring around those pins. An op amp has three signal terminals: inverting, non-inverting and output.
Below, you can see a pin diagram of a typical Op amp used in those circuit drawings.
An ideal Op amp has the following characteristics:
- An infinite open-loop voltage gain.
- Infinite input impedance.
- Zero output impedance.
- Infinite common-mode rejection ratio.
- Infinite bandwidth.
- Zero offset voltage.
Real op amps deviate from that ideal list. More detail is in characteristics about op amps.
This page covers the internal circuit of a typical op-amp IC and the AC equivalent circuit used for analysis.
Pin Diagram of an Op Amp IC
The example IC here is the 8-pin IC 741. Other op-amp ICs use different pin-outs. The 741 pin names are listed below.
PIN 1 – Offset Null
PIN 2 – Inverting input
PIN 3 – non-inverting input
PIN 4 – negative voltage supply
PIN 5 – offset null
PIN 6 – output
PIN 7 – positive voltage supply
PIN 8 – not connected
The internal circuit diagram of an op-amp IC is given below:
As you can see above, the non-inverting terminal and offset null are located at the left. The different terminals are marked in different colors. A 741-class op-amp is a collection of transistors and resistances as you can see from the picture.
The same 741 IC is used as
- Adder
- Subtractor
- Comparator
- Voltage follower
- Integrator
- Differentiator
Here is a circuit diagram of Op-amp IC being used as a comparator
A comparator compares two analog signals: one is a reference voltage and the other is the signal to be compared. The op-amp output saturates toward the supply of the larger input, subject to slew rate and output swing limits.
AC equivalent Circuit of Op amp
The equivalent circuit of an op-amp is the circuit where the op-amp parameters are represented in terms of physical components. That model is used for AC analysis.
Below, you can find the equivalent circuit of the op amp.
Circuit parameters like the internal resistance, output resistance, voltage gain are all represented by circuit components like Rin, Rout, etc.
V1 – non-inverting input voltage with respect to ground;
V2 – inverting input voltage with respect to ground;
Vee – negative supply voltage;
Vcc – positive supply voltage;
Vout – AOLVd = AOL(v1-v2);
Vd – the difference between inverting voltage and non-inverting voltage;
Ri – internal resistance of Op-amp;
R0 – output resistance of op-amp;
AOL – open-loop voltage gain.
The output voltage is proportional to the difference between the input voltages, until the output hits the supply rails.
The voltage AOLVd is the Thevenin’s equivalent voltage source whereas Ro is the Thevenin equivalent resistance as seen from the output terminal. That drawing is the AC equivalent circuit of an Op amp used for small-signal analysis.
