Semiconductor manufacturers produce a wide variety of op-amp ICs. Some are optimized for high speed, others for high precision; some use bipolar junction transistors, others use field-effect transistors; most are of the voltage-feedback type, but some are designed for current feedback. Thus, there are countless variations in the schematics of modern op-amps, and furthermore, we don’t have access to those schematics. That’s why it makes sense to study the 741 op-amp.
The 741 was first introduced in 1968. It is a BJT-based device that has acquired an almost legendary status as one of the first high-performance, user-friendly op-amps. Though no longer a state-of-the-art device, the 741 still provides a good introduction to the basic principles of op-amp design.
This is the schematic provided in the datasheet for the LM741 from Texas Instruments.
The first thing that we need to do when analyzing a circuit like that of the 741 is divide the schematic into stages—that is, into subcircuits that have a coherent function and that work together with other subcircuits to create the overall functionality of the device.
The 741 exemplifies an intuitive and effective architecture that would be a good starting point in the design of many amplifier systems. It consists of an input stage, an intermediate stage, and an output stage.
The input stage accepts two input signals and converts them into a single-ended signal that is delivered to the intermediate stage.
Transistors Q3 and Q4 form a differential pair, and this is why we say that the op-amp has a differential input stage. The input signals are amplified differentially, not as independent signals.
As you can see, the op-amp’s input terminals are connected directly to the base of a bipolar junction transistor. This results in very low input current. The differential pair has an active load and produces a single-ended output signal (at the collector of Q6), and this becomes the input to the next stage.
The intermediate stage includes three transistors (Q15, Q17, Q13), and its purpose is to greatly increase the amplitude of the signal. In other words, this is a high-gain stage.
The transistor (Q15) that receives the output signal from the input stage is configured as an emitter follower (which provides high input impedance), and the output of this emitter follower is delivered to a transistor (Q17) that is configured as a common-emitter amplifier (which provides high gain); the output signal comes from the collector of this transistor.
The load for this common-emitter amplifier is a transistor functioning as a current source (Q13); thus, both the input stage and the intermediate stage benefit from the use of an active load. (If you’d like to understand why active loads are superior to resistive loads, please refer to this article and its sequel.)
You may have noticed that the intermediate stage includes a capacitor (C1). This is actually an extremely important component. It’s called a compensation capacitor, and with the help of the Miller effect it greatly alters the op-amp’s frequency response; for more information, refer to AAC’s article on op-amp frequency compensation.
At this point, we have differentially amplified the input signals, converted them to a single-ended voltage, applied high gain, and beneficially modified the amplifier’s frequency response. Now we need to buffer the signal before it is delivered to the output terminal.
The word “buffer” implies that a circuit provides low output impedance and good current-driving capability, and the 741 achieves these characteristics by means of a class AB output stage. The class AB output configuration combines the high efficiency of the class B configuration with the low distortion of the class A configuration.
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by Jake Hertz
by Jake Hertz