How to read an operational amplifier circuit diagram

The general composition and unit structure of integrated operational amplifier circuits , such as constant current source circuits , differential amplifier circuits, CC-CE, CC-CB circuits and complementary output circuits.

The operational amplifier is mainly composed of four parts: input stage, intermediate amplifier stage, output stage and bias circuit, as shown in Figure 1.

The bias circuit of an operational amplifier is very different from the bias circuit design of a discrete amplifier circuit. It is mainly implemented by various forms of constant current source circuits. Familiarity with various forms of constant current source circuits is the basis for reading operational amplifier circuits.

The input stage of the operational amplifier is usually a differential amplifier circuit, whose main function is to suppress common-mode interference and temperature drift. The differential tube in the bipolar operational amplifier usually adopts CC-CB composite tube to expand the passband;

The intermediate stage of the operational amplifier adopts a common emitter or common source circuit, and uses a constant current source load and a composite tube to increase the voltage amplification factor.

The output stage of the bipolar operational amplifier adopts a complementary output form, and its main function is to improve the load capacity and increase the dynamic range of the output voltage and current. The two output tubes are turned on in turn, and each tube works in the Class B state. In order to eliminate crossover distortion, an appropriate bias current is usually provided to the output tube to make it work in the Class A and B state.

Due to the limitation of integrated circuit technology, direct coupling is used between each stage. In order to ensure that the output DC level is zero when the input is short-circuited, it is sometimes necessary to add a level shifting circuit between stages.

The process of reading the diagram of the operational amplifier is as follows:

(1) Operational amplifier circuit structure decomposition

According to the structural characteristics of the operational amplifier, the operational amplifier is decomposed into four basic parts: input stage, intermediate stage, output stage, and reference current source.

(2) Baseline current analysis

There is a reference current source for biasing in the op amp circuit, which generates the reference current IREF.

(3) Static bias analysis

Based on the reference current, various sizes of DC constant current sources or DC micro constant current sources are generated through mirror DC current or micro-mirror DC current source. These DC constant current sources provide static bias for the amplification transistor.

By replacing the mirror DC source circuit with an equivalent constant current source, an equivalent DC path can be obtained, which is used to analyze the DC bias at each level.

(4) Communication analysis

The main function of the operational amplifier circuit is to perform linear amplification. In addition, there are some additional functional circuits, such as AC mirror current source circuit, output protection circuit, crossover distortion compensation circuit, level shift circuit, etc. These circuits provide auxiliary functions to ensure the amplification function and usually do not affect the calculation of the amplifier circuit indicators. Simplifying the auxiliary circuit can facilitate AC analysis.

After obtaining the simplified AC equivalent circuit, the transistor is replaced by a small signal model, and the dynamic indicators of the amplifier circuit can be calculated.

Figure 2 is the equivalent circuit diagram of the UA741 operational amplifier circuit . Let's analyze its basic working principle.

(1) Structural decomposition of the op amp circuit

(3) Static bias analysis

The mirror DC current source in the circuit is replaced by an equivalent constant current source, and the equivalent DC path is obtained as shown in FIG3 .

(4) Communication analysis

The equivalent AC path after simplifying the auxiliary circuit is shown in Figure 4.

Example 2: Figure 5 is the equivalent circuit diagram of the CMOS operational amplifier C14573. Try to analyze its basic working principle.

The mirror image DC current source is replaced by an equivalent constant current source, and the equivalent DC path is obtained as shown in FIG6 .

(4) Communication analysis