Integrated operational amplifier circuit - Solutions - Huaqiang Electronic Network

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The integrated operational amplifier circuit is a directly coupled multi-stage amplifier circuit, which is a product that combines circuits, circuit systems and components by using an integrated process of semiconductors. Due to the integrated process, the consistency of parameters of adjacent components can be made well, and the complex circuit of multi-transistors is adopted, so that the performance is excellent. The types of integrated op amps vary, but the most common ones are general-purpose integrated op amps, which typically have differential input stages, intermediate stages, and complementary output stages with a variety of current source circuits. This chapter begins with a multi-stage amplifier circuit, analyzes the problems of direct coupling, introduces the characteristics of the differential amplifier circuit, the circuit analysis method and its performance indicators, and briefly introduces the current sources commonly used in integrated operational amplifiers. For integrated op amps, we mainly talk about their external characteristics and usage methods, and provide the necessary knowledge for using op amps in subsequent chapters.
4.1 Coupling mode and dynamic analysis of multi-stage amplifying circuit Chapters 2 and 3 make a detailed analysis of each single-tube amplifying circuit. Any one of the circuit systems is composed of several single-tube amplifying circuits connected in series. The stage amplifying circuit is composed, so as to meet the requirements of the circuit system, such as amplification capability, input resistance and output resistance. Each of the single-tube amplifying circuits constituting the multi-stage amplifying circuit is referred to as a first stage, and the connection mode between the stages is referred to as an inter-stage coupling mode. Common coupling methods for multi-stage amplifier circuits are: RC coupling, direct coupling, transformer coupling, and optocoupler.
The method of connecting the front stage output of the amplifying circuit and the rear stage input through the coupling capacitor is called a RC coupling mode. Figure 4.1.1 shows a two-stage RC coupling amplifying circuit. As shown in the figure, the input and signal source of the first stage and the output and load of the second stage are also connected by a coupling capacitor.
(1) The advantages of RC coupling can be seen from the figure. Since the reactance of the capacitor to the DC is infinite, the quiescent operating points between the RC capacitors are independent of each other. Once the DC path of the circuit is drawn, They are independent, which brings two benefits:
1) Design static working points at all levels without having to consider the influence of the front and rear stages, and it is very convenient to analyze, calculate and adjust the static working points.
2) Due to the blocking effect of the capacitor, the temperature drift of the front working point is not transmitted to the next stage. Although there are some changes in the Q point of each stage when the ambient temperature changes, as the entire circuit system, the static operating point does not change much.
(2) Problems with resistance-capacitance coupling 1) Since the capacity of the capacitor cannot be made infinite, the lower-cutoff frequency of the RC coupling amplifier can only amplify the AC signal with a higher frequency and the DC with a slower change. The signal, the RC coupling amplifier circuit is powerless.
2) Due to the inability to make large-capacity capacitors in the integrated process, the RC coupling method cannot be integrated.
4.1.2 Direct coupling The output of the previous stage is directly connected to the input of the latter stage. This coupling method is called direct coupling, as shown in Figure 4.1.2.
In the figure, the two-stage common-amplitude amplifying circuits are connected by a short-circuit line, and the input and signal source of the first stage are connected with the load of the second stage by a short-circuit line.
(1) Advantages of direct coupling 1) Since there is no coupling capacitor and bypass circuit in the whole circuit, even if the frequency of the signal ui is zero, the signal can be amplified and transmitted step by step.
2) Similarly, there is no large capacitance in the whole circuit, which is easy to integrate.
(2) Problems with direct coupling circuit 1) It can be seen from Figure 4.1.2 that since there is no DC blocking capacitance between the stages and the stages, the points of the various points are mutually restrained and affect each other. We set, calculate and adjust the Q point. Will bring difficulties.
2) The direct coupling circuit is not a simple short-circuit connection between the front stage and the rear stage. It has a DC level configuration problem. As can be seen from Figure 4.1.2, if the second-stage emitter is not connected to the resistor R5, it will cause uCE1=UBE2 when static. The T1 tube is clamped by the T2 base due to the collector potential, and the T1 tube operates in the saturation region. Similarly, since the value of the month 3 is unlikely to be too large, the IB2 is large, and the T2 tube also works in the saturation region. In a multi-stage amplifying circuit, as long as one level of the port is not suitable, the entire circuit will not work properly. The connection of R5 increases the DC potential of the base of the T2 tube. As long as the value is appropriate, the T1 and T2 tubes can all work in the amplification area. It is worth noting that the addition of R5 will reduce the amplification of the second stage due to the inability to connect to the bypass capacitor.
3) Since the output of the first stage is directly connected to the input of the second stage, if the change of the circuit parameters (such as the change of the ambient temperature) causes a small change in the collector potential of the first stage, ΔuC1, it will be used as a "signal". The second stage is amplified, causing zero drift.