Rapid slew rate enhancing system of operational amplifier

Abstract

The invention discloses a fast slew rate enhancement system of an operational amplifier, which comprises a current identification circuit for monitoring the current of a load tube current mirror structure in the operational amplifier; the comparator is used for comparing the current monitored by the current identification circuit or a signal related to the current with a threshold value, judging the size of the monitored current, taking a comparison result of the comparator as a basis for judging that the slew rate is limited, and starting an enable signal of the corresponding slew rate enhancement circuit; and the slew rate enhancing circuit is used for enhancing the slew rate of the operational amplifier by utilizing the slew rate enhancing enable signal identified by the comparator. Whether the operational amplifier is in the slew rate limited state or not is judged according to the chip internal current signal change of the load current mirror tube in the operational amplifier, and interference of off-chip noise signals is effectively prevented; and meanwhile, the influence of the operational amplifier input imbalance on the slew rate triggering system is reduced because the operational amplifier input imbalance is irrelevant to the operational amplifier differential input pair transistor imbalance.

Description

Technical Field

[0001] The invention relates to the technical field of integrated circuits, and in particular to a fast slew rate enhancement system for an operational amplifier. Background Art

[0002] For systems composed of operational amplifiers (op amps), a higher bandwidth is often required to achieve faster small signal response speed; but for large signal input, the system response speed is not only determined by the small signal bandwidth, but also limited by the slew rate. The slew rate refers to the maximum swing rate that the output voltage of the op amp can reach, which is measured in volts per microsecond. In a system with large signal input, a larger slew rate can enable the system to quickly establish a voltage close to the target value, thereby improving the system response speed.

[0003] Usually, the slew rate of the op amp is limited by the size of the internal bias current and the internal compensation capacitor. The compensation capacitor is determined by the loop stability of the system. Therefore, in order to increase the slew rate of the system, it is often necessary to increase the bias current. However, increasing the current increases the static power consumption. Therefore, how to significantly increase the slew rate without significantly increasing the additional static power consumption is a question worth discussing. On the other hand, after the large signal determined by the slew rate is established, it will go through the small signal establishment process determined by the bandwidth. During this period, unreasonable overshoot or slow establishment should be avoided. When the target value is as close as possible, it should be quickly and stably switched to the small signal establishment process. Therefore, the timely intervention and exit of the slew rate enhancement circuit is also very important. The faster identification of the need to start or shut down the slew rate enhancement circuit can greatly improve the signal establishment speed.

[0004] The traditional slew rate enhancement circuit identifies whether it needs to enter the slew rate enhancement mode by identifying the change of the differential signal between the positive and negative terminals of the op amp input. Because there is an offset voltage at the op amp input, the magnitude of the offset voltage will affect the judgment of slew rate enhancement. One end of the judgment signal is connected to the input and the other end is connected to the feedback, which is easily affected by both internal and external factors. The external influence is mainly the high-frequency noise of the input signal, and the internal influence is mainly the transient fluctuation of the signal at the feedback end. Whether it is the input high-frequency noise or the transient fluctuation of the feedback signal, it is a source of interference that interferes with the judgment of slew rate enhancement for the slew rate enhancement circuit, just like the aforementioned offset error. Therefore, the traditional slew rate enhancement circuit will set a corresponding threshold to prevent interference factors from falsely triggering the slew rate enhancement circuit. Therefore, a larger input signal jump is required to trigger the slew rate enhancement circuit. That is, the existence of this threshold reduces the reaction speed of the slew rate enhancement circuit to a certain extent. Whether it is entering the slew rate enhancement mode or exiting the mode, this will obviously cause the final slew rate value to decrease. Summary of the invention

[0005] The object of the present invention is to provide a fast slew rate enhancement system for an operational amplifier to solve the problems in the background technology.

[0006] In order to solve the above technical problems, the present invention provides a fast slew rate enhancement system of an operational amplifier, comprising: A current identification circuit that monitors the current in the load tube current mirror structure of the operational amplifier; A comparator compares the current monitored by the current identification circuit or a signal related to the current with a threshold value to determine the magnitude of the monitored current, and the result of the comparison by the comparator is used as a basis for determining whether the slew rate is limited, and as an enable signal for turning on a corresponding slew rate enhancement circuit; The slew rate enhancement circuit utilizes the slew rate enhancement enable signal recognized by the comparator to enhance the slew rate of the operational amplifier.

[0007] In one embodiment, the current identification circuit includes a first NMOS transistor and a second NMOS transistor, a first threshold current and a second threshold current; The gate of the first NMOS tube and the gate of the second NMOS tube are both connected to the gate of the load tube current mirror structure in the operational amplifier, the source of the first NMOS tube and the source of the second NMOS tube are both grounded, the drain of the first NMOS tube and the drain of the second NMOS tube are respectively connected to one of the input ends of the two comparators; the first threshold current and the second threshold current are respectively connected to the other input ends of the two comparators.

[0008] In one embodiment, the comparators are respectively a first comparator and a second comparator; The positive input terminal of the first comparator is connected to the first threshold current, and the negative input terminal is connected to the drain of the first NMOS tube; The negative input terminal of the second comparator is connected to the second threshold current, and the positive input terminal is connected to the drain of the second NMOS tube.

[0009] In one embodiment, the slew rate enhancement circuit includes a first switch and a second switch, a first current source and a second current source; The first switch is connected to the input end of the first current source, and the output end of the first current source is grounded; the input end of the second current source is connected to the power supply voltage, and the output end is connected to the second switch; The first comparator outputs a negative slew rate enhancement enable signal to control the second switch, and the second comparator outputs a positive slew rate enhancement enable signal to control the first switch.

[0010] In one implementation, the first threshold current is I-Ith, and the second threshold current is I+Ith; wherein I is the quiescent current in the operational amplifier load current mirror, and Ith is the comparator threshold current proportional to I.

[0011] The fast slew rate enhancement system of an operational amplifier provided by the present invention is not only simple to implement, but also different from judging the slew rate state by input voltage, even if the bias current changes, the relative size relationship between the judgment threshold Ith and the internal current variation range ±I can be guaranteed to be proportional, thereby ensuring the correct judgment function. While reducing threshold redundancy, the speed is faster and the adaptability is strong. The slew rate enhancement system can be quickly turned on or off by cooperating with the slew rate enhancement circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 It is a block diagram of a slew rate enhancement related circuit and a schematic diagram of a static operating current provided by the present invention.

[0013] Figure 2 It is a schematic diagram of the forward slew rate limited and limit state working current provided by the present invention.

[0014] Figure 3 It is a schematic diagram of the negative slew rate limited and extreme state working current provided by the present invention. DETAILED DESCRIPTION

[0015] The following is a further detailed description of a fast slew rate enhancement system for an operational amplifier proposed by the present invention in conjunction with the accompanying drawings and specific embodiments. The advantages and features of the present invention will become more apparent from the following description. It should be noted that the accompanying drawings are all in a very simplified form and are not in precise proportions, and are only used to conveniently and clearly assist in explaining the purpose of the embodiments of the present invention.

[0016] The present invention provides a fast slew rate enhancement system for an operational amplifier, such as Figure 1 As shown, there are many amplifier structures. Figure 1 Taking the common NMOS input stage Class AB output folded common source and common gate architecture as an example, the fast slew rate enhancement system of the present invention is also applicable to other amplifier architectures. The main feature is to determine whether to enter the slew rate enhancement mode by detecting the current through the load current mirror tube.

[0017] Please continue reading Figure 1 , the slew rate enhancement system comprises: A current identification circuit extracts current from a load tube current mirror structure of the operational amplifier, and the circuit is used to monitor the current in the operational amplifier; The comparator is used to compare the current monitored by the current identification circuit or the signal related to the current with the threshold value to determine the magnitude of the monitored current. Here, it can be a current comparator that directly compares currents, or it can be a voltage comparator that converts the current into voltage and then compares it. Figure 1Only the current comparator is used as an example. The comparison result of the comparator is used as the basis for judging whether the slew rate is limited, and as the enable signal for starting the corresponding slew rate enhancement circuit.

[0018] The slew rate enhancement circuit utilizes the slew rate enhancement enable signal identified by the comparator to enhance the slew rate of the operational amplifier. Figure 1 The corresponding slew rate enable signal in the circuit controls the corresponding open switches S1 and S2 in the slew rate enhancement circuit to enhance the rate of change of the voltage on the compensation capacitors Cc1 and Cc2.

[0019] The current identification circuit includes a first NMOS tube N1 and a second NMOS tube N2, a threshold current I11 and a threshold current I12; the gate of the first NMOS tube N1 and the gate of the second NMOS tube N2 are both connected to the gate of the load tube current mirror structure in the operational amplifier, the source of the first NMOS tube N1 and the source of the second NMOS tube N2 are both grounded, the drain of the first NMOS tube N1 and the drain of the second NMOS tube N2 are respectively connected to one of the input ends of the two comparators; the threshold current N11 and the threshold current N12 are respectively connected to the other input ends of the two comparators.

[0020] The comparators are respectively a first comparator A1 and a second comparator A2; the positive input terminal of the first comparator A1 is connected to the threshold current I11, and the negative input terminal is connected to the drain of the first NMOS tube N1; the negative input terminal of the second comparator A2 is connected to the threshold current I12, and the positive input terminal is connected to the drain of the second NMOS tube N2.

[0021] The slew rate enhancement circuit includes switches S1 and S2, current sources I21 and I22; the switch S1 is connected to the input end of the current source I21, and the output end of the current source I21 is grounded; the input end of the current source I22 is connected to the power supply voltage, and the output end is connected to the switch S2.

[0022] The first comparator A1 outputs a negative slew rate enhancement enable signal to control the switch S2 , and the second comparator A2 outputs a positive slew rate enhancement enable signal to control the switch S1 .

[0023] The present invention uses the method of identifying the current in the op amp to control the slew rate enhancement function: the first stage first mirrors a current from the load current mirror structure of the op amp to indicate the current size in the load current mirror of the op amp. When the input signal is stable and there is no slew rate limitation, the current in the load current mirror of the op amp is stable at the designed quiescent current, that is, about I, which is between I-Ith and I+Ith, where Ith is the comparator threshold current proportional to I, such as Figure 1 shown.

[0024] When the input changes and the op amp is slew-rate limited, such as Figure 2 or Figure 3 As shown in Figure 1, the current in the op amp load current mirror will change significantly. If the current change is greater than the comparator threshold, that is, Ith, it is determined that the current is in a slew rate limited state, and the slew rate enhancement circuit is enabled. When the identified current is higher than I+Ith, that is, Figure 2 The positive slew rate is limited, or below I-Ith, as shown. Figure 3 In the negative slew rate limited state shown, the slew rate enhancement circuit in the corresponding direction is turned on. Since the current variation range is fixed, in this embodiment, the variation range is between 0 and 2I, it is only necessary to ensure that Ith is a fixed ratio of I to provide a certain ratio of thresholds to prevent the static bias point from falsely triggering the slew rate enhancement function. The characteristic that the threshold is proportional to the working current in the steady state also ensures that when the working current changes, the timing of the slew rate enhancement circuit intervention is relatively stable.

[0025] After the comparator recognizes that the slew rate in a certain direction needs to be enhanced, it transmits the slew rate enable signal to the subsequent slew rate enhancement circuit, which adjusts the rising or falling slew rate according to the slew rate enable signal. Figure 2 or Figure 3 As shown, the circuit node that limits the slew rate in the circuit can be adjusted by injecting current or the like.

[0026] The present invention does not directly monitor the input voltage of the operational amplifier, but determines whether the operational amplifier is in a slew rate limited state according to the change of the chip internal current signal of the load current mirror tube in the operational amplifier, which can effectively prevent the interference of the off-chip noise signal. At the same time, because it is unrelated to the differential input pair offset of the operational amplifier, the input offset of the operational amplifier has a smaller impact on the slew rate trigger system. Due to the use of the current judgment slew rate enhancement circuit, even if the input voltage does not change much, the large input transconductance of the broadband operational amplifier can also cause the internal current to change rapidly. The steady-state value is in a fixed proportion to the change limit change value, and only the proportional judgment threshold current Ith is needed to make a reasonable state judgment. The present invention is not only simple to implement, but also different from judging the slew rate state by the input voltage. Even if the bias current changes, the relative size relationship between the judgment threshold (Ith) and the internal current change range (±I) can be guaranteed to be proportional, ensuring the correct judgment function. While reducing the threshold redundancy, the speed is faster and the adaptability is strong. The slew rate enhancement system can be quickly turned on or off in conjunction with the slew rate enhancement circuit.

[0027] The above description is only a description of the preferred embodiments of the present invention, and is not intended to limit the scope of the present invention. Any changes or modifications made by a person skilled in the art in the field of the present invention based on the above disclosure shall fall within the scope of protection of the claims.

Claims

1. A fast slew rate enhancement system for an operational amplifier, characterized in that: include: A current identification circuit that monitors the current in the load tube current mirror structure of the operational amplifier; A comparator compares the current monitored by the current identification circuit or a signal related to the current with a threshold value to determine the magnitude of the monitored current, and the result of the comparison by the comparator is used as a basis for determining whether the slew rate is limited, and as an enable signal for turning on a corresponding slew rate enhancement circuit; The slew rate enhancement circuit utilizes the slew rate enhancement enable signal recognized by the comparator to enhance the slew rate of the operational amplifier.

2. The fast slew rate enhancement system of an operational amplifier as claimed in claim 1, characterized in that: The current identification circuit includes a first NMOS transistor and a second NMOS transistor, a first threshold current and a second threshold current; The gate of the first NMOS tube and the gate of the second NMOS tube are both connected to the gate of the load tube current mirror structure in the operational amplifier, the source of the first NMOS tube and the source of the second NMOS tube are both grounded, the drain of the first NMOS tube and the drain of the second NMOS tube are respectively connected to one of the input ends of the two comparators; the first threshold current and the second threshold current are respectively connected to the other input ends of the two comparators.

3. The fast slew rate enhancement system of an operational amplifier as claimed in claim 2, characterized in that: The comparators are respectively a first comparator and a second comparator; The positive input terminal of the first comparator is connected to the first threshold current, and the negative input terminal is connected to the drain of the first NMOS tube; The negative input terminal of the second comparator is connected to the second threshold current, and the positive input terminal is connected to the drain of the second NMOS tube.

4. The fast slew rate enhancement system of an operational amplifier as claimed in claim 3, characterized in that: The slew rate enhancement circuit includes a first switch and a second switch, a first current source and a second current source; The first switch is connected to the input end of the first current source, and the output end of the first current source is grounded; the input end of the second current source is connected to the power supply voltage, and the output end is connected to the second switch; The first comparator outputs a negative slew rate enhancement enable signal to control the second switch, and the second comparator outputs a positive slew rate enhancement enable signal to control the first switch.

5. The fast slew rate enhancement system of an operational amplifier as claimed in claim 2, characterized in that: The first threshold current is I-Ith, and the second threshold current is I+Ith; wherein I is the quiescent current in the operational amplifier load current mirror, and Ith is the comparator threshold current proportional to I.

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