CN105978496A - Frequency compensation technology for optimizing conversion speeds of operational amplifiers - Google Patents

Abstract

The invention discloses a frequency compensation technology for optimizing conversion speeds of operational amplifiers. The technology is characterized in that a compensation amplifier is added between a first-stage amplifier and a second-stage amplifier; an input end of the compensation amplifier is connected to an output end of the first-stage amplifier, and an output end of the compensation amplifier is connected to one end of a compensation capacitor; and the other end of the compensation capacitor is connected to one end of a zero setting resistor, a common connecting point is connected to an input end of the second-stage operational amplifier, the other end of the zero setting resistor is connected to one end of a Miller capacitor, and the other end of the Miller capacitor is connected to an output end of the second-stage operational amplifier. The technology disclosed by the invention is characterized in that an extra frequency compensation-stage circuit is added between the two stages of operational amplifiers, so that a left-half plane zero point can be generated by small compensation capacitance and secondary points in transmission functions of the operational amplifiers can be offset. In this way, the operational amplifiers can obtain a large phase margin, and stability can be enhanced. Meanwhile, an adopted compensation capacitance value and an adopted Miller capacitance value are reduced, so that the operational amplifiers obtain the quicker conversion speeds, and thus better compromising effects between the speed and the stability are obtained.

Description

A kind of frequency acquisition and tracking optimizing operational amplifier conversion speed

Technical field

The present invention relates to analogue layout field, particularly relate to a kind of Operational Amplifier Frequency Compensation technology.

Background technology

Operational amplifier is one of module the most basic in analog circuit, most important, be widely used in power management IC, Display drives in the various integrated circuits such as IC, ADC (analog-digital converter)/DAC (digital to analog converter).Therefore, design high-performance Operational amplifier to improve whole analog circuit performance significant.

The performance parameter of operational amplifier includes open-loop gain, unity gain bandwidth, sets up time, switching rate and phase Position nargin etc..In order to obtain high open-loop gain and high unit gain list width, it is typically two-stage by Design of Amplifiers, including defeated Enter high-gain stage and output buffer stage.And in order to obtain less time of setting up and faster switching rate, often sacrificial The stability of domestic animal amplifier, i.e. phase margin.

In order to improve the stability of operational amplifier, compensation technique is one of key.Conventional frequency acquisition and tracking is to adopt With miller-compensated, its schematic diagram is as shown in Figure 1.Wherein R₁、C₁And R₂、C₂For zero-regulator resistor and miller capacitance, they can change The position of zero pole point in operational amplifier transfer function, i.e. produces the effect of limit division, so that the bandwidth of operational amplifier Increasing, phase margin increases, and stability improves.But, when to operational amplifier stability requirement the highest time, it is necessary to use hold It is worth bigger miller capacitance, this time of setting up that can affect operational amplifier and switching rate so that it is slow.Therefore, pass The miller-compensated technology of system is accomplished by trading off consideration between the speed and stability of operational amplifier.

Summary of the invention

In order to solve the problem existing for above-mentioned technology, the invention provides a kind of operational amplifier conversion speed that optimizes Frequency acquisition and tracking, to realize operational amplifier more excellent trading off between speed and stability.

A kind of frequency acquisition and tracking optimizing operational amplifier conversion speed that the present invention provides, it is characterised in that the A compensation amplifier is increased between first stage amplifier and second level amplifier；The input of this compensation amplifier connects the first order The outfan of amplifier, the outfan compensating amplifier connects the one end compensating electric capacity；The other end of this compensation electric capacity with One end of zero-regulator resistor is connected, and this points of common connection connects the input of second level operational amplifier, and zero-regulator resistor is another One end is connected with one end of miller capacitance, and the other end of miller capacitance is connected to the outfan of second level operational amplifier.

Beneficial effects of the present invention, by adding an extra frequency compensated stage circuit between two-stage calculation amplifier, A Left half-plane zero point can be produced, to offset the secondary in operational amplifier transfer function by less compensation electric capacity Point, so that operational amplifier obtains bigger phase margin, improves stability.Simultaneously as the compensation electric capacity used and rice Le capacitance reduces, and operational amplifier can be made to obtain faster switching rate, thus reach more preferable between speed and stability Compromise.

Accompanying drawing explanation

Fig. 1 is the circuit theory diagrams that operational amplifier uses miller-compensated technology.

Fig. 2 is the circuit theory diagrams that operational amplifier uses frequency acquisition and tracking of the present invention.

Fig. 3 be operational amplifier use frequency acquisition and tracking of the present invention implement circuit diagram.

Fig. 4 is the small-signal equivalent circuit figure that operational amplifier uses frequency acquisition and tracking of the present invention.

Detailed description of the invention

In order to make the technical problem to be solved, technical scheme and improvement effect clearer, below tie Close accompanying drawing, the present invention is described in further detail.

Fig. 2 is the circuit theory diagrams that operational amplifier uses frequency compensation method of the present invention, in order to having of the present invention is described Benefit effect, the two-stage calculation amplifier of the Differential Input Single-end output of employing.In addition to using miller-compensated technology, also add Compensate amplifier and compensate electric capacity, and being connected across between first order operational amplifier and second level operational amplifier.

Fig. 3 be operational amplifier use frequency acquisition and tracking of the present invention implement circuit diagram, first order amplifier is High-gain stage, can use any type of differential input amplifier structure.And M7 and M8 constitutes second level operational amplifier knot Structure, this structure is push-pull configuration, has obtained high output driving force.M1, M2, M3 and M4, M5, M6 respectively constitute compensation and amplify Device structure.C₁、R₁、C₂And C₃、R₂、C₄Respectively compensate electric capacity, zero-regulator resistor and miller capacitance.Bias1 and bias2 is respectively The gate bias voltage of M3 and M6.

In order to be expanded on further compensation technique of the present invention in operational amplifier transfer function zero, the impact of pole location, It is small signal circuit by circuit equivalent in Fig. 3, as shown in Figure 4.Wherein, G_m1、G_m2Be respectively first order operational amplifier, second The small-signal transconductance of level operational amplifier, G_mAFor compensating the small-signal transconductance of amplifier；R₁、C₁It is respectively first order operation amplifier The output resistance of device and output node electric capacity；R_AOutput resistance for frequency compensation level circuit；R₂、C_LIt is respectively second level fortune Calculate output resistance and the output load capacitance of amplifier；C_AFor frequency compensation capacitance, C_CFor miller-compensated electric capacity, R_CFor Miller Compensate resistance.Whole operational amplifierTransfer function be:

$\frac{V_O}{V_i}=-G_{m1}{G}_{m_2}{R}_1{R}_2\frac{\[(R_C{C}_A+R_C{C}_C+R_A{C}_A)S+1\]}{(R_A\frac{C_A{C}_C}{C_A+C_C}S+1)(R_2{R}_{L}S+1)(G_{mA}{R}_A{R}_C{C}_{A}S+1)}---\left(1\right)$

According to transfer function, it can be deduced that zero point and dominant pole, secondary point, the 3rd pole location are respectively as follows:

$Z_1\≈-\frac{1}{R_C{C}_A+R_C{C}_C+R_A{C}_A}---\left(2\right)$

$P_1\≈-\frac{1}{G_{mA}{R}_A{R}_C{C}_A}---\left(3\right)$

$P_2\≈-\frac{1}{R_2{C}_L}---\left(4\right)$

$P_3\≈-\frac{1}{R_A\frac{C_A{C}_C}{C_A+C_C}}---\left(5\right)$

Knowable to formula (2), this zero point is Left half-plane zero point, and this dead-center position has two kinds of methods to set up, and one is to be disposed at P₂Near limit, to offset time limit P₂.Due to P₃Limit is high frequency poles, can ignore P₃The impact of limit.The most whole fortune Put and just can become single-stage dot system, reach the phase margin of 90 °.Another kind of method is by Z₁The frequency configuration of zero point is at P₂Pole Between point and unity gain bandwidth GB, and by zero point Z₁Be located proximate to time limit P2.

In view of process fluctuation, zero point Z₁It is difficult under various process corner, be completely counterbalanced by time limit P₂, so taking zero point Z₁ It is arranged on P₂Between limit and unity gain bandwidth GB, and zero point Z₁Close proximity to the method for secondary limit P2, i.e. | P₂|≤|Z₁| ＜ GB.Understand according to formula (2), (4),

$\frac{1}{R_2{C}_L}\&le;\frac{1}{R_C{C}_A+R_C{C}_C+R_A{C}_A}<A_v\left(0\right)\left|P_1\right|=\frac{G_{m1}{G}_{m_2}{R}_1{R}_2}{G_{mA}{R}_A{R}_C{C}_A}---\left(6\right)$

In order to ensure that operational amplifier all keeps stable in whole frequency range, need to ensure that phase margin is more than 60 °, then

Derive for simplifying, it is assumed that ω ＞ ＞ P₁, and C_C=C_A, and conditions above easily reaches, then can be pushed away by formula (6), (7) Following relation:

$\frac{0.18R_2}{2R_C+R_A}{C}_L\&le;C_A\&le;\frac{R_2}{2R_C+R_A}{C}_L---\left(8\right)$

The compensation capacitance size that the most whole operational amplifier needs is C_A+C_C, i.e.

$\frac{0.36R_2}{2R_C+R_A}{C}_L\&le;C_A+C_C\&le;\frac{2R_2}{2R_C+R_A}{C}_L---\left(9\right)$

If R₂=R_C, and R_A=10R_C, this is assumed to be reasonable assumption scope, realizes the most in the design.So can draw

0.03C_L≤C_A+C_C≤0.16C_L (10)

As long as meeting the condition shown in formula (10), then the phase margin of operational amplifier just can reach 60 °～90 °, and protect Keep steady fixed.

In order to the improvement effect of the present invention is expanded on further, miller-compensated to frequency acquisition and tracking of the present invention and tradition technology is carried out Performance comparision, as shown in Table 1 below.

Table 1 frequency acquisition and tracking of the present invention contrasts with tradition miller-compensated technology major parameter

Reducing owing to compensating electric capacity, the performance of operational amplifier can promote in terms of three below:

(1) unity gain bandwidth GB: from unity gain bandwidth GB optimization degree in table 1, if Reasonable adjustment Gm2, GmA, R1, R2, RC, RA are equivalent, and the bandwidth of operational amplifier i.e. can be greatly improved；

(2) conversion rate SR: from Slew Rate formula,Reduce owing to compensating electric capacity, the Slew Rate of operational amplifier Getting a promotion, also reduce amplifier sets up the time simultaneously, even if optimizing the speed of operational amplifier；

(3) PSRR PSRR⁺: according to the PSRR formula of two-level operating amplifierMend Repay electric capacity CC the least, PSRR⁺The biggest.Therefore, benefit from frequency acquisition and tracking of the present invention, the PSRR of operational amplifier PSRR⁺Also improved.

The foregoing is only presently preferred embodiments of the present invention, that is made within every the spirit and principles in the present invention any repaiies Change, equivalent and improvement etc., should be included within the scope of the present invention.

Claims (1)

1. the frequency acquisition and tracking optimizing operational amplifier conversion speed, it is characterised in that at first order amplifier and A compensation amplifier is increased between two-stage amplifier；The input of this compensation amplifier connects the output of first order amplifier End, the outfan compensating amplifier connects the one end compensating electric capacity；The other end of this compensation electric capacity and the one of zero-regulator resistor End is connected, and this points of common connection connects the input of second level operational amplifier, and the other end of zero-regulator resistor is electric with Miller The one end held is connected, and the other end of miller capacitance is connected to the outfan of second level operational amplifier.