CN101355346A - Frequency Compensation Method of Broadband Common Mode Feedback Loop for Two-Stage Operational Amplifier - Google Patents

Abstract

Translated fromChinese

本发明公开了属于模拟集成电路设计领域的一种两级运算放大器的宽带共模反馈环路频率补偿方法。在带有密勒补偿的两级运算放大器中使用一个宽带反馈放大器作为共模反馈环路，以减少共模反馈电路的面积和功耗；两级运算放大器在单位增益带宽内只有一个主极点，采用全差分输入输出结构，差分输出端用来对共模输出电平取样；共模反馈的控制信号通过该可控偏置电路同时控制运算放大器第一级和第二级的共模输出电平。由于宽带反馈放大器的主极点频率远高于运算放大器的单位增益带宽，因此共模环路的相位裕度基本等于运算放大器自身的相位裕度，从而构成了稳定的反馈环路。优点包括更少的共模反馈电路元件、更低的共模反馈电路功耗、更简便的电路设计。

The invention discloses a broadband common-mode feedback loop frequency compensation method for a two-stage operational amplifier, which belongs to the field of analog integrated circuit design. A wideband feedback amplifier is used as a common-mode feedback loop in a two-stage operational amplifier with Miller compensation to reduce the area and power consumption of the common-mode feedback circuit; the two-stage operational amplifier has only one dominant pole within the unity gain bandwidth, Using a fully differential input and output structure, the differential output terminal is used to sample the common-mode output level; the common-mode feedback control signal simultaneously controls the common-mode output levels of the first and second stages of the operational amplifier through the controllable bias circuit. . Since the dominant pole frequency of the broadband feedback amplifier is much higher than the unity gain bandwidth of the operational amplifier, the phase margin of the common-mode loop is basically equal to the phase margin of the operational amplifier itself, thus forming a stable feedback loop. Advantages include fewer common-mode feedback circuit components, lower common-mode feedback circuit power consumption, and simpler circuit design.

Description

The method for compensating frequency of wideband common mode feedback loop of two-stage calculation amplifier

Technical field

The invention belongs to the analog integrated circuit design field, particularly the method for compensating frequency of wideband common mode feedback loop of two-stage calculation amplifier.

Background technology

Integrated amplifier generally adopts the fully differential structure, and this structure can suppress common mode disturbances and noise.Yet the common mode output level of differential amplifier since factors such as manufacturing process, operating voltage and temperature influence meeting off-design value, therefore need common mode feedback circuit to determine that output DC is flat.When the design common mode feedback circuit, need consider the stability of loop.

Integrated operational amplifier is the circuit unit of using always.In order to realize high-gain and big output signal range, generally adopt two-layer configuration.Can steady operation for the feedback circuit that guarantees to constitute by operational amplifier, adopt the method for miller compensation to improve the phase margin of operational amplifier usually.Fig. 1 is the structure chart that adopts the two-stage calculation amplifier of miller compensation, and Fig. 2 is the open-loop frequency response Bode diagram of operational amplifier.Under suitable miller compensation, have only a dominant pole within the UGB of operational amplifier, and operational amplifier has bigger phase margin.

For two-stage calculation amplifier, the first order and partial output node are " high resistant " node, and its DC level is affected and the off-design value easily, therefore need common mode feedback circuit stablize simultaneously each grade DC level.As the two-stage calculation amplifier of Fig. 3, need two common mode feedback loops, thereby increased the manufacturing cost and the power consumption of circuit.

Summary of the invention

The method for compensating frequency of wideband common mode feedback loop that the purpose of this invention is to provide a kind of two-stage calculation amplifier, it is characterized in that, in having the two-stage calculation amplifier of miller compensation, use a broadband feedback amplifier as common mode feedback loop, to reduce the area and the power consumption of common mode feedback circuit; Two-stage calculation amplifier has only a dominant pole in unity gain bandwidth, adopt the fully differential input/output structure, and difference output end is used for the common mode output level is taken a sample; There is controlled biasing circuit in its first order operation amplifier circuit, and the control signal of common-mode feedback is controlled the operational amplifier first order and partial common mode output level simultaneously by this controlled biasing circuit.

As Fig. 4, the open-loop transmission function of described operational amplifier is H₁(s)=v_Out(s)/v_In(s), unity gain bandwidth is UGB₁, phase margin is PM₁The transfer function of this operational amplifier in common mode feedback loop is H₂(s)=v_Out(s)/v_Cmfb(s), unity gain bandwidth is UGB₂, phase margin is PM₂The transfer function of broadband feedback amplifier is F (s)=v_Cmfb(s)/v_Out(s); Then the overall transfer function of common mode feedback loop is CML (s)=H₂(s) F (s), unity gain bandwidth is UGB₃, phase margin is PM₃

The dominant pole frequency of described broadband feedback amplifier is being designed to far above UGB₁And UGB₂Under the condition, its dominant pole is less to the influence of common mode feedback loop phase margin, H₁(s) UGB₁And H₂(s) UGB₂Satisfy UGB₁≈ UGB₂, PM₁≈ PM₂, so the phase margin of CML (s) satisfies PM₃≈ PM₂≈ PM₁, make the phase margin of common mode feedback loop and the phase margin approximately equal of operational amplifier self, satisfy the requirement of loop stability.

Beneficial effect of the present invention is the unity gain bandwidth of the dominant pole frequency of broadband feedback amplifier far above operational amplifier, so the phase margin of common mode loop equals the phase margin of operational amplifier self substantially, thereby has constituted stable feedback control loop.Thereby the common mode feedback circuit element still less, lower common mode feedback circuit power consumption, easier circuit design.

Description of drawings

Fig. 1 is the two-stage calculation amplifier structure chart that adopts miller compensation.

Fig. 2 is the frequency response curve schematic diagram of Fig. 1 operational amplifier.

Fig. 3 is to use the operational amplifier structure chart of two common mode feedback loops.

Fig. 4 is to use the operational amplifier structure chart of a common mode feedback loop.

Fig. 5 is an exemplary circuit of the compensating frequency of wideband common mode feedback loop scheme that proposes of the present invention.

Embodiment

Below in conjunction with accompanying drawing the present invention is illustrated.

Among Fig. 4, the open-loop transmission function of establishing operational amplifier is H₁(s)=v_Out(s)/v_In(s), unity gain bandwidth is UGB₁, phase margin is PM₁The transfer function of this operational amplifier in common mode feedback loop is H₂(s)=v_Out(s)/v_Cmfb(s), unity gain bandwidth is UGB₂, phase margin is PM₂The transfer function of broadband feedback amplifier is F (s)=v_Cmfb(s)/v_Out(s); Then the overall transfer function of common mode feedback loop is CML (s)=H₂(s) F (s), unity gain bandwidth is UGB₃, phase margin is PM₃

Under suitable design, H₁(s) UGB₁And H₂(s) UGB₂Satisfy UGB₁≈ UGB₂, PM₁≈ PM₂Because the dominant pole frequency of broadband feedback amplifier is far above UGB₁And UGB₂, so broadband feedback amplifier dominant pole is less to the influence of common mode feedback loop phase margin, the phase margin of CML (s) satisfies PM₃≈ PM₂≈ PM₁The phase margin approximately equal of the phase margin of common mode feedback loop and operational amplifier self satisfies the requirement of loop stability like this.

Fig. 5 has described a two-stage calculation amplifier circuit that has the broadband common mode feedback circuit.The left side is the two-stage calculation amplifier that has miller compensation, wherein V_In+And V_In-Be the differential input end of operational amplifier, V_Out+And V_Out-It is the difference output end of operational amplifier.The difference input of the first order circuit of operational amplifier is M to pipe₁, establishing its mutual conductance is g_M1The differential load of first order circuit is M to pipe₂, establishing its mutual conductance is g_M2The input pipe of second level circuit is M₃, establishing its mutual conductance is g_M3The load capacitance of operational amplifier is C_L, miller compensation electric capacity is C_C

The right is the broadband feedback amplifier in the common mode feedback loop.V_Out+And V_Out-Be added in pipe M₄Grid, realize the sampled functions of common mode output level, establish M₄The mutual conductance of pipe is g_M41. the broadband feedback amplifier exports control voltage V at node_Cmfb, this voltage is added in the operational amplifier differential load to pipe M₂Grid.Thereby operational amplifier and broadband feedback amplifier constitute closed common mode feedback loop.

When operational amplifier is operated in open loop difference modes following time, input signal is V_In+And V_In-, output signal is V_Out+And V_Out-, adopt capacitor C_CBehind the miller compensation, its unity gain bandwidth (UGB₁) be:

${\mathrm{UGB}}_1=\frac{g_{m1}}{C_C}$

Its non-dominant pole frequency (angular frequency) is:

${\mathrm{\ω}}_{n1}=\frac{g_{m3}}{C_L}$

The phase margin of operational amplifier under difference modes is:

When considering the common mode loop gain of entire circuit, this loop comprises operational amplifier and broadband feedback amplifier two parts.Wherein the input signal of operational amplifier is V_Cmfb, output signal is V_Out+And V_Out-(this moment V_Out+=V_Out-), its unity gain bandwidth (UGB₂) be:

${\mathrm{UGB}}_2=\frac{g_{m2}}{C_C}$

The non-dominant pole frequency (angular frequency) of operational amplifier part is:

${\mathrm{\ω}}_{n2}=\frac{g_{m3}}{C_L}$

The phase margin of operational amplifier part is in common mode feedback loop:

In circuit design, g is arranged generally_M2＜=g_M1, so UGB₁＞UGB₂, PM₁＜=PM₂

For the broadband feedback amplifier, establish node 1. corresponding amplifier dominant pole angular frequency be ω_N3, it is much larger than the UGB of operational amplifier₁, have so:

$\mathrm{arctg}\left(\frac{{\mathrm{UGB}}_1}{{\mathrm{\ω}}_{n3}}\right)\≈0$

Then the phase margin of whole common mode feedback loop is:

The two-stage calculation amplifier that then comprises common mode feedback circuit, the phase margin of its common mode loop are approximately equal to the phase margin under the operational amplifier open loop differential mode, and loop is stable.

Claims (3)

1. the method for compensating frequency of wideband common mode feedback loop of a two-stage calculation amplifier, it is characterized in that, in having the two-stage calculation amplifier of miller compensation, use a broadband feedback amplifier as common mode feedback loop, to reduce the area and the power consumption of common mode feedback circuit; Two-stage calculation amplifier has only a dominant pole in unity gain bandwidth, adopt the fully differential input/output structure, and difference output end is used for the common mode output level is taken a sample; There is controlled biasing circuit in its first order operation amplifier circuit, and the control signal of common-mode feedback is controlled the operational amplifier first order and partial common mode output level simultaneously by this controlled biasing circuit.

2. according to the method for compensating frequency of wideband common mode feedback loop of the described two-stage calculation amplifier of claim 1, it is characterized in that the open-loop transmission function of described operational amplifier is H₁(s)=v_Out(s)/v_In(s), unity gain bandwidth is UGB₁, phase margin is PM₁The transfer function of this operational amplifier in common mode feedback loop is H₂(s)=v_Out(s)/v_Cmfb(s), unity gain bandwidth is UGB₂, phase margin is PM₂The transfer function of broadband feedback amplifier two-stage calculation amplifier is F (s)=v_Cmfb(s)/v_Out(s); Then the overall transfer function of common mode feedback loop is CML (s)=H₂(s) F (s), unity gain bandwidth is UGB₃, phase margin is PM₃

3. according to the method for compensating frequency of wideband common mode feedback loop of the described two-stage calculation amplifier of claim 1, it is characterized in that the dominant pole frequency of described broadband feedback amplifier is far above UGB₁And UGB₂Under the condition, its dominant pole is less to the influence of common mode feedback loop phase margin, H₁(s) UGB₁And H₂(s) UGB₂Satisfy UGB₁≈ UGB₂, PM₁≈ PM₂, so the phase margin of CML (s) satisfies PM₃≈ PM₂≈ PM₁, make the phase margin of common mode feedback loop and the phase margin approximately equal of operational amplifier self, satisfy the requirement of loop stability.

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