CN103095231A - Novel common mode feedback circuit - Google Patents

Abstract

The invention relates to a novel common mode feedback circuit. The novel common mode feedback circuit comprises an output common mode current detection circuit, an error amplifier and a reference current generating circuit. When a differential input end of the novel common mode feedback circuit is connected with an output end of a fully differential operational amplifier, the output common mode current detection circuit generates an electric current related to an output common mode electric level of the fully differential operational amplifier, the electric current is enabled to be equal to a reference electric current generated by the reference current generating circuit through the error amplifier and negative feedback, so that the output common mode electric level of the fully differential operational amplifier is enabled to be equal to one reference voltage. When used for achieving common mode feedback, the novel common mode feedback circuit does not need to be refreshed regularly, can not reduce the open loop gain, the output swing, the input common mode range and the like of the fully differential operational amplifier, and simultaneously has the advantages of being small in area and parasitic effect and the like.

Description

A kind of novel common mode feedback circuit

Technical field

The present invention relates to the Full differential operational amplifier technical field, relate in particular to a kind of novel common mode feedback circuit.

Background technology

It is important module in analog integrated circuit that prior art discloses operational amplifier.Compare with the operational amplifier of Single-end output, Full differential operational amplifier has larger output voltage swing and stronger antijamming capability, is widely used in the circuit such as analog to digital converter, filter.

Studies show that, because the output common mode level of high-gain Full differential operational amplifier is very responsive to the mismatch of device, so must stablize its output common mode level by increasing common mode feedback circuit.Common mode feedback circuit will be realized following functions usually: the output common mode level is detected, testing result and a certain reference voltage are compared, produce error voltage or error current, being used for a certain road bias current of resonance-amplifier, is this reference voltage by negative feedback with the output common mode level equalization.

Existing circuit for the output common mode level detection has following a few class, one class is to use resistance detection, but for detection of resistance must be more much larger than the output impedance of operational amplifier, otherwise the open-loop gain of meeting step-down amplifier, and the resistance of high value can take large chip area and bring large ghost effect; One class is to detect with the metal-oxide-semiconductor that is operated in dark linear zone, but there are following two problems in this method, a problem is output voltage swing or the input common-mode range that can reduce amplifier, and another problem is can not be with the output common mode voltage stabilization a definite value; Also having a class is to use capacitance detecting, but this class circuit need to periodically refresh.

Summary of the invention

Main purpose of the present invention is to overcome the defective that prior art exists, and a kind of novel common mode feedback circuit is provided.

In order to achieve the above object, technical scheme of the present invention is: a kind of novel common mode feedback circuit (as shown in Figure 1) is provided, has been comprised of output common mode current detection circuit (1), error amplifier (2) and reference current generating circuit (3); Four inputs, wherein V are arranged _inpAnd V _innBe two differential input ends, V _bpBe bias voltage input, V _refBe reference voltage input terminal; An output V is arranged _Cmfb

Described output common mode current detection circuit (1) is comprised of four metal-oxide-semiconductors, wherein PMOS pipe M ₁And M ₂Grid respectively with input V _inpAnd V _innBe connected; PMOS manages M ₁And M ₂The drain electrode concurrent, and with NMOS pipe M ₃Drain and gate be connected; NMOS manages M ₃Source electrode be connected with ground GND; PMOS manages M ₄Grid and input V _bpBe connected; M ₄Source electrode be connected with power vd D; PMOS manages M ₁And M ₂The source electrode concurrent, and with PMOS pipe M ₄Drain electrode, V _{In_cm}An input of end and error amplifier is connected;

Described reference current generating circuit (3) is comprised of four metal-oxide-semiconductors, wherein PMOS pipe M ₆And M ₇Grid respectively with input V _refBe connected; PMOS manages M ₆And M ₇The drain electrode concurrent, and with NMOS pipe M ₅Drain and gate be connected; NMOS manages M ₅Source electrode be connected with ground GND; PMOS manages M ₈Grid and input V _bpBe connected; M ₈Source electrode be connected with power vd D; PMOS manages M ₆And M ₇The source electrode concurrent, and with PMOS pipe M ₈Drain electrode, V _{Ref_cm}An input of end and error amplifier is connected;

The output of described error amplifier (2) and output V _CmfbBe connected.

When realizing the common-mode feedback function, differential input end V _inpAnd V _innRespectively with the output V of Full differential operational amplifier _OutpAnd V _OutnBe connected, output V _CmfbBe used for controlling a certain road bias current of Full differential operational amplifier, consist of negative feedback loop, make the output common mode level of operational amplifier equal reference voltage V _ref

When the output common mode level of Full differential operational amplifier changes, as working as the output common mode level higher than reference voltage V _refThe time, the V of output common mode current detection circuit (1) _{In_cm}Terminal voltage rises, by the V of error amplifier (2) with itself and reference current generating circuit (3) _{Ref_cm}Terminal voltage compares, and error voltage is amplified, and passes through V _CmfbThe a certain road bias current of output control algorithm amplifier descends the output common mode level, finally makes it equal reference voltage V _refEqually, when the output common mode level lower than reference voltage V _refThe time, the V of output common mode current detection circuit (1) _{In_cm}Terminal voltage descends, by the V of error amplifier (2) with itself and reference current generating circuit (3) _{Ref_cm}Terminal voltage compares, and error voltage is amplified, and passes through V _CmfbThe a certain road bias current of output control algorithm amplifier makes the output common mode electrical level rising by negative feedback, finally makes it equal reference voltage V _ref

Common mode feedback circuit of the present invention has the following advantages:

1, the common mode feedback circuit that utilizes the present invention to realize does not need timing circuit to be refreshed, can not reduce open-loop gain, output voltage swing and the input common-mode range etc. of operational amplifier;

2, have simultaneously the advantages such as less area and ghost effect.

Description of drawings

Fig. 1 is the circuit diagram of common mode feedback circuit of the present invention.

Embodiment

The present invention is described in more detail below in conjunction with accompanying drawing.

Figure 1 shows that the circuit diagram of a kind of novel common mode feedback circuit provided by the invention, formed by output common mode current detection circuit (1), error amplifier (2) and reference current generating circuit (3).PMOS manages M ₁, M ₂, M ₆And M ₇Identical, PMOS manages M ₄And M ₈Identical, NMOS manages M ₃And M ₅Identical.

When the present invention is used for common-mode feedback, differential input end V _inpAnd V _innRespectively with the output V of Full differential operational amplifier _OutpAnd V _OutnBe connected.Difference input in common mode current testing circuit (1) is to M ₁And M ₂The common mode incoming level of pipe, i.e. the output common mode level V of Full differential operational amplifier _cmFor:

V _cm＝0.5*(V _outp+V _outn) (1)

PMOS manages M ₄Electric current be:

$I_{M4}={\mathrm{\μ}}_p{C}_{\mathrm{ox}}\frac{W_4}{L_4}{(V_{\mathrm{cm}}-V_{\mathrm{in}\_\mathrm{cm}}-V_{\mathrm{th}})}^2---\left(2\right)$

μ in formula (2) _p, C _ox, V _thWith

Be respectively M ₁/ M ₂The carrier mobility of pipe, gate oxide electric capacity, threshold voltage and the breadth length ratio of unit are.

Differential pair M in reference current generating circuit (3) ₆And M ₇The common mode incoming level of pipe is V _ref, flow through metal-oxide-semiconductor M ₈Electric current be:

$I_{M8}={\mathrm{\μ}}_p{C}_{\mathrm{ox}}\frac{W_6}{L_6}{(V_{\mathrm{ref}}-V_{\mathrm{ref}\_\mathrm{cm}}-V_{\mathrm{th}})}^2---\left(3\right)$

μ in formula (3) _p, C _ox, V _thWith

Be respectively M ₆/ M ₇The carrier mobility of pipe, gate oxide electric capacity, threshold voltage and the breadth length ratio of unit are.

Due to degenerative effect, two input terminal voltages of error amplifier (2) are equated, i.e. V _{Ref_cm}=V _{In_cm}Because metal-oxide-semiconductor M ₄And M ₈Have identical size and identical grid voltage, drain voltage and source voltage, so I is arranged _M8=I _M4, namely formula (2) equals formula (3).Due to metal-oxide-semiconductor M ₁, M ₂, M ₆And M ₇Be all the PMOS pipe, and have identical width and length, therefore can be got by formula (2)=formula (3):

V _cm＝V _ref (4)

Be the output common mode level equalization of Full differential operational amplifier like this, just reference voltage V _ref

Compare with existing common-mode feedback technology, a kind of novel common mode feedback circuit of the present invention does not need regularly circuit to be refreshed, can not reduce open-loop gain, output voltage swing and the input common-mode range etc. of operational amplifier; Have simultaneously the advantages such as less area and ghost effect.

Claims (4)

1. a novel common mode feedback circuit, is characterized in that, this circuit is comprised of output common mode current detection circuit (1), error amplifier (2) and reference current generating circuit (3); Four inputs, wherein V are arranged _inpAnd V _innBe two differential input ends, V _bpBe bias voltage input, V _refBe reference voltage input terminal; An output V is arranged _Cmfb

Described output common mode current detection circuit (1) is comprised of four metal-oxide-semiconductors, wherein PMOS pipe M ₁And M ₂Grid respectively with input V _inpAnd V _innBe connected; PMOS manages M ₁And M ₂The drain electrode concurrent, and with NMOS pipe M ₃Drain and gate be connected; NMOS manages M ₃Source electrode be connected with ground GND; PMOS manages M ₄Grid and input V _bpBe connected; M ₄Source electrode be connected with power vd D; PMOS manages M ₁And M ₂The source electrode concurrent, and with PMOS pipe M ₄Drain electrode, V _{In_cm}An input of end and error amplifier is connected;

Described reference current generating circuit (3) is comprised of four metal-oxide-semiconductors, wherein PMOS pipe M ₆And M ₇Grid respectively with input V _refBe connected; PMOS manages M ₆And M ₇The drain electrode concurrent, and with NMOS pipe M ₅Drain and gate be connected; NMOS manages M ₅Source electrode be connected with ground GND; PMOS manages M ₈Grid and input V _bpBe connected; M ₈Source electrode be connected with power vd D; PMOS manages M ₆And M ₇The source electrode concurrent, and with PMOS pipe M ₈Drain electrode, V _{Ref_cm}An input of end and error amplifier is connected;

The output of described error amplifier (2) and output V _CmfbBe connected.

2. by common mode feedback circuit claimed in claim 1, it is characterized in that, when realizing the common-mode feedback function, described differential input end V _inpAnd V _innRespectively with the output V of Full differential operational amplifier _OutpAnd V _OutnBe connected, output V _CmfbBe used for controlling a certain road bias current of Full differential operational amplifier, consist of negative feedback loop, make the output common mode level of operational amplifier equal reference voltage V _ref

3. by common mode feedback circuit claimed in claim 1, it is characterized in that, when the output common mode level of Full differential operational amplifier changes, wherein, when the output common mode level higher than reference voltage V _refThe time, the V of output common mode current detection circuit (1) _{In_cm}Terminal voltage rises, by the V of error amplifier (2) with itself and reference current generating circuit (3) _{Ref_cm}Terminal voltage compares, and error voltage is amplified, and passes through V _CmfbThe a certain road bias current of output control algorithm amplifier descends the output common mode level, finally makes it equal reference voltage V _ref

4. by common mode feedback circuit claimed in claim 1, it is characterized in that, when the output common mode level of Full differential operational amplifier changes, wherein, when the output common mode level lower than reference voltage V _refThe time, the V of output common mode current detection circuit (1) _{In_cm}Terminal voltage descends, by the V of error amplifier (2) with itself and reference current generating circuit (3) _{Ref_cm}Terminal voltage compares, and error voltage is amplified, and passes through V _CmfbThe a certain road bias current of output control algorithm amplifier makes the output common mode electrical level rising by negative feedback, finally makes it equal reference voltage V _ref

Images