CN114900139B - Common-mode feedback circuit of fully differential operational amplifier - Google Patents
Common-mode feedback circuit of fully differential operational amplifier Download PDFInfo
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- H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
- H03F3/45—Differential amplifiers
- H03F3/45071—Differential amplifiers with semiconductor devices only
- H03F3/45076—Differential amplifiers with semiconductor devices only characterised by the way of implementation of the active amplifying circuit in the differential amplifier
- H03F3/45179—Differential amplifiers with semiconductor devices only characterised by the way of implementation of the active amplifying circuit in the differential amplifier using MOSFET transistors as the active amplifying circuit
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Abstract
The invention discloses a common-mode feedback circuit of a fully differential operational amplifier, and relates to a semiconductor integrated circuit. The amplifier comprises a first-stage amplifying unit, a second-stage amplifying unit, a common-mode level detecting unit and a common-mode feedback amplifier; the common-mode level detection unit is used for acquiring two differential output signals of the second-stage amplification unit and acquiring a common-mode signal according to the two differential output signals; the common mode feedback amplifier is used for comparing the level of the common mode signal with a reference level to obtain an error signal, and feeding the error signal back to an output node of the first-stage amplification unit so as to adjust the current of the output node of the first-stage amplification unit. The invention reduces the number of stages of the amplifying circuit in the common mode feedback loop, thereby reducing the number of poles in the common mode feedback loop and improving the stability of the common mode feedback loop.
Description
Technical Field
The present invention relates to semiconductor integrated circuits, and more particularly, to a common mode feedback circuit of a fully differential operational amplifier.
Background
Operational amplifiers are basic design units in integrated circuits, and their performance plays a crucial role in simulating the overall performance of the integrated circuit. Compared with a single-ended output circuit, the fully differential operational amplifier has the advantages of larger output swing amplitude, stronger common-mode noise suppression capability, capability of eliminating high-order harmonic distortion and the like. The output common mode level of the high-gain operational amplifier is very sensitive to the factors such as device mismatch, a small offset may cause the operating point of the MOS transistor of the operational amplifier to shift, and once the operating point deviates from a saturation region, the operational amplifier will work unstably or even cannot work, and cannot be stabilized by differential feedback. An additional common-mode feedback mechanism needs to be introduced to stabilize the common-mode output level, reduce the common-mode gain, and improve the common-mode rejection ratio. And the stability of the common mode feedback loop plays an important role in the normal operation of the operational amplifier.
The traditional operational amplifier common mode feedback circuit adopts a voltage feedback scheme. And the output node of the common mode feedback circuit is connected to the grid electrode of the load tube of the first-stage amplification circuit or the tail current source. Namely, the common mode feedback circuit feeds back an error signal detected by the common mode level detection circuit to the grid electrode of the first stage load current source or the tail current source to form a common mode feedback loop. Therefore, three stages of amplifying circuits, namely a first-stage amplifying circuit load tube, an operational amplifier second-stage amplifying circuit and a common-mode feedback amplifier, exist in the common-mode feedback loop. In the loop design, the more the amplification stages are, the worse the loop stability is, and the phase margin of the common mode feedback loop affected by each pole is poor.
Disclosure of Invention
The technical problem to be solved by the present invention is to provide a common mode feedback circuit of a fully differential operational amplifier, which reduces the number of stages of an amplifier circuit in a common mode feedback loop, thereby reducing the number of poles in the common mode feedback loop and improving the stability of the common mode feedback loop.
The invention relates to a common mode feedback circuit of a fully differential operational amplifier, which comprises a first-stage amplification unit A1 and a second-stage amplification unit A2; the output node of the first-stage amplification unit A1 is connected with the input node of the second-stage amplification unit A2; also comprises
The common mode level detection unit is used for acquiring two differential output signals of the second-stage amplification unit A2 and acquiring a common mode signal according to the two differential output signals;
the common mode feedback amplifier is used for comparing the level of the common mode signal with a reference level to obtain an error signal, and feeding the error signal back to an output node of the first-stage amplification unit A1 so as to adjust the current of the output node of the first-stage amplification unit A1.
The current of the output node of the first-stage amplification unit A1 is adjusted in a manner that,
if the level of the common-mode signal is higher than the reference level, the common-mode feedback amplifier increases the current injected into the first-stage amplification unit A1 so as to raise the potential of the output node of the first-stage amplification unit A1;
if the level of the common mode signal is lower than the reference level, the common mode feedback amplifier reduces the current injected into the first-stage amplification unit A1, so that the potential of the output node of the first-stage amplification unit A1 is reduced.
The common mode feedback amplifier consists of an eleventh field effect transistor M11, a fifteenth field effect transistor M15 and a tenth field effect transistor M10; a common mode signal is input to the gate of the eleventh field effect transistor M11, and a PMOS current source bias voltage is input to the drain of the eleventh field effect transistor M11; reference levels are input to the grids of the fifteenth field-effect tube M15 and the tenth field-effect tube M10, and the drain electrode of the fifteenth field-effect tube M15 and the drain electrode of the tenth field-effect tube M10 are respectively connected with two output nodes of the first-stage amplification unit A1 in a one-to-one correspondence manner; the sources of the eleventh field effect transistor M11, the fifteenth field effect transistor M15 and the tenth field effect transistor M10 are all grounded.
The source electrodes of the eleventh field effect transistor M11, the fifteenth field effect transistor M15 and the tenth field effect transistor M10 are connected with each other, and the source electrode connection ends of the eleventh field effect transistor M11, the fifteenth field effect transistor M15 and the tenth field effect transistor M10 are grounded through a current source.
The current source is a sixteenth field effect transistor M16; the drain electrode of the sixteenth field effect transistor M16 is connected with the source electrode connecting ends of the eleventh field effect transistor M11, the fifteenth field effect transistor M15 and the tenth field effect transistor M10, and an NMOS current source bias voltage is input to the gate electrode of the sixteenth field effect transistor M16; the source of the sixteenth fet M16 is grounded.
The common mode signal is one half of the sum of the two differential output signals of the second stage amplification unit A2.
The common mode level detection unit consists of a first resistor R1 and a second resistor R2; one end of the first resistor R1 is connected with one differential output end of the second-stage amplification unit A2, and one end of the second resistor R2 is connected with the other differential output end of the second-stage amplification unit A2; the other end of the first resistor R1 is connected with the other end of the second resistor R2, and the other end of the first resistor R1 is used as the output end of the common mode level detection unit to output the common mode signal.
The common mode feedback circuit further comprises an amplifier load current source for providing current for the output node of the first stage amplifying unit A1.
The amplifier load current source consists of a thirteenth field effect tube M13, a fourteenth field effect tube M14 and a twelfth field effect tube M12; the grid electrodes of the thirteenth field effect transistor M13, the fourteenth field effect transistor M14 and the twelfth field effect transistor M12 are all inputted with a PMOS current source bias voltage; the source electrodes of the thirteenth field effect transistor M13, the fourteenth field effect transistor M14 and the twelfth field effect transistor M12 are connected with each other and serve as the power supply input end of the amplifier load current source; the drain of the thirteenth field-effect tube M13 is connected to the gate thereof, the drain of the fourteenth field-effect tube M14 is connected to one output node of the first-stage amplification unit A1, and the drain of the twelfth field-effect tube M12 is connected to the other output node of the first-stage amplification unit A1.
The common-mode gain of the common-mode feedback circuit is,
in the formula,
is the common mode gain;
for the first field-effect transistor M1 in the first-stage amplification unit A1A transconductance;
is the transconductance of a sixth field effect transistor M6 in the second-stage amplification unit A2;
outputting impedance for a fifth field effect transistor M5 in the first-stage amplification unit A1;
the output impedance of the third field effect transistor M3 or the fourth field effect transistor M4 in the first-stage amplification unit A1 is obtained;
the output impedance of the eighth field effect transistor M8 or the ninth field effect transistor M9 in the second-stage amplification unit A2;
is the output impedance of the twelfth fet M12.
Advantageous effects
The invention has the advantages that: compared with the traditional common mode feedback loop with the three-stage amplification circuit, the output end of the common mode feedback amplifier is connected with the output node of the first-stage amplification unit, the common mode feedback loop is formed in a current feedback mode, the stage number of the amplification circuit in the common mode feedback loop is reduced, the pole number in the common mode feedback loop is reduced, and the stability of the common mode feedback loop is improved.
Drawings
FIG. 1 is a schematic diagram of a common mode feedback circuit according to the present invention;
FIG. 2 is a schematic diagram of a common mode feedback circuit of the present invention;
FIG. 3 is a schematic diagram of a half-equivalent circuit of a common mode feedback circuit schematic diagram according to the present invention;
FIG. 4 is a schematic diagram of a small signal model of a half-equivalent circuit of the present invention;
FIG. 5 is a diagram illustrating a simulation result of common mode gain of a common mode feedback circuit according to various embodiments of the present invention;
FIG. 6a is a diagram illustrating a simulation result of a common mode loop gain and phase of a conventional voltage common mode feedback circuit;
FIG. 6b is a diagram illustrating the simulation results of the common-mode loop gain and phase of the current common-mode feedback circuit according to the present invention.
Detailed Description
The invention is further described below with reference to examples, but not to be construed as being limited thereto, and any number of modifications which can be made by anyone within the scope of the claims are also within the scope of the claims.
The invention discloses a common-mode feedback circuit of a fully differential operational amplifier, which comprises a first-stage amplifying unit A1 and a second-stage amplifying unit A2. The output node of the first-stage amplification unit A1 is connected to the input node of the second-stage amplification unit A2. The specific circuit structures of the first-stage amplification unit A1 and the second-stage amplification unit A2 are shown in fig. 2. The first-stage amplification unit A1 consists of five field effect transistors and is divided into a first field effect transistor M1, a second field effect transistor M2, a third field effect transistor M3, a fourth field effect transistor M4 and a fifth field effect transistor M5. The source electrode connecting ends of the first field effect tube M1 and the second field effect tube M2 are grounded through a fifth field effect tube M5. The second-stage amplification unit A2 also comprises four field effect transistors, and is divided into a sixth field effect transistor M6, a seventh field effect transistor M7, an eighth field effect transistor M8 and a ninth field effect transistor M9. Since the present invention does not improve the original two-stage amplifying unit in the common mode feedback circuit, the specific connection mode and the working principle thereof are not discussed further herein.
Referring to fig. 1-2, the common mode feedback circuit of the present invention further includes a common mode level detection unit, a common mode feedback amplifier, and an amplifier load current source.
In the common mode feedback circuit of the invention, the common mode level detection unit is used for acquiring two differential output signals of the second-stage amplification unit A2 and acquiring a common mode signal according to the two differential output signals.
Specifically, the common mode level detection unit is composed of a first resistor R1 and a second resistor R2. One end of the first resistor R1 is connected with one differential output end of the second-stage amplification unit A2, and one end of the second resistor R2 is connected with the other differential output end of the second-stage amplification unit A2; the other end of the first resistor R1 is connected with the other end of the second resistor R2, and the other end of the first resistor R1 is used as an output end of the common mode level detection unit and outputs a common mode signal. The corresponding differential output signals are divided by the first resistor R1 and the second resistor R2, so that the common mode signal is one half of the sum of the two differential output signals of the second stage amplification unit A2. That is, the common mode signal of the present embodiment is:
in the above formula, the first and second carbon atoms are,
is a common mode signal;
is a differential output signal of the second stage amplification unit A2;
is the other differential output signal of the second stage amplification unit A2.
The common mode feedback amplifier is used for comparing the level of the common mode signal with a reference level to obtain an error signal, and feeding the error signal back to the output node of the first-stage amplification unit A1 so as to adjust the current of the output node of the first-stage amplification unit A1. That is, in the present embodiment, the common mode feedback amplifier and the second-stage amplification unit form a common mode feedback loop in which only two stages of amplification circuits exist. Compared with the traditional common mode feedback loop with the three-stage amplification circuit, the output end of the common mode feedback amplifier of the embodiment is connected with the output node of the first-stage amplification unit A1, the common mode feedback loop is formed in a current feedback mode, the stage number of the amplification circuit in the common mode feedback loop is reduced, the pole number in the common mode feedback loop is reduced, and the stability of the common mode feedback loop is improved.
As for the manner of current adjustment to the output node of the first-stage amplification unit A1, specifically,
if the level of the common-mode signal is higher than the reference level, the common-mode feedback amplifier increases the current injected into the first-stage amplification unit A1, so that the potential of the output node of the first-stage amplification unit A1 is raised, and the purpose of reducing the common-mode level between the two differential output signals is achieved.
If the level of the common mode signal is lower than the reference level, the common mode feedback amplifier reduces the current injected into the first-stage amplification unit A1, so that the potential of the output node of the first-stage amplification unit A1 is reduced, and the common mode level between the two differential output signals is increased.
In the present embodiment, the common mode feedback amplifier is composed of an eleventh field effect transistor M11, a fifteenth field effect transistor M15, and a tenth field effect transistor M10. The common mode signal is input to the gate of the eleventh fet M11, and a PMOS current source bias voltage is input to the drain of the eleventh fet M11. As shown in fig. 2, the PMOS current source bias voltage is denoted as VPM. Reference levels are input to the grids of the fifteenth field-effect tube M15 and the tenth field-effect tube M10, and the drain electrode of the fifteenth field-effect tube M15 and the drain electrode of the tenth field-effect tube M10 are respectively connected with the two output nodes of the first-stage amplification unit A1 in a one-to-one correspondence manner. The sources of the eleventh field effect transistor M11, the fifteenth field effect transistor M15 and the tenth field effect transistor M10 are all grounded.
In a further improvement of the common mode feedback amplifier, the sources of the eleventh field effect transistor M11, the fifteenth field effect transistor M15 and the tenth field effect transistor M10 are connected with each other, and the source connection terminals of the eleventh field effect transistor M11, the fifteenth field effect transistor M15 and the tenth field effect transistor M10 are grounded through a current source.
Specifically, the current source is a sixteenth field effect transistor M16; the drain of the sixteenth fet M16 is connected to the source connection terminals of the eleventh fet M11, the fifteenth fet M15, and the tenth fet M10, and an NMOS current source bias voltage is input to the gate of the sixteenth fet M16. As shown in fig. 2, the NMOS current source bias voltage is denoted Vbn. The source of the sixteenth fet M16 is grounded.
The amplifier load current source is used for providing current for the output node of the first-stage amplification unit A1.
Specifically, the amplifier load current source is composed of a thirteenth field effect transistor M13, a fourteenth field effect transistor M14, and a twelfth field effect transistor M12. The gates of the thirteenth fet M13, the fourteenth fet M14 and the twelfth fet M12 all input a signal VPM. The source electrodes of the thirteenth field effect transistor M13, the fourteenth field effect transistor M14 and the twelfth field effect transistor M12 are mutually connected and are used as the power supply input end of the load current source of the amplifier; the drain of the thirteenth field effect transistor M13 is connected to the gate thereof, the drain of the fourteenth field effect transistor M14 is connected to one output node of the first-stage amplification unit A1, and the drain of the twelfth field effect transistor M12 is connected to the other output node of the first-stage amplification unit A1.
The improved common-mode gain of the common-mode feedback circuit will be analyzed below.
The output end of the current mode common mode feedback amplifier of the improved common mode feedback circuit is connected with the output node of the first-stage amplifying unit A1, the schematic diagram of the common mode feedback circuit based on the invention is shown in FIG. 2, and the half-side equivalent circuit is shown in FIG. 3. A small signal model for calculating the common-mode response of the operational amplifier can be obtained according to the half-edge equivalent circuit, as shown in fig. 4. The common-mode gain of the common-mode feedback circuit according to the invention can be obtained from a small-signal model as follows:
in the formula (I), the compound is shown in the specification,
is the common mode gain;
is transconductance of a first field effect tube M1 in a first stage amplifying unit A1;
is the transconductance of a sixth field effect transistor M6 in the second-stage amplification unit A2;
outputting impedance for a fifth field effect transistor M5 in the first-stage amplification unit A1;
the output impedance of the third field effect transistor M3 or the fourth field effect transistor M4 in the first-stage amplification unit A1 is obtained;
the output impedance of the eighth field effect transistor M8 or the ninth field effect transistor M9 in the second-stage amplification unit A2;
is the output impedance of the twelfth fet M12.
If the common mode feedback amplifier is not added to the common mode feedback circuit, the common mode gain is as follows:
according to the formula, the compound has the advantages of,
<
. Therefore, the common mode feedback amplifier not only has the function of stabilizing the static operating point of the operational amplifier, but also can improve the common mode rejection capability of the operational amplifier.
As shown in fig. 5, 6a, 6 b. From simulation results, the transistor size of the common mode feedback circuit is reasonably designed, so that the improved common mode feedback circuit can obtain lower common mode gain and higher feedback loop phase margin under the same power consumption. In addition, simulation results also show that the common-mode gain of the invention is far smaller than that of the traditional voltage common-mode feedback scheme, and the phase margin of the common-mode feedback loop is far larger than that of the traditional voltage common-mode feedback scheme.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that those skilled in the art can make various changes and modifications without departing from the structure of the invention, which will not affect the effect of the invention and the practicability of the patent.
Claims (9)
1. A common mode feedback circuit of a fully differential operational amplifier comprises a first-stage amplification unit A1 and a second-stage amplification unit A2; the output node of the first-stage amplification unit A1 is connected with the input node of the second-stage amplification unit A2; it is characterized by also comprising:
the common mode level detection unit is used for acquiring two differential output signals of the second-stage amplification unit A2 and acquiring a common mode signal according to the two differential output signals;
the common-mode feedback amplifier is used for comparing the level of the common-mode signal with a reference level to obtain an error signal, and feeding the error signal back to an output node of the first-stage amplification unit A1 so as to adjust the current of the output node of the first-stage amplification unit A1;
the common mode feedback amplifier consists of an eleventh field effect tube M11, a fifteenth field effect tube M15 and a tenth field effect tube M10; a common mode signal is input to the gate of the eleventh field effect transistor M11, and a PMOS current source bias voltage is input to the drain of the eleventh field effect transistor M11; reference levels are input to the grids of the fifteenth field-effect tube M15 and the tenth field-effect tube M10, and the drain electrode of the fifteenth field-effect tube M15 and the drain electrode of the tenth field-effect tube M10 are respectively connected with two output nodes of the first-stage amplification unit A1 in a one-to-one correspondence manner; the sources of the eleventh field effect transistor M11, the fifteenth field effect transistor M15 and the tenth field effect transistor M10 are all grounded.
2. The common-mode feedback circuit of a fully differential operational amplifier as claimed in claim 1, wherein the current at the output node of the first stage A1 is adjusted in such a way that,
if the level of the common-mode signal is higher than the reference level, the common-mode feedback amplifier increases the current injected into the first-stage amplification unit A1 so as to raise the potential of the output node of the first-stage amplification unit A1;
if the level of the common mode signal is lower than the reference level, the common mode feedback amplifier reduces the current injected into the first-stage amplification unit A1, so that the potential of the output node of the first-stage amplification unit A1 is reduced.
3. The common-mode feedback circuit of the fully differential operational amplifier according to claim 1, wherein the sources of the eleventh fet M11, the fifteenth fet M15 and the tenth fet M10 are connected to each other, and the source connection terminals of the eleventh fet M11, the fifteenth fet M15 and the tenth fet M10 are grounded through a current source.
4. The common-mode feedback circuit of a fully differential operational amplifier as claimed in claim 3, wherein the current source is a sixteenth fet M16; the drain electrode of the sixteenth field effect transistor M16 is connected with the source electrode connecting ends of the eleventh field effect transistor M11, the fifteenth field effect transistor M15 and the tenth field effect transistor M10, and an NMOS current source bias voltage is input to the gate electrode of the sixteenth field effect transistor M16; the source of the sixteenth fet M16 is grounded.
5. The common-mode feedback circuit of a fully-differential operational amplifier as claimed in claim 1 or 2, wherein the common-mode signal is one half of the sum of two differential output signals of the second stage amplification unit A2.
6. The common-mode feedback circuit of the fully-differential operational amplifier according to claim 5, wherein the common-mode level detecting unit is composed of a first resistor R1 and a second resistor R2; one end of the first resistor R1 is connected with one differential output end of the second-stage amplification unit A2, and one end of the second resistor R2 is connected with the other differential output end of the second-stage amplification unit A2; the other end of the first resistor R1 is connected with the other end of the second resistor R2, and the other end of the first resistor R1 is used as the output end of the common mode level detection unit to output the common mode signal.
7. The common-mode feedback circuit of claim 1, further comprising an amplifier load current source for supplying current to the output node of the first stage A1 amplifying unit.
8. The common-mode feedback circuit of a fully differential operational amplifier as claimed in claim 7, wherein the amplifier load current source is composed of a thirteenth fet M13, a fourteenth fet M14 and a twelfth fet M12; the grid electrodes of the thirteenth field effect transistor M13, the fourteenth field effect transistor M14 and the twelfth field effect transistor M12 are all inputted with a PMOS current source bias voltage; the source electrodes of the thirteenth field effect transistor M13, the fourteenth field effect transistor M14 and the twelfth field effect transistor M12 are connected with each other and serve as the power supply input end of the amplifier load current source; the drain of the thirteenth field effect transistor M13 is connected to the gate thereof, the drain of the fourteenth field effect transistor M14 is connected to one output node of the first-stage amplification unit A1, and the drain of the twelfth field effect transistor M12 is connected to the other output node of the first-stage amplification unit A1.
9. The common-mode feedback circuit of a fully-differential operational amplifier according to claim 8, wherein the common-mode gain of the common-mode feedback circuit is,
in the formula (I), the compound is shown in the specification,
is the common mode gain;
is transconductance of a first field effect tube M1 in a first stage amplifying unit A1;
is the transconductance of a sixth field effect transistor M6 in the second-stage amplification unit A2;
outputting impedance for a fifth field effect transistor M5 in the first-stage amplification unit A1;
the output impedance of the third field effect transistor M3 or the fourth field effect transistor M4 in the first-stage amplification unit A1;
the output impedance of the eighth field effect transistor M8 or the ninth field effect transistor M9 in the second-stage amplification unit A2;
is the output impedance of the twelfth fet M12.
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