Analog amplifier for recovering abnormal operation of common mode feedback

Abstract

An analog amplifier for recovering an abnormal operation of a common-mode feedback is provided. An analog variable amplifier includes a first input transistor and a second input transistor, a first output transistor and a second output transistor, a third transistor and a fourth transistor, a first current source, a fifth transistor and a sixth transistor, and a second current source. The first input transistor and the second input transistor amplify a bias current depending on a magnitude of a first input voltage and a second input voltage. The first output transistor and the second output transistor output the amplified bias current. The third transistor and the fourth transistor receive an output voltage of the first output transistor as an input and amplifying the received output voltage. The first current source provides a predetermined current between the first output transistor and the third transistor.

Claims

1. An analog variable amplifier comprising: at least one input resistor; at least one feedback resistor; and a differential amplifier comprising a first circuit and a second circuit that have symmetric structures, wherein the first circuit comprises: a first transistor coupled to a first input terminal of the differential amplifier and configured to amplify a bias current depending on a magnitude of a first input voltage, a second transistor configured to generate a voltage signal corresponding to the amplified bias current, a third transistor coupled to a first output terminal of the differential amplifier and configured to amplify the voltage signal and output the amplified voltage signal, and a current source configured to provide a current to a node between a drain of the second transistor and a gate of the third transistor, so as to increase a voltage of the node while the first transistor is turned off, wherein, if the at least one input resistor is greater than the at least one feedback resistor by a predetermined ratio, the first transistor is off, and wherein the first transistor, the second transistor, and the third transistor are arranged so that the voltage of the node decreases and an operation point of an output voltage at the first output terminal increases if an input bias voltage of the second transistor increases or decreases, while the first transistor is off.

2. An operational amplifier comprising: a first stage and a second stage of the operational amplifier; and a current source disposed between the first stage and the second stage, so as to increase a voltage of a node while a first input transistor of the first stage is turned off and to prevent a voltage of an input transistor of the operational amplifier from dropping down to a threshold or less, wherein the current source is configured to provide a predetermined current between a drain terminal of an output transistor of the first stage and a gate terminal of a transistor of the second stage, and wherein, if an input bias voltage of a first output transistor of the first stage increases, an output voltage of a third transistor of the second stage reduces and an output voltage of the first output transistor of the first stage increases, so that an operation point of an output voltage increases at an output terminal.

3. The operational amplifier of claim 2, wherein the first stage comprises: a first input transistor and a second input transistor configured to amplify a bias current depending on a magnitude of a first input voltage and a second input voltage; and the first output transistor and a second output transistor configured to output the amplified bias current to form a differential amplifier, and wherein the second stage comprises: the third transistor and a fourth transistor configured to receive an output voltage of the first output transistor as an input and to amplify the received output voltage; and a fifth transistor and a sixth transistor configured to receive an output voltage of the second output transistor as an input and to amplify the received output voltage, and wherein the current source one of provides a predetermined current between the first output transistor and the third transistor and provides a predetermined current between the second output transistor and the fifth transistor.

4. The operational amplifier of claim 3, wherein, if an input bias voltage of the first output transistor reduces, an output voltage of the third transistor increases and an output voltage of the first output transistor reduces, so that an operation point of an output voltage reduces at an output terminal.

5. The operational amplifier of claim 3, wherein the bias current is greater than a current magnitude of the current source.

6. An analog variable amplifier comprising: a input resistor; a feedback resistor; and a differential amplifier comprising: a first input terminal, a second input terminal, a first input transistor, a second input transistor, a first output transistor, a second output transistor, a first current source, a second current source, a first output terminal, and a second output terminal, wherein the first current source is configured to provide a predetermined current to a first node between a drain of the first input transistor and a drain of the first output transistor, so as to increase a voltage of the first node while the first transistor is turned off, wherein the second current source is configured to provide a predetermined current to a second node between a drain of the second input transistor and a drain of the second output transistor, so as to increase a voltage of the second node while the second input transistor is turned off, wherein, if the input resistor is greater than the feedback resistor by a predetermined ratio, the first input transistor is off, and wherein the first input transistor and the first output transistor are arranged so that the voltage of the first node decreases and an operation point of an output voltage at the first output terminal increases if an input bias voltage of the first output transistor increases or decreases, while the first transistor is off.

7. The analog variable amplifier of claim 6, wherein the predetermined current is less than a bias current of the differential amplifier.

8. The analog variable amplifier of claim 1, wherein the first circuit further comprises a fourth transistor coupled to the third transistor and the first output terminal.

9. The analog variable amplifier of claim 8, wherein the third transistor and the fourth transistor are configured for class A amplification.

10. The analog variable amplifier of claim 1, wherein the bias current is greater than ten (10) times of the current provided by the current source and less than one hundred (100) times of the current provided by the current source.

11. The analog variable amplifier of claim 1, wherein a source of the first transistor is coupled to a source of the second transistor, wherein the source of the second transistor is coupled to the gate of the third transistor, and wherein a drain of the third transistor and a drain of the second transistor are grounded.

12. The analog variable amplifier of claim 1, wherein the at least one input resistor is coupled to a gate of the first transistor and the input terminal.

13. The analog variable amplifier of claim 1, wherein the at least one input resistor comprises at least one variable resistor.

14. The analog variable amplifier of claim 1, wherein the at least one feedback resistor is coupled to a gate of the first transistor and the first output terminal.

15. The analog variable amplifier of claim 1, wherein the at least one feedback resistor comprises at least one variable resistor.

16. The analog variable amplifier of claim 1, wherein the first transistor is turned off, if a voltage between a gate of the first transistor and a source of the first transistor becomes lower than a first threshold.

17. The analog variable amplifier of claim 16, wherein the current source provides the current to maintain the voltage of the node to be greater than a second threshold while the voltage between the gate of the first transistor and the source of the first transistor is lower than the first threshold.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other aspects, features, and advantages of certain embodiments of the present disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a view illustrating an analog filter structure where an analog variable amplifier is configured in two stages according to the related art;

(3) FIG. 2 is a view illustrating an analog variable amplifier according to an embodiment of the present disclosure;

(4) FIG. 3 is a view illustrating a circuit of an operational amplifier according to an embodiment of the present disclosure; and

(5) FIG. 4 is a view illustrating a cut-off circuit of an operational amplifier according to an embodiment of the present disclosure.

(6) Throughout the drawings, like reference numerals will be understood to refer to like parts, components and structures.

DETAILED DESCRIPTION

(7) The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of various embodiments of the present disclosure as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the various embodiments described herein can be made without departing from the scope and spirit of the present disclosure. In addition, descriptions of well-known functions and constructions may be omitted for clarity and conciseness.

(8) The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the present disclosure. Accordingly, it should be apparent to those skilled in the art that the following description of various embodiments of the present disclosure is provided for illustration purpose only and not for the purpose of limiting the present disclosure as defined by the appended claims and their equivalents.

(9) It is to be understood that the singular forms a, an, and the include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a component surface includes reference to one or more of such surfaces.

(10) The present disclosure relates to an analog amplifier having a wide gain range using one amplification stage by preventing a phenomenon that an input transistor does not operate in the case where it has a very low gain.

(11) Various embodiments of the present disclosure provide an analog amplifier for recovering an abnormal operation of a common-mode feedback.

(12) FIG. 2 illustrates an analog variable amplifier according to an embodiment of the present disclosure.

(13) Referring to FIG. 2, the analog variable amplifier has an analog filter structure configured in a first stage. The analog variable amplifier includes an operational amplifier 200, an input resistor R.sub.1 201, and a feedback resistor R.sub.2 202. Here, the input resistor R.sub.1 201 and the feedback resistor R.sub.2 202 may be at least one variable resistor.

(14) Depending on the implementation, the analog variable amplifier may include one variable resistor and one variable capacitor instead of the operational amplifier and the two variable resistors R.sub.1 and R.sub.2 (201, 202).

(15) In an analog variable amplifier configured in the first stage, a gain is determined by a ratio of the input resistor R.sub.1 to the feedback resistor R.sub.2. Assuming a transfer function of the operational amplifier 200 is A(s), the gain is given below by Equation (2). In an ideal operational amplifier, a gain has a value of R.sub.2/R.sub.1 in its infinity.

(16) $\begin{array}{cc}\mathrm{Gain}=\frac{R_{2}A\left(s\right)}{R_1+R_2-R_{1}A\left(s\right)}& \mathrm{Equation}\left(2\right)\end{array}$

(17) In Equation (2), a variable gain amplifier is configured by controlling a value of the input resistor R.sub.1 or the feedback resistor R.sub.2. In the analog variable amplifier 200, when R.sub.1 is greater than R.sub.2, the variable gain amplifier performs attenuation. When R.sub.1 is less than R.sub.2, the variable gain amplifier performs amplification.

(18) The operational amplifier 200 used inside the analog variable amplifier may have a Complementary Metal-Oxide Semiconductor (CMOS) structure as illustrated in FIG. 3.

(19) FIG. 3 is a view illustrating a circuit of an operational amplifier according to an embodiment of the present disclosure.

(20) Referring to FIG. 3, the operational amplifier 200 is a differential amplifier and includes a common source amplifier configured in two stages. For ease in explanation, the operational amplifier may be divided into half circuits 320 and 330 having a symmetric structure.

(21) In the first common source amplifier circuit 320 and the second common source amplifier circuit 330, a P-type Metal-Oxide Semiconductor (PMOS) bias voltage and an N-type Metal-Oxide Semiconductor (NMOS) bias voltage are supplied to VP (331, 321) and VN (324, 334), respectively. For convenience in description, a common-mode feedback circuit and compensation elements have been omitted.

(22) The differential amplifier includes a first input terminal (i.e., gate of CMOS transistor 302) and a second input terminal (i.e., gate of CMOS transistor 303) to receive two voltage signals, that is, a first voltage and a second voltage. Here, the first voltage and the second voltage may be a positive voltage and a negative voltage, respectively. A bias current input from a current source 301 is amplified depending on each input voltage magnitude and forms an amplified voltage at output resistor terminals 324 and 334, first output transistor and second output transistor, respectively, and is input to an input terminal of a third transistor and a fifth transistor (i.e., gate of CMOS transistors 322 and 332, respectively) of the second common-source amplifier. Also, this voltage is amplified by a fourth transistor 321 and the third transistor 322, or a sixth transistor 331 and the fifth transistor 332 forming the second amplifier, and then is output via output terminals 323 and 333.

(23) In the general case, a voltage differentially input to the input terminals 302 and 303 is amplified by a designed gain and output as a voltage form at the output terminals 323 and 333. A common component of this signal always maintains a constant voltage via a common-mode feedback circuit. That is, a common voltage which is an average of the voltages of the positive output terminal 323 and the negative output terminal 333, and may operate in the normal range. For example, in detail, when the common voltage of the output terminals 323 and 333 is raised simultaneously by an environmental factor (e.g., temperature, operation voltage, etc.) and becomes a voltage close to a voltage source VDD, a VN value (that is, bias voltage provided to gate of CMOS transistor 324 and gate of CMOS transistor 334) is lowered by the common-mode feedback circuit, which raises the voltage of a node A whose phase is opposite to that of the VN, so that the voltage of output terminals 323 and 333 whose phase is opposite to that of the node A is lowered consequently and balance is maintained. In contrast, when the common voltage of the output terminals lowers simultaneously and becomes a voltage close to the ground, a VN voltage is raised by the common-mode feedback circuit, which lowers the voltage of the node A, so that the voltage of the output terminals 323 and 333 is raised and balance is restored.

(24) FIG. 4 is a view illustrating a cut-off circuit of an operational amplifier according to an embodiment of the present disclosure.

(25) FIG. 4 illustrates the half of the differential amplifier 300 and the second common source amplifier circuit 330 in the operational amplifier of FIG. 3. Also, FIG. 4 additionally illustrates the input resistor R.sub.1 and the feedback resistor R.sub.2 of FIG. 2.

(26) In the case where set attenuation of the differential amplifier is large and so the input resistor R.sub.1 is considerably greater than the feedback resistor R.sub.2, a phenomenon that the input transistor 303 of the differential amplifier 300 does not operate due to a resistance ratio when one amplification stage processes a signal may occur. For example, in the case where a gain is 20 dB, a signal is attenuated by 1/10, and R.sub.1 should be ten times greater than R.sub.2. Likewise, in the case where a gain is 40 dB, R.sub.1 becomes a hundred times greater than R.sub.2. That is, when an attenuation range is widened, a ratio of R.sub.1 to R.sub.2 increases very much. In the case where R.sub.1 becomes excessively larger than R.sub.2 as described above, a possibility occurs that the input transistor 303 of the differential amplifier 300 in the analog variable amplifier does not operate.

(27) In a case of the general operation, a balance point is restored by controlling the voltage of the VN via the common-mode feedback circuit as described above.

(28) For example, when the VN which is a bias voltage of the first stage is raised (410) by an environmental factor (e.g., temperature, operation voltage, etc.) and becomes a voltage close to a voltage source VDD, the voltage of the node A is lowered by the common-mode feedback circuit (400, 412). When the voltage of the node A is lowered, an output voltage Vo may be raised (414). To maintain a balance, the voltage of the node A whose phase is opposite to that of the VN is raised (402), and consequently the voltage of the output terminal 333 whose phase is opposite to that of the node A is lowered and a balance may be maintained (404). In contrast, when the common voltage of the output terminal is lowered simultaneously (404) and becomes a voltage close to the ground, the VN value is raised by the common-mode feedback circuit (410), which lowers the voltage of the node A (412), so that the voltage of the output terminal 333 is raised and a balance is restored.

(29) However, in the case where the input resistor R.sub.1 is considerably larger than the feedback resistor R.sub.2, compensation by the common-mode feedback circuit does not operate, which will be described below.

(30) A voltage supplied to a node VG which is the virtual ground is given below by Equation (3) via a resistance divider. When R.sub.1 becomes excessively larger than R.sub.2, when V.sub.0 becomes a voltage close to VDD, the voltage close to the voltage source VDD is applied to the node VG which is a gate of the input transistors 302 and 303 of the differential amplifier.

(31) $\begin{array}{cc}{V}_{\mathrm{VG}}=\frac{R_1{V}_{0-}+R_2{V}_{\mathrm{in}}}{R_1+R_2}& \mathrm{Equation}\left(3\right)\end{array}$

(32) At this point, a voltage Vgs applied to the gate and the source of the input NMOS transistor 303 of the differential amplifier 300 becomes lower than a threshold voltage of the input NMOS transistor 303, so that the input transistor is turned off. Accordingly, a current does not flow through the first PMOS transistor 332, so that even when the VN voltage of the second PMOS transistor 334 is lowered, the voltage of the node A is not restored. This phenomenon conspicuously occurs in the case where a supply voltage is low and temperature is low, and direct relation to yield occurs. Therefore, to avoid this phenomenon, the related-art connects amplifier stages having a gain whose operation range is small in series and uses the same as illustrated in FIG. 1. As described above, the related-art has disadvantages of consuming much power and occupying a large area.

(33) Therefore, current sources 340 and 350 allowing a fine current to flow through the node A are provided inside the operational amplifier.

(34) The current sources I.sub.leak supplied to the node A are added to the first common-source amplifier circuit 320 and the second common-source amplifier circuit 330, and 1/10 1/100 of the bias current I.sub.B 301 supplied to the main amplifier is appropriate for the magnitude of the current. When the added fine current source is too large, a mismatch increases at both terminals and the entire power consumption may increase. However, in the present disclosure, the magnitude of the current source supplied to the node A is not limited.

(35) As described above, under an environment where the output voltage V.sub.0 is raised, even in the state where a node VG approaches VDD due to a resistance divider and the input transistor 303 of the differential amplifier does not operate, a current is supplied to the node A via the current source I.sub.leak and the voltage of the node A is raised again under control of the second PMOS transistor 334 and consequently the operation range of the output voltage V.sub.0 is lowered.

(36) Though the present disclosure has exemplarily described a common-source amplifier using a CMOS transistor, it is applicable to a common-gate amplifier and a common-drain amplifier, and the analog amplifier of FIG. 4 may be configured using a Bi-polar Junction Transistor (BJT) instead of a CMOS transistor.

(37) As described above, the present disclosure processes an analog signal using a single stage or a smaller number of stages via an operational amplifier circuit that may apply amplification and attenuation of an analog signal over a wide range simultaneously, so that power consumption may be reduced, a circuit area may be reduced, and so manufacturing costs may be saved.

(38) Also, the present disclosure may increase reliability of a circuit operation and also expect yield improvement by including a current source supplying a fine current inside the circuit to prevent the operation of an input transistor from being cut-off. Particularly, the present disclosure has an effect of securing operation reliability of an operational amplifier under a poor environment such as a low voltage and low temperature.

(39) While the present disclosure has been shown and described with reference to various embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present disclosure as defined by the appended claims and their equivalents.