Patent Application
20090045870
1. Field of the Invention
The present invention relates to a reference voltage circuit for generating a constant reference voltage.
2. Description of the Related Art
The ED type reference voltage circuit includes a depletion NMOS transistor 84 and an NMOS transistor 85. The gate and source of the depletion NMOS transistor 84 are connected with the reference voltage output terminal 83 and the drain thereof is connected with the power supply terminal 81. The gate and drain of the NMOS transistor 85 are connected with the reference voltage output terminal 83 and the source thereof is connected with the ground terminal 82 (see, for example, JP 04-065546 B (
According to the ED type reference voltage circuit, even when a power supply voltage of the power supply terminal 81 varies, a reference voltage of the ED type reference voltage circuit 86 does not easily vary while each of the NMOS transistors operates in saturation.
Assume that a mutual conductance of the NMOS transistor 85 is expressed by gm85 and an output resistance of the depletion NMOS transistor 84 is expressed by ro84. In this case, a power supply rejection ratio (ratio between variation in power supply voltage and variation in reference voltage due to variation in power supply voltage) PSRRLF in the reference voltage output terminal 83 at low frequency is calculated by the following expression.
PSRRLF=gm85×ro84 (2)
However, because of, for example, a channel length modulation effect of the depletion NMOS transistor 84, when the power supply voltage of the power supply terminal 81 varies, the reference voltage of the ED type reference voltage circuit 86 also varies. Therefore, the power supply rejection ratio PSRRLF does not become larger.
In order to take measures against such a situation, there is a case where a cascode circuit is added to the power supply terminal 81.
This reference voltage circuit includes a bias voltage supplying circuit 89, an NMOS transistor 88, and the ED type reference voltage circuit 86. The gate of the NMOS transistor 88 is connected with the bias voltage supplying circuit 89, the source thereof is connected with the ED type reference voltage circuit 86, and the drain thereof is connected with the power supply terminal 87.
According to the reference voltage circuit, even when a power supply voltage of the power supply terminal 87 varies, the reference voltage of the ED type reference voltage circuit 86 does not easily vary because the NMOS transistor 88 operates such that the power supply voltage of the power supply terminal 81 is constant.
Assume that a mutual conductance of the NMOS transistor 88 is expressed by gm88, a substrate bias mutual conductance of the NMOS transistor 88 is expressed by gmb88, and an output resistance of the NMOS transistor 88 is expressed by ro88. In this case, the power supply rejection ratio PSRRLF in the reference voltage output terminal 83 at low frequency is calculated by the following expression.
PSRRLF={(gm88+gmb88)×ro88}×(gm85×ro84) (3)
In other words, the power supply rejection ratio PSRRLF is multiplied by “(gm88+gmb88)×ro88”.
An application example of the reference voltage circuit will be described.
This reference voltage circuit includes depletion NMOS transistors 91 to 93, an NMOS transistor 94, the reference voltage output terminal 83, and the ED type reference voltage circuit 86. The gate of the depletion NMOS transistor 91 is connected with the source of the depletion NMOS transistor 92, the source thereof is connected with the ED type reference voltage circuit 86, and the drain thereof is connected with the power supply terminal 87. The gate of the depletion NMOS transistor 92 is connected with the source of the depletion NMOS transistor 91, the source thereof is connected with the drain of the depletion NMOS transistor 93, and the drain thereof is connected with the power supply terminal 87. The gate of the depletion NMOS transistor 93 is connected with the source thereof. The gate of the NMOS transistor 94 is connected with the drain thereof and the source of the depletion NMOS transistor 93. The source of the NMOS transistor 94 is connected with the ground terminal 82 (see, for example, JP 2003-295957 A (FIG. 1)).
According to the reference voltage circuit, even when the power supply voltage of the power supply terminal 87 varies, the reference voltage of the ED type reference voltage circuit 86 does not easily vary because the depletion NMOS transistor 91 operates such that the power supply voltage of the power supply terminal 81 is constant.
When the depletion NMOS transistor 92 operates such that a gate voltage of the depletion NMOS transistor 91 is equal to a source voltage thereof, a mutual conductance of the depletion NMOS transistor 91 does not contribute to the power supply rejection ratio. Therefore, assume that a substrate bias mutual conductance of the depletion NMOS transistor 91 is expressed by gmb91 and an output resistance of the depletion NMOS transistor 91 is expressed by ro91. In this case, the power supply rejection ratio PSRRLF in the reference voltage output terminal 83 at low frequency is calculated by the following expression.
PSRRLF=(gmb91×ro91)×(gm85×ro84) (4)
In other words, the power supply rejection ratio PSRRLF is multiplied by “gmb91×ro91”.
However, when the power supply voltage of the power supply terminal 87 lowers and thus the depletion NMOS transistor 91 operates in non-saturation, the output resistance ro91 of the depletion NMOS transistor 91 becomes smaller to reduce the power supply rejection ratio PSRRLF.
The present invention has been made in view of such a problem. An object of the present invention is to provide a reference voltage circuit in which a power supply rejection ratio is large even when a power supply voltage is low.
In order to solve the above-mentioned problem, the present invention provides A reference voltage circuit for generating a constant reference voltage, comprising: a power supply terminal; a reference voltage output terminal; an ED type reference voltage circuit including a depletion type transistor and an enhancement type transistor for outputting a reference voltage to the reference voltage output terminal; a control transistor for supplying an internal power supply voltage based on a power supply voltage of the power supply terminal to the ED type reference voltage circuit; and a differential amplifier circuit for inputting the reference voltage and the internal power supply voltage, and outputting a control signal to the control transistor, wherein the differential amplifier circuit has an input offset voltage to the reference voltage for operating the depletion type transistor in saturation, and controls the control transistor so that the internal power supply voltage becomes a constant voltage.
Besides, in order to solve the above-mentioned problem, the present invention provides A reference voltage circuit for generating a constant reference voltage, comprising: a power supply terminal; a reference voltage output terminal; a constant voltage circuit including a junction type transistor and a resistor for outputting a reference voltage to the reference voltage output terminal; a control transistor for supplying an internal power supply voltage based on a power supply voltage of the power supply terminal to the constant voltage circuit; and a differential amplifier circuit for inputting the reference voltage and the internal power supply voltage, and outputting a control signal to the control transistor, wherein the differential amplifier circuit has an input offset voltage to the reference voltage for operating the junction type transistor in saturation, and controls the control transistor so that the internal power supply voltage becomes a constant voltage.
According to the present invention, even in a case where the power supply voltage of the power supply terminal becomes lower and thus the control transistor operates in non-saturation, when a gain of the differential amplifier circuit is large, the power supply rejection ratio is also large.
In the accompanying drawings:
Hereinafter, a concept and embodiments of the present invention will be described with reference to the accompanying drawings.
A conceptual structure of a reference voltage circuit for generating a constant reference voltage will be described.
The reference voltage circuit of the present invention includes a constant voltage circuit 50, a differential amplifier circuit 60, and a control transistor 70.
The constant voltage circuit 50 includes an input terminal connected with the internal power supply terminal 40 and an output terminal connected with the reference voltage output terminal 30. The differential amplifier circuit 60 includes a non-inverted input terminal connected with the reference voltage output terminal 30, an inverted input terminal connected with the internal power supply terminal 40, and an output terminal connected with an input terminal of the control transistor 70. An output terminal of the control transistor 70 is connected with the internal power supply terminal 40.
The differential amplifier circuit 60 has a predetermined gain and an input offset voltage. The differential amplifier circuit 60 and the control transistor 70 serve as a negative feedback circuit for the internal power supply terminal 40.
Next, a conceptual operation of the reference voltage circuit will be described.
The constant voltage circuit 50 outputs, to the reference voltage output terminal 30, the reference voltage based on the power supply voltage of the internal power supply terminal 40. The differential amplifier circuit 60 outputs a control signal to the control transistor 70 based on the power supply voltage of the internal power supply terminal 40 and the reference voltage of the Constant voltage circuit 50. The control transistor 70 operates in response to the control signal to adjust the power supply voltage of the internal power supply terminal 40 to a constant value.
Next, a structure of a reference voltage circuit according to a first embodiment will be described.
An ED type reference voltage circuit as the constant voltage circuit 50 includes a depletion NMOS transistor 51 and an NMOS transistor 52. The control transistor 70 includes an NMOS transistor 71.
The gate and source of the depletion NMOS transistor 51 are connected with the reference voltage output terminal 30, the drain thereof is connected with the internal power supply terminal 40, and the back gate thereof is connected with the ground terminal 20. The gate and drain of the NMOS transistor 52 are connected with the reference voltage output terminal 30, the source thereof is connected with the ground terminal 20, and the back gate thereof is connected with the ground terminal 20. The gate of the NMOS transistor 71 is connected with the output terminal of the differential amplifier circuit 60, the source thereof is connected with the internal power supply terminal 40, the drain thereof is connected with the power supply terminal 10, and the back gate thereof is connected with the ground terminal 20.
The non-inverted input terminal and inverted input terminal of the differential amplifier circuit 60 are imaginarily short-circuited. The differential amplifier circuit 60 has the predetermined gain and the input offset voltage for operating the depletion NMOS transistor 51 in saturation. Because of the input offset voltage, a source-drain voltage of the depletion NMOS transistor 51 becomes equal to or larger than a saturation voltage at which the depletion NMOS transistor 51 can operate in saturation, and hence, the depletion NMOS transistor 51 operates in saturation. In other words, in view of circuit design, the input offset voltage is set to a value equal to or larger than the saturation voltage. The differential amplifier circuit 60 and the NMOS transistor 71 serve as the negative feedback circuit for the internal power supply terminal 40. Because of the negative feedback circuit, the apparent output resistance of the NMOS transistor 71 increases to a value obtained by being multiplied by the gain of the differential amplifier circuit 60.
Assume that a mutual conductance of the NMOS transistor 71 is expressed by gm71, a substrate bias mutual conductance of the NMOS transistor 71 is expressed by gmb71, the gain of the differential amplifier circuit 60 is expressed by Ao, the output resistance of the NMOS transistor 71 is expressed by ro71, a mutual conductance of the NMOS transistor 52 is expressed by gm52, and an output resistance of the depletion NMOS transistor 51 is expressed by ro51. In this case, the power supply rejection ratio PSRRLF in the reference voltage output terminal 30 at low frequency is calculated by the following expression and becomes larger than a conventional power supply rejection ratio.
PSRRLF=[(gm71+gmb71)×Ao×ro71]×(gm52×ro51) (1)
Next, an operation of the reference voltage circuit according to the first embodiment will be described.
When the power supply voltage of the reference voltage circuit is applied to the power supply terminal 10, the power supply voltage of the Constant voltage circuit 50 is generated in the internal power supply terminal 40 to generate the reference voltage in the reference voltage output terminal 30. The power supply voltage of the Constant voltage circuit 50 and the reference voltage of the Constant voltage circuit 50 are input to the differential amplifier circuit 60 to be compared with each other by the differential amplifier circuit 60. The differential amplifier circuit 60 operates such that the power supply voltage of the Constant voltage circuit 50 is equal to a voltage obtained by adding the input offset voltage to the reference voltage of the Constant voltage circuit 50. Therefore, a gate voltage of the NMOS transistor 71 is controlled such that the power supply voltage of the Constant voltage circuit 50 is constant. The NMOS transistor 71 operates to output the constant power supply voltage of the Constant voltage circuit 50 to the internal power supply terminal 40 based on the gate voltage of the NMOS transistor 71 and the power supply voltage of the power supply terminal 10. To be specific, when the power supply voltage of the Constant voltage circuit 50 is higher than the voltage obtained by adding the input offset voltage to the reference voltage of the Constant voltage circuit 50, the voltage of the output terminal of the differential amplifier circuit 60 (gate of NMOS transistor 71) lowers to turn off the NMOS transistor 71, thereby reducing the power supply voltage of the Constant voltage circuit 50. When the power supply voltage of the Constant voltage circuit 50 is lower than the voltage obtained by adding the input offset voltage to the reference voltage of the Constant voltage circuit 50, the power supply voltage of the Constant voltage circuit 50 increases. In other words, the power supply voltage of the Constant voltage circuit 50 is controlled to a constant value. The depletion NMOS transistor 51 operates to flow a constant current into the NMOS transistor 52 based on the power supply voltage of the Constant voltage circuit 50. The NMOS transistor 52 operates to generate the reference voltage which is a constant voltage in the reference voltage output terminal 30.
Next, the differential amplifier circuit 60 will be described.
An input terminal of a current mirror circuit including PMOS transistors 61 and 62 is connected with the drain of a depletion NMOS transistor 63 and an output terminal thereof is connected with the drain of an NMOS transistor 65. The gate of the depletion NMOS transistor 63 is connected with the non-inverted input terminal of the differential amplifier circuit 60 and the gate of an NMOS transistor 66. The source of the depletion NMOS transistor 63 is connected with the drain of an NMOS transistor 64. The back gate of the depletion NMOS transistor 63 is connected with the ground terminal 20. The gate of the NMOS transistor 64 is connected with the drain thereof and the source thereof is connected with the drain of the NMOS transistor 66. The back gate of the NMOS transistor 64 is connected with the ground terminal 20. The gate of the NMOS transistor 65 is connected with the inverted input terminal of the differential amplifier circuit 60 and the source thereof is connected with the drain of the NMOS transistor 66. The back gate of the NMOS transistor 65 is connected with the ground terminal 20. The source and back gate of the NMOS transistor 66 are connected with the ground terminal 20. The gate of the depletion NMOS transistor 63 corresponds to the non-inverted input terminal of the differential amplifier circuit 60. The gate of the NMOS transistor 65 corresponds to the inverted input terminal of the differential amplifier circuit 60. The output terminal of the current mirror circuit corresponds to the output terminal of the differential amplifier circuit 60.
The NMOS transistor 66 operates as a constant current circuit for maintaining a constant sum of a current flowing into the depletion NMOS transistor 63 and a current flowing into the NMOS transistor 65. A threshold voltage between the non-inverted input terminal and the drain of the NMOS transistor 66 is a sum of a threshold voltage of the depletion NMOS transistor 63 and a threshold voltage of the NMOS transistor 64. A threshold voltage between the inverted input terminal and the drain of the NMOS transistor 66 is a threshold voltage of the NMOS transistor 65. In this case, when the NMOS transistor 64 and the NMOS transistor 65 have the same drive capability, the differential amplifier circuit 60 has a positive input offset voltage based on an absolute value of the threshold voltage of the depletion NMOS transistor 63 at the non-inverted input terminal because the threshold voltage of the depletion NMOS transistor 63 is negative. When the NMOS transistor 64 and the NMOS transistor 65 have different drive capabilities from each other, the positive input offset voltage is adjusted by a difference therebetween. The reference voltage output terminal 30 is connected with the gate of the NMOS transistor 66, and hence, a current based on a current flowing through the Constant voltage circuit 50 flows into the NMOS transistor 66.
In such a case, as is apparent from Expression (1), the mutual conductance gm71 of the NMOS transistor 71, the substrate bias mutual conductance gmb71 of the NMOS transistor 71, the gain Ao of the differential amplifier circuit 60, and the output resistance ro71 of the NMOS transistor 71 contribute to the power supply rejection ratio PSRRLF. Therefore, the power supply rejection ratio PSRRLF becomes larger by the contribution.
Even in a case where the power supply voltage of the power supply terminal 10 becomes lower and thus the NMOS transistor 71 operates in non-saturation to reduce the output resistance ro71 of the NMOS transistor 71, when the gain Ao of the differential amplifier circuit 60 is large, the power supply rejection ratio PSRRLF is also large. Therefore, even when a minimum operating voltage of the reference voltage circuit is low, the power supply rejection ratio PSRRLF can be made larger. In other words, since the gain Ao of the differential amplifier circuit 60 contributes to the power supply rejection ratio PSRRLF, when the gain Ao of the differential amplifier circuit 60 increases, the power supply rejection ratio PSRRLF also becomes larger by the increase.
The reference voltage of the Constant voltage circuit 50 is not determined only based on a voltage applied from the outside and the threshold voltages of the MOS transistors. Since the negative feedback circuit is used, the power supply voltage of the Constant voltage circuit 50 is determined based on the power supply voltage and the reference voltage of the Constant voltage circuit 50, and the reference voltage of the Constant voltage circuit 50 is determined based on the determined power supply voltage. Therefore, the reference voltage of the Constant voltage circuit 50 is adjusted for determination and thus not easily affected by a variation in threshold voltage of the depletion NMOS transistor 51 and a variation in threshold voltage of the NMOS transistor 52 in the Constant voltage circuit 50.
The NMOS transistor 71 is used, but a PMOS transistor (not shown) of a grounded-source circuit may also be used. In this case, a connection point of the non-inverted input terminal of the differential amplifier circuit 60 and a connection point of the inverted input terminal thereof are interchanged to negatively feed back to the internal power supply terminal 40.
The example of the circuit structure of the Constant voltage circuit 50 has been described. The circuit structure disclosed in JP 04-065546 B (not shown) may be employed. In this case, the power supply voltage of the Constant voltage circuit 50 and the reference voltage thereof are input to the differential amplifier circuit 60. The differential amplifier circuit 60 operates such that the power supply voltage of the Constant voltage circuit 50 is equal to the voltage obtained by adding the input offset voltage to the reference voltage of the Constant voltage circuit 50.
In
The gate of the NMOS transistor 66 may be connected with the ground terminal 20 and a depletion NMOS transistor (not shown) may be used instead of the NMOS transistor 66.
The internal circuit structure of the differential amplifier circuit 60 may be modified.
When the differential amplifier circuit 60 shown in
The NMOS transistor 66 operates as the constant current circuit for maintaining a constant sum of a current flowing into the depletion NMOS transistor 63 and a current flowing into the NMOS transistor 65. The threshold voltage between the non-inverted input terminal and the drain of the NMOS transistor 66 is the threshold voltage of the depletion NMOS transistor 63. The threshold voltage between the inverted input terminal and the drain of the NMOS transistor 66 is the threshold voltage of the NMOS transistor 65. In this case, since the threshold voltage of the depletion NMOS transistor 63 is negative, the differential amplifier circuit 60 has a positive input offset voltage based on an absolute value of a difference voltage between the threshold voltage of the depletion NMOS transistor 63 and the threshold voltage of the NMOS transistor 65 at the non-inverted input terminal.
The internal circuit structure of the differential amplifier circuit 60 may be modified.
When the differential amplifier circuit 60 shown in
The NMOS transistor 66 operates as the constant current circuit for maintaining a constant sum of a current flowing into the depletion NMOS transistor 63 and a current flowing into the NMOS transistor 65. The threshold voltage between the non-inverted input terminal and the drain of the NMOS transistor 66 is the threshold voltage of the depletion NMOS transistor 63. The threshold voltage between the inverted input terminal and the drain of the NMOS transistor 66 is a sum of the threshold voltage of the NMOS transistor 65 and the threshold voltage of the NMOS transistor 64c. In this case, since the threshold voltage of the depletion NMOS transistor 63 is negative, the differential amplifier circuit 60 has a positive input offset voltage based on an absolute value of a difference voltage between the threshold voltage of the depletion NMOS transistor 63 and the voltage of the above-mentioned sum at the non-inverted input terminal.
The internal circuit structure of the differential amplifier circuit 60 may be modified.
When the differential amplifier circuit 60 shown in
The NMOS transistor 66 operates as the constant current circuit for maintaining a constant sum of a current flowing into the NMOS transistor 63d and a current flowing into the NMOS transistor 65. The threshold voltage between the non-inverted input terminal and the drain of the NMOS transistor 66 is the threshold voltage of the NMOS transistor 63d. The threshold voltage between the inverted input terminal and the drain of the NMOS transistor 66 is a sum of the threshold voltage of the NMOS transistor 65 and the threshold voltage of the NMOS transistor 64c. In this case, the differential amplifier circuit 60 has a positive input offset voltage based on an absolute value of a difference voltage between the threshold voltage of the NMOS transistor 63d and the voltage of the above-mentioned sum at the non-inverted input terminal.
The internal circuit structure of the differential amplifier circuit 60 may be modified.
When the differential amplifier circuit 60 shown in
The NMOS transistor 66 operates as the constant current circuit for maintaining a constant sum of a current flowing into the NMOS transistor 63e and a current flowing into the NMOS transistor 65e. The threshold voltage between the non-inverted input terminal and the drain of the NMOS transistor 66 is the threshold voltage of the NMOS transistor 63e. The threshold voltage between the inverted input terminal and the drain of the NMOS transistor 66 is the threshold voltage of the NMOS transistor 65e. In this case, the differential amplifier circuit 60 has a positive input offset voltage based on an absolute value of a difference voltage between the threshold voltage of the NMOS transistor 63e and the threshold voltage of the NMOS transistor 65e at the non-inverted input terminal.
Next, a structure of a reference voltage circuit according to a second embodiment will be described.
An ED type reference voltage circuit as the constant voltage circuit 50 is the same circuit as in the first embodiment. The control transistor 70 includes a depletion NMOS transistor 71b.
The gate of the depletion NMOS transistor 71b is connected with the output terminal of the differential amplifier circuit 60, the source thereof is connected with the internal power supply terminal 40, the drain thereof is connected with the power supply terminal 10, and the back gate thereof is connected with the ground terminal 20.
Next, a structure of a reference voltage circuit according to a third embodiment will be described.
An ED type reference voltage circuit as the constant voltage circuit 50 includes a depletion NMOS transistor 51c and the NMOS transistor 52. The control transistor 70 includes an NMOS transistor 71c.
The gate, source and back gate of the depletion NMOS transistor 51c are connected with the reference voltage output terminal 30, the drain thereof is connected with the internal power supply terminal 40. The gate of the NMOS transistor 71c is connected with the output terminal of the differential amplifier circuit 60, the source and back gate thereof are connected with the internal power supply terminal 40, and the drain thereof is connected with the power supply terminal 10.
Next, a structure of a reference voltage circuit according to a fourth embodiment will be described.
An ED type reference voltage circuit as the constant voltage circuit 50 is the same circuit as in the third embodiment. The control transistor 70 includes a depletion NMOS transistor 71d.
The gate of the depletion NMOS transistor 71d is connected with the output terminal of the differential amplifier circuit 60, the source and back gate thereof are connected with the internal power supply terminal 40, and the drain thereof is connected with the power supply terminal 10.
Next, a structure of a reference voltage circuit according to a fifth embodiment will be described.
The Constant voltage circuit 50 includes a junction NMOS transistor 51e and a resistor 52e. The control transistor 70 includes an NPN transistor 71e.
The gate and source of the junction NMOS transistor 51e are connected with the reference voltage output terminal 30 and the drain thereof is connected with the internal power supply terminal 40. One end of the resistor 52e is connected with the reference voltage output terminal 30 and the other end thereof is connected with the ground terminal 20. The base of the NPN transistor 71e is connected with the output terminal of the differential amplifier circuit 60, the emitter thereof is connected with the internal power supply terminal 40, and the collector thereof is connected with the power supply terminal 10.
The NPN transistor 71e is used, but a PNP transistor (not shown) may also be used. In this case, the connection point of the non-inverted input terminal of the differential amplifier circuit 60 and the connection point of the inverted input terminal thereof are interchanged to negatively feed back to the internal power supply terminal 40.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-212070 | Aug 2007 | JP | national |
