Low noise current buffer circuit and I-V converter

A low noise current buffer circuit includes a first transistor, for receiving an input current, a second transistor, for draining a first current from a drain of the second transistor according to the input current received by the first transistor, a third transistor, for outputting first current, a fourth transistor, for outputting a second current to an output resistor, to generate an output voltage, and a feedback capacitor, for eliminating impacts of noise of a system voltage on the output voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a low noise current buffer circuit and current voltage (I-V) converter, and more particularly, to a low noise current buffer circuit and current voltage converter capable of reducing impact of noise of a system voltage on an output voltage.

2. Description of the Prior Art

A current voltage converter, such as a bandgap reference circuit, utilizes a current source to output an input current to an output resistor to generate a required output voltage. In such a conventional structure, since the current source likely experiences interference from noise of a system voltage, the output voltage is affected and can not stay within a stable range.

Please refer toFIG. 1AandFIG. 1B.FIG. 1Ais a schematic diagram of a bandgap reference circuit10for generating a zero temperature coefficient (zero-TC) voltage in the prior art, andFIG. 1Bis a schematic diagram of a bandgap reference circuit12for generating zero-TC current in the prior art. In the bandgap reference circuit10, a transistor102, which can be considered a current source, outputs an input current Iin to an output resistor Ro and a diode Q1, to generate a zero-TC output voltage Vout. Similarly, in the bandgap reference circuit12, a transistor104, which can be considered a current source as well, outputs an zero-TC input current Iin′ to an output resistor Ro′, to generate an output voltage Vout′. In such a situation, a system voltage VDD experiences interference from noise, and the input currents Iin, Iin′ experience interference as well, such that the output voltages Vout, Vout′ are affected, and thus can not stay within a stable range.

For example, when the system voltage VDD rises rapidly due to noise, the transistors102,104output corresponding greater input currents Iin, Iin′, which increases the output voltages Vout, Vout′, such that the output voltages Vout, Vout′ are greater than the stable range. Thus, there is a need for improvement of the prior art.

SUMMARY OF THE INVENTION

It is therefore an objective of the present invention to provide a low noise current buffer circuit and current voltage converter.

The present invention discloses a low noise current buffer circuit for reducing impacts of noise of a system voltage on an output voltage in a current voltage converter. The low noise current buffer circuit includes a first current mirror, a second current mirror and a feedback capacitor. The first current mirror includes a first transistor, including a gate, a drain and a source, the gate coupled to the drain, and the drain receiving an input current, and a second transistor, including a gate, a drain and a source, the gate coupled to the gate of the first transistor, for draining a first current from the drain according to the input current received by the first transistor. The second current mirror includes a third transistor, including a gate, a drain and a source, the gate coupled to the drain, and the drain coupled to the drain of the second transistor, for outputting the first current, and a fourth transistor, including a gate, a drain and a source, the gate coupled to the gate of the third transistor, for outputting a second current to an output resistor according to the first current outputted by the third transistor, to generate the output voltage. The feedback capacitor includes a terminal coupled between the drain of the second transistor and the drain of the third transistor, and another terminal coupled between the drain of the fourth transistor and the output resistor, for forming a negative feedback loop, to eliminate the impacts of the noise of the system voltage on the output voltage.

The present invention further discloses a current voltage converter capable of reducing impacts of noise of a system voltage on an output voltage. The current-to-voltage converter includes a current source, for generating an input current, an output resistor, for generating an output voltage according to a second current, and a low noise current buffer circuit, coupled between the current source and the output resistor. The low noise current buffer circuit includes a first current mirror, a second current mirror and a feedback capacitor. The first current mirror includes a first transistor, including a gate, a drain and a source, the gate coupled to the drain, and the drain receiving an input current, and a second transistor, including a gate, a drain and a source, the gate coupled to the gate of the first transistor, for draining a first current from the drain according to the input current received by the first transistor. The second current mirror includes a third transistor, including a gate, a drain and a source, the gate coupled to the drain, and the drain coupled to the drain of the second transistor, for outputting the first current, and a fourth transistor, including a gate, a drain and a source, the gate coupled to the gate of the third transistor, for outputting the second current to the output resistor according to the first current outputted by the third transistor, to generate the output voltage, The feedback capacitor includes a terminal coupled between the drain of the second transistor and the drain of the third transistor, and another terminal coupled between the drain of the fourth transistor and the output resistor, for forming a negative feedback loop, to eliminate the impacts of the noise of the system voltage on the output voltage.

DETAILED DESCRIPTION

Please refer toFIG. 2AandFIG. 2B,FIG. 2AandFIG. 2Bare schematic diagrams of bandgap reference circuits20,22according to an embodiment of the present invention, respectively. The bandgap reference circuits20,22are utilized for generating a zero temperature coefficient (zero-TC) voltage and current, respectively. Partial structures of the bandgap reference circuits20,22are the same as those of the bandgap reference circuits10,12, and thus elements with the same functions and structures are denoted by the same figures and symbols for simplicity. In short, a main difference between the bandgap reference circuit22and the bandgap reference circuit12is that a low noise current buffer circuit214is added between transistors208,210,212, which can be considered current sources, and the output resistor Ro′ of the bandgap reference circuit22. The low noise current buffer circuit214receives input currents Iin1′, Iin2′, Iin3′, and outputs a current I2to the output resistor Ro′ after reducing impact of noise of the system voltage VDD through negative feedback, so as to generate an output voltage Vout′ unaffected by the noise of the system voltage VDD, such that the output voltage Vout′ can stay within a stable range. Similarly, differences between the bandgap reference circuit20and the bandgap reference circuit10can be referred from the above description.

Please refer toFIG. 3, which is a schematic diagram of circuitry of the low noise current buffer circuit214shown inFIG. 2B. The low noise current buffer circuit214mainly includes transistors MNR1, MNR2, MNR3, MPR1, MN1, MN2, MN3, MP1, MP2, MP3and feedback capacitors CM1, CM2, and detailed structure and connection configuration are as shown inFIG. 3, where a gate of the transistor MNR1is coupled to a drain of the transistor MNR1, a gate of the transistor MN1is coupled to the gate of the transistor MNR1, a source of the transistor MN2is coupled between a drain of the transistor MN1and feedback capacitor CM1, a source of the transistor MN3is coupled to a drain of the transistor MN2, a gate of the transistor MP1is coupled to a drain of the transistor MP1, the drain of the transistor MP1is coupled to a drain of the transistor MN3, a gate of the transistor MP2is coupled to the gate of the transistor MP1, a terminal of the feedback capacitor CM1is coupled between the drain of the transistor MN1and the drain of the transistor MN2, another terminal of the feedback capacitor CM1is coupled between a drain of the transistor MP3and output resistor Ro′, and the feedback capacitor CM2is coupled between a gate and the drain of the transistor MN2. The transistors MNR1, MNR2, MNR3, MN1, MN2, MN3are N-type metal oxide semiconductor (MOS) transistors, and the transistors MPR1, MP1, MP2, MP3are P-type MOS transistors.

In short, the transistors MNR1, MN1and the transistors MP1, MP2form current mirrors, respectively. The feedback capacitor CM1can form a negative feedback loop FB to eliminate the impact of the noise of the system voltage VDD on the output voltage Vout′. The transistors MN2, MN3, MP3form a cascade stage to reduce the channel-length-modulation and provide better current matching of the transistors MN1, MP2. The feedback capacitor CM2can perform Miller compensation to prevent the noise of the system voltage VDD from generating feed-forward noise to the output voltage Vout′ along a feed-forward path FFP1through the feedback capacitor CM1. The transistors MNR2, MNR3, MPR1correspond to the transistors MN2, MN3, MP3of the cascade stage, respectively.

In detail, the transistor MNR1receives the input current Iin3′, such that the transistor MN1drains a current I1from the drain of the transistor MN1according to the input current Iin3′. Since the transistor MP1and the transistor MN1are cascaded, a current of the transistor MN1is substantially the same with the current I1, such that the transistor MP2can output current I2to the output resistor Ro′ according to the current I1to generate the output voltage Vout′. The feedback capacitor CM1forms the negative feedback loop FB to eliminate the impact of the noise of the system voltage VDD on the output voltage Vout′, such that the output voltage Vout′ can stay within a stable range. For example, as shown inFIG. 4, assume that the low noise current buffer circuit214only includes the transistors MNR1, MN1, MP1, MP2and the feedback capacitor CM1. When the system voltage VDD rises rapidly due to noise, the transistor MP2outputs a greater current I2, which increases the output voltage Vout′. At this moment, a drain voltage VDN1of the transistor MN1can rise due to a feedback path formed by the feedback capacitor CM1, i.e. a gate voltage VGP2of the transistor MP2can rise, to reduce the current I2outputted by the transistor MP2, so as to achieve an effect of negative feedback.

However, if the low noise current buffer circuit214only includes the transistors MNR1, MN1, MP1, MP2and the feedback capacitor CM1, the noise of the system voltage VDD will generate feed-forward noise to the output voltage Vout′ along a feed-forward path FFP2through the feedback capacitor CM1as shown inFIG. 4. Therefore, the low noise current buffer circuit214can include the transistor MN2, MN3acting as the cascade stage to eliminate the feed-forward path FFP2.

Please continue to refer toFIG. 3. The transistor MN2prevents the noise of the system voltage VDD from generating feed-forward noise to the output voltage Vout′ along the feed-forward path FFP2through the feedback capacitor CM1as shown inFIG. 4. The feedback capacitor CM2performs Miller compensation to prevent the noise of the system voltage VDD from generating feed-forward noise to the output voltage Vout′ along the feed-forward path FFP1through the feedback capacitor CM1. The transistor MN3prevents the noise of the system voltage VDD from affecting operations of the feedback capacitor CM2. For example, when the system voltage VDD rises due to noise, a gate voltage VGN2of the transistor MN2rises as well. Since the current I1of the transistor MN2is fixed, which can be considered a fixed current source, a source voltage VSN2of the transistor MN2rises as well, which increases the output voltage Vout′ via the feedback capacitor CM1. At this moment, the feedback capacitor CM2performs Miller compensation to reduce the gate voltage VGN2of the transistor MN2, so as to reduce the output voltage Vout′, such that the output voltage Vout′ stays within a stable range. Noticeably, if the noise of the system voltage VDD is high frequency noise, the noise of the system voltage VDD can generate feed-forward noise along a feed-forward path FFP3through the feedback capacitor CM2as shown inFIG. 3. However, the feed-forward noise along the feed-forward path FFP3is in phase with the negative feedback signal in the negative feedback loop FB formed by the feedback capacitor CM1. Therefore, the feed-forward noise can strengthen negative feedback, so as to facilitate eliminating the impact of the noise of the system voltage VDD on the output voltage Vout′, such that the output voltage Vout′ can stay within a stable range.

On the other hand, please refer toFIG. 5A, which is a schematic diagram of a small signal model of the low noise current buffer circuit214shown inFIG. 3. Transformation from a schematic diagram of the circuit of the low noise current buffer circuit214shown inFIG. 3to the small signal model of the low noise current buffer circuit214shown inFIG. 5Ais known by those skilled in the art, and is not narrated hereinafter. InFIG. 5A, a dotted line of the negative feedback loop FB corresponds to the negative feedback loop FB shown inFIG. 3, and transconductors gmN1, gmN2, gmN3, gmP2, gmP3correspond to the transistors MN1, MN2, MN3, MP2, MP3, respectively. Other resistors and capacitors correspond to parasitic resistors and parasitic capacitors. As can be seen fromFIG. 5A, after the feedback capacitor CM1forms the negative feedback loop FB, the transconductors gmN2, gmN3, gmP2, gmP3can act as a gain stage, and the transconductor gmP2performs an inverting operation, so as to eliminate the impact of the noise of the system voltage VDD on the output voltage Vout′.

Please refer toFIG. 5BandFIG. 5C, which are schematic diagrams of noise of the small signal model shown inFIG. 5A. Dotted lines shown inFIG. 5Bdenote noise entering from the transconductors gmN1, gmN2, gmN3, gmP2, gmP3. The transconductor gmP2is directly connected to the system voltage VDD, such that the noise entering from the transconductor gmP2is greater. The noise of the dotted line shown inFIG. 5Bcan be eliminated by the negative feedback loop FB shown inFIG. 5A. On the other hand, the feed-forward paths FFP1, FFP3of the dotted lines shown inFIG. 5Ccorrespond to the feed-forward paths FFP1, FFP3shown inFIG. 3, respectively. In other words, after entering from the gate of transistor MN2, the noise of the system voltage VDD generates feed-forward noise to the output voltage Vout′ along the feed-forward paths FFP1, FFP3.

InFIG. 5C, since the transistor MN2is a source follower, a source voltage VSN2of the transistor MN2is a division voltage of the gate voltage VGN2, i.e.

VSN⁢⁢2=roN⁢⁢1roN⁢⁢1+1/gmN⁢⁢2,
such that the noise of the system voltage VDD affects the output voltage Vout′ via the feed-forward path FFP1. At this moment, the feedback capacitor CM2performs Miller compensation to eliminate the impact of the noise of the system voltage VDD on the output voltage Vout'. If the noise of the system voltage VDD is high frequency noise, the noise of the system voltage VDD generates feed-forward noise along the feed-forward path FFP3through the feedback capacitor CM2, but the feed-forward noise along the feed-forward path FFP3is in phase with the negative feedback signal in the negative feedback loop FB formed by the feedback capacitor CM1. Therefore, the feed-forward noise can strengthen negative feedback, so as to facilitate eliminating the impact of the noise of the system voltage VDD on the output voltage Vout′, such that the output voltage Vout′ can stay within a stable range.

Furthermore, an open loop transfer function Aopen*f can be derived from the negative feedback loop FB shown inFIG. 5Ato clarify characteristics of the negative feedback loop FB. A frequency response of forward transfer function Aopencan denoted as follows:

And a frequency response of feedback transfer function f can be denoted as:

Then, the whole open loop transfer function Aopen*f can be derived as follows:

In addition, in order to prevent the transistors MNR1, MN1, MP1, MP2forming the current mirrors from generating the currents I1, I2with too much variation due to process mismatch, sizes of the transistors MNR1, MN1, MP1, MP2are greater than those of the other transistors. Therefore, the feedback capacitor CM1in the negative feedback loop FB forms a dominant pole, and a parasitic capacitor CGR2of the transistor MP2is greater than those of other transistors and thus forms a second pole. As a result, the open loop transfer function Aopen*f of the low noise current buffer circuit214is shown inFIG. 6AandFIG. 6B. As can be seen fromFIG. 6AandFIG. 6B, the open loop transfer function Aopen*f has a zero when the frequency is 0, which means the negative feedback loop FB does not operate when frequency is 0, i.e. the feedback capacitor CM1is open. Therefore, the gain rises as frequency increases until the pole 1/Ro′CM1, and stays the same after the pole 1/Ro′CM1, and then starts falling after the second pole gmP1/CGP2, and poles can be derived by the same token. As can be seen from the above, a main operating frequency range of the negative feedback loop FB is 1/Ro′CM1to gmP1/CGP2, and since a numerator Ro′CM1of the open loop transfer function Aopen*f is cancelled by a denominator of the open loop transfer function Aopen*f within this range, the loop gain is gmP2/gmP1, which means the noise of the system voltage VDD is eliminated. As a result, by adjusting 1/Ro′CM1and gmP1/CGP2, i.e. a resistance of the output resistor Ro′, a capacitance of the feedback capacitor CM1and a size of the transistor MP1, the present invention can adjust the main operating frequency range. Besides, by adjusting gmP2/gmP1, i.e. a ratio of a size of the transistor MP2to a size of the transistor MP1, the present invention can adjust the loop gain.

Noticeably, the spirit of the present invention is to utilize the low noise current buffer circuit214to receive the noisy input current of the current source, and then to output the current I2to the output resistor Ro′ after reducing the impact of the noise of the input current and system voltage VDD by negative feedback, so as to generate the output voltage Vout′ unaffected by the noise of the input current and system voltage VDD, such that the output voltage can stay within a stable range. Those skilled in the art should make modifications or alterations accordingly. For example, the present invention is not limited to being applied in a bandgap reference circuit, and can be applied in any current voltage converter utilizing a current source to generate an output voltage. Besides, the bandgap reference circuit22outputs the current I2to the output resistor Ro′ to generate the output voltage Vout′, but methods for generating an output voltage can be similar to that of the bandgap reference circuit20, which outputs the current I2to the output resistor Ro and the diode Q1, or other elements, and are not limited to these. In addition, the low noise current buffer circuit214can be as shown inFIG. 4and only include the transistors MNR1, MN1, MP1, MP2and the feedback capacitor CM1as well. However, the noise of the system voltage VDD will generate feed-forward noise to the output voltage Vout′ along the feed-forward path FFP2as shown inFIG. 4, and the low noise current buffer circuit214can not preferably eliminate the impact of the noise of the system voltage VDD on the output voltage Vout′ as shown inFIG. 3.

In the prior art, since a current source is likely to experience interference by noise of a system voltage, an output voltage is affected as well and thus can not stay within a stable range. In comparison, the present invention utilizes the low noise current buffer circuit214to receive the input current of the current source, and then to output a current I2to generate the output voltage unaffected by the noise of the input current and system voltage VDD, such that the output voltage can stay within a stable range.