JP3250884B2 - Operational amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an operational amplifier, and more particularly to an operational amplifier such as an AD converter, a DA converter, a switched capacitor filter, a sample hold circuit, and an active filter. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an operational amplifier suitable for applications requiring high output amplitude at low power supply voltage in addition to high gain and high bandwidth.

[0002]

2. Description of the Related Art Generally, an operational amplifier has a structure of 1
There is a one-stage type having a stage amplifier and a two-stage type in which two stage amplifiers are cascaded.

When a high gain is required in a single-stage configuration, a cascode circuit is usually provided as a means for increasing the gain. For details, see PR Gray et al., John Wiley & Son
s Second edition published in 1984, "Analysis and Design of Analo
g Integrated Circuits ”. In this configuration, since the high impedance terminal is the output only, phase compensation is achieved by connecting a suitable capacitor to the output terminal. However, the output amplitude of the amplifier is greatly limited by the cascode circuit, and it is extremely difficult to achieve a high output amplitude particularly at a low power supply voltage.

When a high gain is required in a two-stage configuration, each amplification stage usually does not require as much gain as the one-stage configuration, so a cascode circuit for increasing the gain is required. Not done. Therefore, a high output amplitude can be achieved with a low power supply voltage. However, in the case of a two-stage configuration, as a result of the presence of two high impedance terminals (the second stage input and the second stage output), there are two poles. Polar separation must be performed. The details are also described in the above-mentioned reference.

FIG. 3 shows an outline of the configuration of a two-stage operational amplifier which has performed the above-mentioned mirror phase compensation. Mirror phase compensation is
This is performed by returning the output of the second-stage inverting amplifier to the input of the amplifier via a capacitor. This feedback capacitance appears equivalently increased at the output terminal of the first-stage amplifier due to the Miller effect caused by the gain of the second-stage amplifier, thereby achieving pole separation. Further, by connecting a resistor in series with the feedback capacitor, zero can be eliminated.

[0006]

As described above, a single-stage operational amplifier having a cascode circuit has a relatively simple phase compensation and is suitable for applications requiring high gain and high bandwidth. However, on the other hand, it is difficult to achieve a high output amplitude at a low power supply voltage. On the other hand, the conventional two-stage operational amplifier does not require a cascode circuit for limiting the output amplitude, and thus has a high output power even at a low power supply voltage. It is possible to achieve amplitude, but mirror-phase compensation must be applied. The drawback of Miller phase compensation is that the second pole is limited to low frequencies by the relatively large feedback capacitance and the load capacitance of the second stage output, resulting in a one-stage configuration with bandwidth. It cannot be stretched as much.

An object of the present invention is to eliminate the drawbacks of the conventional one-stage configuration and the two-stage configuration as described above, and to have both of these advantages, that is, high bandwidth, high gain, and low power supply voltage. An object of the present invention is to provide an operational amplifier that can obtain a high output amplitude and has a simple circuit configuration.

[0008]

SUMMARY OF THE INVENTION The present invention achieves the above object by improving the mirror phase compensation in a two-stage operational amplifier and increasing the loop band of the mirror feedback. is there. That is, the operational amplifier of the present invention includes a transconductance amplifier, a first constant current source connected to an output terminal of the transconductance amplifier via at least a first transistor, and at least a second constant current source connected to the output terminal. An amplifier circuit having a second constant current source connected via a transistor of the second type, an output stage amplifier circuit having an input terminal connected to a connection point between the second transistor and the second constant current source, and A capacitor connected between a connection point between the first transistor and the first constant current source and an output terminal of the output stage amplifier circuit;

If the transconductance amplifier is of an input / output fully differential type, and each of the pair of output terminals is connected to the same circuit as described above, an input / output fully differential type operational amplifier can be obtained.

[0010]

In the operational amplifier according to the present invention, the loop band of the mirror phase compensation is increased by the gain from the first transistor to the second transistor. The gain is higher by the gain, and therefore the frequency band in which the stability is obtained is higher.
In this way, the band limitation, which is a drawback of the conventional two-stage operational amplifier, is greatly eased, and the object described above is achieved.

[0011]

An embodiment of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a circuit diagram showing one embodiment of an operational amplifier according to the present invention, which can be realized using CMOS processing technology. In the figure, M3, M7 and M16 are PMOS transistors, M5 is an NMOS transistor, 1 is an input terminal, 2 to 4 are terminals inside the amplifier,
5 is an output terminal, 6 (VDD) is a voltage source terminal, 7 is a ground terminal, 8 and 9 are constant current sources, 10 is an input transconductance amplifier, CC is a capacitance for phase compensation, CL is a load capacitance at the output terminal, VB1, VB2 and VB3
Represents a bias power supply. Bias power supply VB
1, VB2 and VB3 are transistors M3,
M16 and M7 are biased in the saturation region.

Next, the operation of the operational amplifier will be described.
The analog voltage input from the input terminal 1 is converted into a current signal by the input transconductance amplifier 10 and output to the terminal 2. This current signal flows to the transistor M7 having a lower impedance than the constant current source constituted by the transistors M3 and M16, is converted into a voltage signal by the impedance at the terminal 4, and
The signal is amplified by a source ground circuit composed of a transistor M5 and a constant current source 8 and output to an output terminal 5.

Since this amplifier is composed of two amplification stages, it is necessary to perform phase compensation in order to obtain stability. Conventional phase compensation is equivalent to providing a capacitance / resistance circuit between terminals 5 and 4. The phase compensation in this embodiment is performed by feeding back the output signal of the terminal 5 to the terminal 4 via the phase compensation capacitor CC and the transistors M16 and M7. Therefore, the phase compensation capacitor CC in the present embodiment is equivalently increased at the terminal 4 by the gain of the source ground circuit composed of the transistor M5 and the constant current source 8. As a result, the first and second amplifiers
Are separated to obtain a stable frequency characteristic. In this phase compensation, unlike the conventional mirror phase compensation, since there is no feedforward path from the terminal 4 to the output terminal 5, zero cannot be achieved, and therefore, there is no need to insert a resistor in the feedback path.

In the phase compensation of the above embodiment, the loop band of the mirror feedback has a voltage gain from the terminals 3 to 4, that is, the transistors M16 and M7 and the constant current source 9 as compared with the conventional circuit. Therefore, the second pole frequency is higher by the gain of the grounded gate circuit than in the conventional circuit. That is, the frequency band in which the stability is obtained is higher than that of the conventional circuit by the gain of the gate ground circuit.

FIG. 2 shows a second embodiment of the present invention, which is an input / output fully differential operational amplifier having the same phase compensation circuit as that shown in FIG. M1, M2, M
5, M8, M9, M12 to M15 and M18 are NMOS
Transistors, M3, M4, M6, M7, M10, M1
1, M16 and M17 are PMOS transistors, CC1
And CC2 are phase compensation capacitors, VIN + and VIN- are input terminal pairs, VO + and VO- are output terminal pairs, VDD and VSS are power supply terminals, VCF1, VCF2, VBN1,.
VBN2 and VBP1 to VBP3 represent bias power supply terminals, respectively. Transistors M1, M2, M18
And M13 are differential input transconductance
Configure the amplifier.

Transistors M3, M16, M7 and M5
Corresponds to the transistors with the same reference numerals in FIG.
The transistor M6 also corresponds to the constant current source 8, and the transistors M8 and M14 also correspond to the constant current source 9.
Circuits equivalent to this are transistors M4, M17, M1
1, M12, M15, M10 and M9. The operation of the present embodiment configured as described above is the first operation.
Since the operation can be easily inferred from the operation of the embodiment, the description is omitted.

FIG. 4 shows the result obtained by computer simulation of the frequency characteristics of the operational amplifier according to the present invention. The graph connecting □ represents gain, and the graph connecting ○ represents phase. As is clear from this figure,
The second pole occurs at about 200 MHz, indicating that the bandwidth has been greatly extended compared to the second pole of a conventional two-stage operational amplifier occurring at about tens of MHz. ing.

The above embodiments are merely examples of the present invention, and it goes without saying that the present invention is not limited to these embodiments. For example, while the above embodiment is configured using FETs, other elements can also be used, and the above embodiment is a circuit based on CMOS processing technology, but other processing technology, for example, BiCMOS processing technology It can also be done by a circuit. Various modifications are also possible in the circuit configuration. For example, in FIG. 1, another transistor may be inserted between the transistor M16 and the terminal 3 and / or between the transistor M7 and the terminal 4.

[0019]

As described in detail above, according to the present invention, a stable frequency band can be obtained by extending the loop band of the mirror phase compensation required for the two-stage operational amplifier. And an operational amplifier capable of obtaining a high output amplitude with a high bandwidth, a high gain, and a low power supply voltage can be realized without significantly complicating the circuit configuration.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an operational amplifier according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing a fully differential operational amplifier according to a second embodiment of the present invention.

FIG. 3 is a circuit diagram showing a conventional two-stage operational amplifier using mirror phase compensation.

FIG. 4 is a graph showing frequency characteristics of the operational amplifier according to the present invention.

[Explanation of symbols]

4: Output terminal of first-stage amplifier circuit 5: Transistor as second-stage amplifier circuit 10: Input transconductance amplifier CC: Capacitor for phase compensation M16, M7: Capacitor for phase compensation and output terminal of first-stage amplifier circuit Transistor provided between

Claims (2)

(57) [Claims] 1. A transconductance amplifier, a first constant current source connected to an output terminal of the transconductance amplifier through at least a first transistor, and an output terminal connected to at least a second transistor. An amplifier circuit having a second constant current source connected to
An output stage amplifier circuit having an input terminal connected to a connection point between the second transistor and a second constant current source, and a connection point between the first transistor and the first constant current source and the output stage amplification circuit An operational amplifier comprising a capacitor connected between output terminals of a circuit. 2. An input / output fully differential transconductance amplifier having first and second input terminals and first and second output terminals, and at least a first transistor connected to the first output terminal. A first constant current source connected via the first output terminal, a second constant current source connected to the first output terminal via at least a second transistor, and at least a third current source connected to the second output terminal. An amplifier circuit having a third constant current source connected via a transistor, and a fourth constant current source connected to the second output terminal via at least a fourth transistor; the second transistor And the second
A first output stage amplifying circuit having an input terminal connected to a connection point with the constant current source, and a second output having an input terminal connected to a connection point between the fourth transistor and the fourth constant current source. Stage amplifier circuit, a first capacitive element connected between a connection point between the first transistor and the first constant current source and an output terminal of the first output stage amplifier circuit, and the third transistor An operational amplifier comprising a second capacitive element connected between a connection point between the second constant current source and a third constant current source and an output terminal of the second output stage amplifier circuit.