JPH0794978A - Operational amplifier - Google Patents

Abstract

PURPOSE:To obtain an operational amplifier having high band and high gain, capable of obtaining high output amplitude by low power supply voltage and simplified at its circuit constitution by improving mirror phase compensation in a two-stage type operational amplifier so as to improve a mirror feedback loop band. CONSTITUTION:Analog voltage inputted from an input terminal 1 is converted into a current signal by an input transformer conductance amplifier 10 and the current signal is outputted to a terminal 2. The current signal is allowed to flow into a transistor (TR) M7 whose impedance is lower than that of a constant current source constituted of TRs M3, M16 and converted into a voltage signal by the impedance of the terminal 4, the voltage signal is amplified by a source ground circuit constiuted of a TR M5 and a constant current source 8 and the amplified signal is outputted to an output terminal 5. Phase compensation is executed by feeding back an output signal from a terminal 5 to the terminal 4 through a phase compensating capacitor CC and the TRs M16, M7. Thereby the capacitance of the capacitor CC is equivalently increased on the terminal 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to operational amplifiers, and more particularly to operational amplifiers such as AD converters, DA converters, switched capacitor filters, sample-hold circuits, active filters, etc. The present invention relates to an operational amplifier suitable for applications requiring high gain, high band, and high output amplitude at low power supply voltage.

[0002]

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

When a high gain is required in a one-stage configuration, a Cascade circuit is usually used as a means for increasing the gain. For details, see PR Gray et al., John Wiley & Son.
s 2nd edition, 1984, "Analysis and Design of Analo
g Integrated Circuits ”. In this configuration, the high impedance terminal is the only output, so phase compensation is achieved by connecting an appropriate capacitance to the output terminal. However, the output amplitude of the amplifier is significantly limited due to the cascode circuit, and it is extremely difficult to achieve a high output amplitude especially at a low power supply voltage.

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

FIG. 3 shows an outline of the configuration of the two-stage operational amplifier which has been subjected to the mirror phase compensation. Mira-phase compensation is
This is done by feeding back the output of the second stage inverting amplifier via a capacitor to the input of the amplifier. 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 realizing pole separation. In addition, the zero can be removed by connecting the resistor in series with the feedback capacitance.

[0006]

As described above, the one-stage operational amplifier having the cascade circuit is relatively simple in 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 need a cascade circuit for limiting the output amplitude, so that a high output power is obtained even at a low power supply voltage. It is possible to achieve amplitude, but Miller-phase compensation must be applied. The drawback of the mirror phase compensation is that the second pole is limited to a low frequency by the relatively large feedback capacitance and the load capacitance of the second stage output, and as a result, the band is possible with the one-stage configuration. The point is that it cannot be stretched as much.

The object of the present invention is that it does not have the drawbacks of the conventional one-stage type configuration and the two-stage type configuration as described above, and has the advantages of both, namely, high bandwidth, high gain, and low power supply voltage. It is an object of the present invention to provide an operational amplifier which 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 through at least a first transistor, and at least a second terminal connected to the output terminal. An amplifier circuit having a second constant current source connected via a transistor, 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 capacitive element is 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 the input / output fully differential type and each of the pair of output terminals is connected to a circuit similar to the 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, so that the second pole frequency is The gain is higher, and thus the frequency band in which stability is obtained is higher.
In this way, the limitation on the area, which has been a drawback of the conventional two-stage operational amplifier, is greatly relaxed, and the above-mentioned object is achieved.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of an operational amplifier according to the present invention, which can be realized by 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 internal terminals of the amplifier,
5 is an output terminal, 6 (VDD) is a voltage source terminal, 7 is a Guland terminal, 8 and 9 are constant current sources, 10 is an input transconductance amplifier, CC is a phase compensation capacitance, CL is a load capacitance at the output terminal, VB1, VB2 and VB3
Denote bias power supplies, respectively. Bias power supply VB
1, VB2 and VB3 are transistors M3,
Bias M16 and M7 in the saturation region.

Next, the operation of this 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 through the transistor M7, which has a lower impedance than the constant current source composed of the transistors M3 and M16, is converted into a voltage signal by the impedance at the terminal 4, and
The signal is amplified by the source ground circuit composed of the transistor M5 and the constant current source 8 and output to the output terminal 5.

Since this amplifier is composed of two amplification stages, it is necessary to perform phase compensation in order to obtain stability. The conventional phase compensation corresponds to the one in which a capacitance / resistance circuit is provided between the terminals 5 and 4. The phase compensation in the present embodiment is performed by feeding back the output signal of the terminal 5 to the terminal 4 via the phase compensating capacitor CC and the transistors M16 and M7. Therefore, the phase compensating capacitor CC in this 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
The poles of are separated, and stable frequency characteristics are obtained. 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 made, and therefore, it is not necessary to insert a resistor in the feedback path.

In the phase compensation of the above embodiment, the loop band of the mirror feedback is compared with the conventional circuit, and the voltage gain from the terminal 3 to the terminal 4, that is, the transistors M16 and M7 and the constant current source 9 are provided. And the second pole frequency becomes higher than that in the conventional circuit by the gain of the gate ground 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 type operational amplifier having the same phase compensation circuit as shown in FIG. M1, M2, M
5, M8, M9, M12 to M15 and M18 are NMOS
Transistor, 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 an amplifier.

Transistors M3, M16, M7 and M5
Corresponds to the transistor with the same reference numeral 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.
A circuit equivalent to this is the transistor M4, M17, M1.
1, M12, M15, M10 and M9. The operation of the present embodiment configured as described above is
Since it can be easily inferred from the operation of the above embodiment, the description thereof will be omitted.

FIG. 4 shows results obtained by computer simulation of frequency characteristics of the operational amplifier according to the present invention. The graph connecting the □ marks represents the gain, and the graph connecting the ○ marks represents the phase. As you can see from this figure,
The second pole occurs at about 200 MHz, which indicates that the band has been extended significantly compared to the second pole of a conventional two-stage operational amplifier at about tens of MHz. ing.

It is needless to say that the above-mentioned embodiments are examples of the present invention, and the present invention is not limited to these. For example, although the above-mentioned embodiment is configured by using the FET, other elements can be used, and the above-mentioned embodiment is a circuit by the CMOS processing technology, but other processing technology, for example, BiCMOS processing technology. You can also use the circuit by. Various modifications are possible in the circuit configuration as well, and 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 above in detail, according to the present invention, a stable frequency band can be obtained by expanding the loop band of the mirror phase compensation required for the two-stage operational amplifier. It is possible to realize an operational amplifier which can obtain a high output amplitude with a wide band, a high gain, and a low power supply voltage without considerably complicating the circuit configuration, which is a remarkable effect.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an operational amplifier as 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 a mirror phase compensation.

FIG. 4 is a graph showing frequency characteristics of an 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: Phase compensation capacitance M16, M7: Phase compensation capacitance and output terminal of first-stage amplification circuit Transistor provided between

Claims (2)

[Claims] 1. A transconductance amplifier, a first constant current source connected to an output terminal of the transconductance amplifier via at least a first transistor, and an output terminal via at least a second transistor. An amplifier circuit having a second constant current source connected in parallel,
An output stage amplification 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 An operational amplifier comprising a capacitive element 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 at the first output terminal. A first constant current source connected to the first output terminal, a second constant current source connected to the first output terminal via at least a second transistor, and a second constant current source connected to the second output terminal at least a third 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 amplifier 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 of the first transistor and a first constant current source and an output terminal of the first output stage amplifier circuit, and the third transistor And a third constant current source, and a second capacitance element connected between the connection point of the third constant current source and the output terminal of the second output stage amplifier circuit.