JPH11340751A - Operational amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an operational amplifier improving an operation band to the limit of a higher operation band decided by the source follower of an output stage and the load capacity of output. SOLUTION: This operational amplifier is provided with a differential amplifier circuit 1 for amplifying the differential input of inverted input and non- inverted input and first and second source follower circuits 2 and 3 receiving the output of the differential amplifier circuit. Then, the output of the first source follower circuit 1 is used as first output and the output of the second source follower circuit 2 is used as second output.

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 a high-band operational amplifier using CMOS technology and suitable for an image signal processing circuit.

[0002]

2. Description of the Related Art Various circuit configurations have been proposed for operational amplifiers. However, as for the operational amplifier for image signal processing, the one mainly configured by a circuit of a bipolar element which operates at a high speed has been mainly used from the viewpoint of an operation band. In the case of a bipolar operational amplifier, other CMs
It was difficult to form the logic circuit of the OS on the same chip, and the logic circuit portion had to be configured as a separate chip.

On the other hand, there are products in which a bipolar analog circuit and a CMOS logic circuit are integrated on the same chip using Bi-CMOS technology (hybrid of bipolar and CMOS), but a pure CMOS process is generally used. As compared with the above, the Bi-CMOS process has a problem that the process is complicated and the chip manufacturing cost is high.

An operational amplifier using CMOS technology has characteristics such as an input impedance being almost infinite, and is suitable for a so-called logic analyzer mixed IC formed on the same chip together with a logic circuit composed of CMOS. have. However, a CMOS operational amplifier has a lower operating frequency band (hereinafter, referred to as a band) than an operational amplifier made by a bipolar process, and it is difficult to use the operational amplifier for processing an image signal.

FIG. 3 shows a circuit diagram of a general CMOS operational amplifier. This operational amplifier has an inverting input terminal NEG,
A non-inverting input terminal POS, a differential amplifier stage 100 having an output from a drain terminal of an N-type (channel) MOS transistor having the non-inverting input terminal POS connected to a gate terminal,
It comprises an output stage 200 composed of a CMOS inverter circuit to which the output of the differential amplification stage 100 is input, and a load capacitance 300. In the figure, OUT is an output terminal of the operational amplifier, P is a P-type MOS transistor, and N is an N-type MOS transistor.
A transistor, 400 indicates a constant current source, and 500 indicates a phase compensation capacitance.

What limits the band of this operational amplifier is the output impedance of the output stage 200 and the load capacitance 30.
0. When the output stage 200 has an inverter configuration, a high open-loop gain can be obtained, but the band becomes narrow, resulting in a low-band operational amplifier as a whole.

As a measure for improving this, it is conceivable to configure the output stage 210 as shown in FIG. 4 with a source follower having a low output impedance. In this case, although the open-loop gain is lower than when the output stage 200 of FIG. 3 has an inverter configuration, the band can be expanded to a significantly higher frequency. In the figure, reference numeral 110 denotes a differential amplification stage;
Reference numeral 10 denotes a load capacity, 410 denotes a constant current source, and 510 denotes a phase compensation capacity.

[0008]

FIG. 5 shows the frequency characteristics (Board diagram) of the differential amplifier stage 110, the output stage 210, and the entire operational amplifier of the operational amplifier of FIG. The horizontal axis is frequency and the vertical axis is gain. A broken line 150 indicates the characteristics of the differential amplifier stage 110, a thin solid line 250 indicates the characteristics of the output stage (source follower) 210, and a thick solid line 350 indicates the characteristics of the entire operational amplifier.

The highest frequency in the operating band is determined by the second pole frequency determined by the characteristic 250 of the source follower of the output stage 210 and the load capacitance 310, and a band higher than this frequency cannot be obtained. Further, in order to secure a phase margin of 45 °, the gain of the differential amplifier stage 110 is set to 0 at the second pole frequency.
It must be less than dB. For this purpose, the first pole frequency by the differential amplifier stage 110 must be shifted to a lower frequency side by performing phase compensation, and the gain of the differential amplifier stage at the second pole frequency must be 0 dB or less. Since this phase compensation uses a capacitor built on a chip, the pole frequency of the differential amplifier stage 110 must be shifted to a low frequency band with a certain margin in consideration of process variations. The entire operating band is determined by the first pole frequency, and does not become the maximum band determined by the second pole frequency of the output stage 210.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an operational amplifier capable of improving an operation band up to a limit of a maximum operation band determined by a source follower of an output stage and a load capacity of an output.

[0011]

An operational amplifier according to the present invention comprises:
A differential amplifier circuit for amplifying a differential input between an inverting input and a non-inverting input, and first and second source follower circuits to which an output of the differential amplifier circuit is input; The output of the follower circuit is a first output, and the output of the second source follower circuit is a second output.

[0012]

FIG. 1 shows a CMOS according to an embodiment of the present invention.
1 shows a circuit diagram of an operational amplifier. In FIG. 1, reference numeral 1 denotes a first differential amplifier stage, and the output terminal of the differential amplifier stage 1 has a drain terminal of an N-type MOS transistor 11 whose inverting input terminal NEG is connected to a gate terminal. The output is
Both the first source follower 2 and the second source follower 3 are connected to the gates of the drive transistors 21 and 31. A feedback loop (not shown) including a feedback coefficient from the feedback output FBO of the operational amplifier is connected to the inverting input terminal NEG of the differential amplifier stage 1.

The source follower circuit is a circuit in which a load such as a constant current source or a resistor is connected to the source, and a voltage corresponding to the input voltage applied to the gate is output from the source.

The first source follower 2 is a source follower for feedback output, and the second source follower 3 is a source follower for output of the operational amplifier. First
The source follower 2 and the second source follower 3 are configured such that transistors (L) (gate length) of the source followers are the same and only W (gate width) is different. In this embodiment, W of the output second source follower 3 is set to be larger than that of the first source follower 2. That is, the first source follower 2 for feedback is the second source follower 3
Output impedance. That is, when the conductivity coefficients of the transistors 22 and 21 of the source follower 2 for feedback are Z ₁ and Z ₂ , Z ₁ = W / L, Z ₂
= W / L, and the conductivity coefficients of the transistors 32 and 31 of the output source follower 3 are W / L = kZ ₁ , W / L =
represented by kZ _2. Here, k is a coefficient having a value larger than 1.

Regarding the input / output characteristics, the first and second source followers 2 and 3 have the ratio of the conduction coefficient of the drive transistor and the current source transistor so that the same output voltage can be obtained when the same input voltage is applied. They are set to be equal.

[0016] 4 is a parasitic capacitance C _pa associated with the first source follower 2 outputs FBO for feedback. 5
Is the load capacitance C associated with the output terminal OUT of the operational amplifier.
_load . In the figure, 6 is a constant current source, 7 is a phase compensation capacitor, and 22 and 32 are drive transistors 2 respectively.
These are current source transistors that constitute a source follower in pairs with 1, 31.

FIG. 2 shows a Bode diagram of the operational amplifier of FIG. A dashed line 10 is the frequency characteristic of the differential amplifier stage 1, a solid line 20 is a frequency characteristic of the source follower 2 for feedback, a two-dot chain line 30 is a frequency characteristic of the source follower 3 for output, and the thick solid line 40 is The frequency characteristics of the combination of the differential amplifier stage 1 and the source follower 2 for feedback are shown, and the dotted line 50 shows the frequency characteristics of the combination of the differential amplifier stage 1 and the source follower 3 for output.

In FIG. 2, a source follower 3 for output is provided.
The pole frequency f _po, determined by the mutual conductance g _mo of the drive transistor 31 and the load capacitance C _load, f
_po = 1 / [2 × π × (C _load / g _mo )] The pole frequency f _pf of the source follower 2 for feedback is determined by the transconductance g _mf of the drive transistor 21 and the parasitic capacitance C _pa, and f _pf = 1 / [2 ×
π × (C _pa / g _mf )].

Here, the parasitic capacitance C _pa is the load capacitance C _{load
Therefore} , even if g _mf is smaller than g _mo , that is, k is larger than 1, the pole frequency f _pf of the source follower 2 for feedback is equal to the pole frequency f _pf of the source follower 3 for output. Can be higher than _po .

Since the feedback loop of the operational amplifier is formed between the inverting input terminal NEG of the differential amplifier stage 1 and the feedback output terminal FBO, the phase compensation uses the frequency of the differential amplifier stage 1 at the time of 0 dB for feedback. Since the pole frequency may be set to be lower than the pole frequency determined by the source follower 2, even if the first pole frequency is shifted to a lower frequency side with some allowance, the operating band of the entire operational amplifier is still lower than that of FIG. As is apparent, the pole frequency of the output source follower 3 can be set higher than the pole frequency.

The output of the feedback source follower 2 is determined by the output voltage of the differential amplifier stage 1. This output voltage is input to the output source follower 3, and the feedback source follower 2 and the output source follower 3
Since the input / output characteristics of the amplifiers are the same, the output of the feedback source follower 2 and the output of the operational amplifier become equal, and the same output as when a feedback is applied between the inverted input terminal and the output terminal by the conventional operational amplifier. Is obtained.

In an actual integrated circuit, a transistor pair (a drive transistor 21 and a current source transistor 2) constituting a source follower 2 for feedback is provided.
2) and transistor pair 3 of output source follower 2
Reference numerals 1 and 32 each have a configuration in which a plurality of unit transistor pairs are connected in parallel. By changing the number of unit transistors, two source followers having the same input / output characteristics and different output impedances are used. Make up.

It should be noted that the present invention is not limited to the embodiment described above, and that various modifications and improvements can be made by those skilled in the art based on the disclosure of the embodiment.

[0024]

According to the present invention, the output side source follower connected to the differential amplifier stage has two source follower configurations for feedback and output, so that the input and output are the same as those of the conventional operational amplifier. It is possible to obtain an operational amplifier that operates in a band higher than the conventional one while maintaining the characteristics and the same phase margin, and can be a suitable operational amplifier for processing video signals with a CMOS configuration.

[Brief description of the drawings]

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

FIG. 2 is a Bode diagram of an embodiment of the operational amplifier of the present invention.

FIG. 3 is a circuit diagram of a conventional operational amplifier.

FIG. 4 is a circuit diagram of another operational amplifier according to the related art.

FIG. 5 is a Bode diagram of the conventional operational amplifier of FIG. 4;

[Brief description of reference numerals]

REFERENCE SIGNS LIST 1 Differential amplification stage 2 Feedback source follower 3 Output source follower 4 Parasitic capacitance 5 Load capacitance 6 Constant current source 7 Phase compensation capacitance 10 Frequency characteristics of differential amplification stage 20 Frequency characteristics of feedback source follower 30 Output source follower Frequency characteristics of differential amplifier stage and source follower for feedback 50 Frequency characteristics of differential amplifier stage and source follower for output

Claims (4)

[Claims] 1. A differential amplifier circuit for amplifying a differential input between an inverting input and a non-inverting input, and first and second source follower circuits to which an output of the differential amplifier circuit is input, An operational amplifier in which an output of the first source follower circuit is a first output and an output of the second source follower circuit is a second output. 2. The operational amplifier according to claim 1, wherein the first and second source follower circuits have substantially the same input / output voltage characteristics and different output impedances. 3. A feedback loop is formed between an output of one of the first and second source follower circuits and an inverting input terminal of the differential amplifier circuit.
An operational amplifier as described. 4. A source follower output which forms a feedback loop with an inverting input terminal of the differential amplifier circuit is an output of a source follower circuit whose output impedance is not small among the first and second source follower circuits. The operational amplifier according to claim 3, wherein