JP2002118427A - Operational amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an operational amplifier in which the waveform is prevented from being distorted by suppressing generation of operational delay of an output stage transistor. SOLUTION: Transistors M9 and M10 constituting the current mirror of a folded cascode stage 11 have drains connected with the drains of transistors M8 and M11 having connected gate-drain. The transistors M8 and M11 have sources connected with the same potential as the sources of the transistors M9 and M10 constituting the current mirror. According to the arrangement, output voltage of a folded cascode type operational amplifier can be limited within a level not causing saturation of an output transistor M12 when the folded cascode type operational amplifier tries to output a voltage deviating from the output voltage range.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an operational amplifier circuit, and more particularly to a folded cascode operational amplifier circuit in which a differential stage, a folded cascode stage, and an output stage are constituted by MOS transistors.

[0002]

2. Description of the Related Art In a CMOS transistor process, an N-channel transistor generally operates at a higher speed if the P-channel transistor and the N-channel transistor have the same size. When processing a high-frequency signal using an operational amplifier, it is relatively easy to increase the gain in order to take advantage of it and to realize a two-stage amplifier in comparison with a two-stage amplifier having a simple configuration. For this reason, a folded cascode operational amplifier is often used.

[0003] The simplest configuration of a folded cascode amplifier is classified into three stages: a differential stage, a folded cascode stage, and an output stage. The transistors functioning as the amplifying circuits of the moving stage and the output stage can both be of N-channel type, which is advantageous for high speed operation.

FIG. 3 is a block diagram showing an inverting amplifier circuit. A reference voltage (VRE) is applied to the positive input terminal of the operational amplifier 1.
F), and an input signal (Vin) is applied to a negative input terminal via a resistor (R1). Note that a feedback resistor Rf is connected to the negative input terminal.

FIG. 4 shows an example in which a folded cascode amplifier is used in the inverting amplifier circuit as shown in FIG. As shown in FIG. 3, the folded cascode type amplifier includes a differential stage 10, a folded cascode stage 11, and an output stage 12. The differential stage 10
It includes two P-channel transistors M1, M2 and three N-channel transistors M3, M4, M5. The folded cascode stage 11 includes two P-channel transistors M6 and M7 and two N-channel transistors M
9, M10. Then, the output stage 11 has one P
It includes a channel transistor M12 and one N-channel transistor M13. These circuits are respectively CMO
S.

[0006] The gates of the transistors M1 and M2 of the differential stage 10 are connected, and a first bias potential (VBIAS1) is applied to these gates. These gates are also connected to the gate of the transistor M12 in the output stage, and the first bias potential (V
BIAS1). Further, the transistor M1
, M2 and the transistor M12 are supplied with the power supply potential VDD.

The transistors M1 and M3 are connected to each other, and the transistors M2 and M4 are connected to each other, and their connection nodes are respectively connected to the transistors M6 and M7 in the folded cascode stage. The transistors M3 and M4 are connected, and are connected to the lowest potential VSS via the transistor M5.

The gate of the transistor M3 has a negative input (V
IN-) is applied to the positive input (V
IN +). Further, a third bias potential (VBIAS3) is applied to the gate of the transistor M5.

The gates of the transistors M6 and M7 are connected, and these gates are connected to a second bias potential (VBIAS).
2) is given. The transistors M6 and M9, the transistors M7 and M10 are connected, and the transistors M7 and M1 are connected.
The connection node of 0 is given to the gate of the transistor M13 in the output stage. The gates of the transistors M9 and M10 are connected, and the respective sources are supplied with the lowest potential (VSS). The gate and the drain of the transistor M9 are connected. Then, the transistors M9 and M10
Form a current mirror circuit.

The drain and gate of the transistor M13 are connected via a capacitor C1 and a resistor R1.
An output potential (VOUT) is output from a connection node between the transistors M12 and M13.

[0011]

For example, in the inverting amplifier circuit as shown in FIG. 3, when a folded cascode type amplifier as shown in FIG. There is. That is, FIG.
When a voltage higher than the output voltage range of the operational amplifier is to be output by using a folded cascode type amplifier as shown in (1), current does not flow between the source and the drain of the transistor M13 in the output stage 12. The gate potential of the transistor M13 decreases toward the lowest potential VSS. The potential of the drain of the transistor M12 (the output VOUT of the operational amplifier) cannot rise above the maximum potential VDD, and is limited. In this state, the transistor M12 in the output stage inside the operational amplifier becomes saturated.

In this state, in the circuit shown in FIG. 3, since the potential of VOUT is limited by VDD, the feedback by the resistor Rf does not work properly, and the voltage of the VIN− terminal becomes lower than the VIN + terminal. . This causes a decrease in the current between the source and the drain of the transistor M3 in FIG. 4, and the reduced current becomes an increase in the current between the source and the drain of the transistor M4, and the transistor M10 constituting the current mirror of the folded cascode stage 10 No current flows between the source and the drain of the transistor.

When transitioning from this state to a state in which a voltage within the output voltage range is output, the transistor M12 of the output stage 12 requires more time than normal operation to return from the saturated state. FIG. 5 shows the relationship between the input voltage and the output voltage in the configuration described above. As shown in FIG.
If the recovery time of the transistor M12 is long, the waveform is distorted.

SUMMARY OF THE INVENTION The present invention has been made in order to solve the above-mentioned problems, and has as its object to provide an operational amplifier circuit which suppresses an operation delay of an output-stage transistor and does not generate a waveform distortion. I do.

[0015]

According to the present invention, a current mirror of a folded cascode stage is formed in a folded cascode type operational amplifier circuit in which a differential stage, a folded cascode stage and an output stage are formed by MOS transistors. A drain terminal of the transistor whose gate and drain are connected to a drain terminal of the transistor is connected, and a source of the transistor is connected to the same potential as a source of a transistor forming a current mirror.

[0016] The invention is characterized in that the transistor is an N-channel transistor.

According to the present invention, the drain of the transistor whose gate and drain are connected is connected to the drain of the transistor forming the current mirror of the folded cascode stage, and the source of the transistor is the source of the transistor forming the current mirror. When the folded cascode type operational amplifier tries to output a voltage higher than the output voltage range,
The limit can be applied at a level that does not saturate the output transistor.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. The same parts as those of the conventional example are denoted by the same reference numerals. FIG. 1 is a circuit diagram showing a folded cascode type operational amplifier circuit in which a differential stage, a folded cascode stage, and an output stage are constituted by MOS transistors according to an embodiment of the present invention, and FIG. 2 is an input signal of the circuit. FIG. 4 is a diagram showing a relationship between the output signal and the output signal;

As shown in FIG. 1, the operational amplifier circuit according to the present invention is basically the same as the circuit shown in FIG. 4 except for the differential stage 10, folded cascode stage 11 and output stage 12.
Are composed of MOS transistors. Then, in the present invention, the drains of the transistors M8 and M10 whose gates and drains are connected are connected to the drains of the transistors M9 and M10 constituting the current mirror of the folded cascode stage 11. The sources of the transistors M8 and M10 are connected to the same potential as the sources of the transistors M9 and M10 forming a current mirror.

That is, as shown in FIG. 1, the drain of an N-channel transistor M8 having a drain-gate connected to the drain side node of the transistor M9 between the transistors M6 and M9 of the folded cascode stage 11 is connected. Further, the drain of the N-channel transistor M11 whose drain and gate are connected is connected to the drain node of the transistor M10 between the transistors M7 and M10 of the folded cascode stage 11. The sources of the transistors M8 and M11 are connected to the lowest potential V.
Connected to SS.

As described above, the N-channel transistor M
5, M8, M9. The sources of M10, M11, and M13 are connected to the lowest potential VSS.

In the operational amplifier configured as described above, even if the output VOUT attempts to output a voltage exceeding the output voltage range, the transistors M8 and M11
Since the current flows between the source and the drain of the transistor M10, the drain potential of the transistor M10 does not become the minimum potential VSS. Therefore, a voltage sufficient to operate the transistor M13 is applied to the gate of the transistor M13.

Therefore, the drain of the transistor M13
Since the current always flows between the sources,
The drain potential of the transistor M12 is
The voltage is limited by the voltage (VDD-VSAT) determined by the current flowing through No. 3 so that the voltage is not saturated. As a result, as shown in FIG. 2, when the output VOUT of the operational amplifier tries to output a voltage exceeding the output voltage range, the voltage is limited by the voltage (VDD-VSAT), and the output state within the output voltage range is reduced from there. When there is a transition, no operation delay occurs and no waveform distortion occurs.

[0024]

As described above, according to the present invention, even when the folded cascode type operational amplifier attempts to output a voltage higher than the output voltage range, the output transistor is limited at a level that does not saturate the output transistor. The occurrence of delay is suppressed, and the occurrence of distortion in the output waveform can be prevented.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing a relationship between an input signal and an output signal of the circuit shown in FIG.

FIG. 3 is a block diagram illustrating an inverting amplifier circuit.

FIG. 4 is a circuit diagram showing a folded cascode type operational amplifier circuit as a premise of the present invention.

5 is a diagram showing a relationship between an input signal and an output signal of the circuit shown in FIG.

[Description of Signs] 10 Differential stage 11 Folded cascode stage 12 Output stage M9 N-channel transistor (for current mirror) M10 N-channel transistor (for current mirror) M8 N-channel transistor M11 N-channel transistor M12 P-channel transistor (output) M13 N-channel transistor (output)

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5J066 AA01 AA46 AA47 CA21 CA65 FA01 HA10 HA17 HA25 HA29 KA02 KA09 MA11 MA17 ND01 ND14 ND22 ND23 PD02 TA01 TA06 5J090 AA01 AA46 AA47 CA21 CA65 FA01 GN01 HA10 HA17 HA17 MN01 TA01 TA06 5J091 AA01 AA46 AA47 CA21 CA65 FA01 HA10 HA17 HA25 HA29 KA02 KA09 MA11 MA17 TA01 TA06

Claims (2)

[Claims] In a folded cascode type operational amplifier circuit in which a differential stage, a folded cascode stage and an output stage are constituted by MOS transistors, a gate terminal is connected to a drain terminal of a transistor constituting a current mirror of the folded cascode stage. An operational amplifier circuit in which the drain of a transistor to which the drain is connected is connected, and the source of the transistor is connected to the same potential as the source of the transistor forming the current mirror. 2. The operational amplifier according to claim 1, wherein the transistor is an N-channel transistor.