Operational amplifier and related method of enhancing slew rate

1. Field of the Invention

The present invention relates to an operational amplifier and related method of enhancing slew rate, and more particularly, to an operational amplifier and related method that reallocates current intensities of an amplification stage bias current and an output stage drive current to increase drive capability of the amplification stage circuit.

2. Description of the Prior Art

An operational amplifier is a widely used element for realizing a variety of circuit functions. Taking driving circuits of a liquid crystal display (LCD) as an example, the operational amplifier can be used as an output buffer, which charges or discharges loads, i.e. liquid crystals, according to analog signals outputted by a front stage digital to analog converter (DAC), for driving corresponding pixel units on the LCD. However, with increases in size and resolution of the LCD, data quantity processed by the driving circuits is also increasing significantly, so that response speed of the operational amplifier, also called slew rate, has to be enhanced as well.

In a conventional driver chip, the operational amplifier generally has a two-stage structure, which includes a first stage amplification circuit (amplification stage) and a second stage output circuit (output stage). The first stage amplification circuit is utilized for increasing current or voltage gain of the operational amplifier, while the second stage output circuit is utilized for driving capacitive or resistive loads connected to the operational amplifier. In addition, since the operational amplifier may suffer loop instability problems, Miller compensation capacitors are commonly used to perform frequency compensation for improving loop stability.

Please refer to FIG. 1 . FIG. 1 is a schematic diagram of a conventional operational amplifier 100 . For brevity, the operational amplifier 100 is an operational amplifier with a simplest circuit structure, and merely has an N-type input differential pair. The operational amplifier 100 mainly includes an amplification stage circuit 110 , an output stage circuit 120 , a first bias current source 115 and a second bias current source 125 . The amplification stage circuit 110 is formed by transistors MP 1 , MP 2 , MN 1 and MN 2 . The output stage circuit 120 is formed by a transistor MPO that has a common emitter configuration. The first bias current source 115 and the second bias current source 125 are utilized for providing fixed static currents (or driving currents) IT 1 and IT 2 for the amplification stage circuit 110 and the output stage circuit 120 , respectively. Furthermore, an output terminal Vout of the output stage circuit 120 is further coupled to an input terminal AVN of the amplification stage circuit 110 for forming an output buffer with unit gain and negative feedback. In addition, a compensation capacitor C M is coupled between an output terminal of the amplification stage circuit 110 , i.e. a drain of the transistor MN 2 , and the output terminal Vout of the output stage circuit 120 , and is utilized for performing pole-splitting for output signals of the amplification stage circuit 110 and the output stage circuit 120 , so as to enhance loop stability. Detailed operation of the operational amplifier 100 is well-known by those skilled in the art, and not narrated herein.

Specifically, the response speed of the operational amplifier 100 is decided by the bias currents of the amplification stage circuit and the output stage circuit. However, in order to drive external loads of the operational amplifier 100 , the output stage drive current is generally greater than the amplification stage bias current. In this case, when the operational amplifier drives a heavy load, the response speed is often restricted by the bias current of the amplification stage circuit.

Please refer to FIG. 2 . FIG. 2 illustrates an internal current path of the operational amplifier 100 when an input signal of the operational amplifier 100 is converted from low to high. Since a voltage level of the input terminal AVP is raised immediately when the input signal of the operational amplifier 100 is converted, the transistors MN 1 , MP 1 and MP 2 are then switched off, so that the first bias current source 115 can only draw the bias current IT 1 from the compensation capacitor C M through the transistor MN 2 for pulling up the voltage level of the output terminal Vout. The above current path is shown as PATH_ 1 in FIG. 2 . On the other hand, please refer to FIG. 3 , which illustrates an internal current path of the operational amplifier 100 when the input signal of the operational amplifier 100 is converted from high to low. Since the voltage level of the input terminal AVP is dropped immediately when the input signal is converted, the transistor MN 2 is then switched off, so that the bias current IT 1 is drawn entirely through the transistor MN 1 . Additionally, since the transistors MP 1 and MP 2 have a current mirror structure, a current with intensity the same as the bias current IT 1 also simultaneously flows to the ground through the compensation capacitor C M , so as to pull down the voltage level of the output terminal Vout. The above current path is shown as PATH_ 2 in FIG. 3 .

Thus, it can be seen that the response speed of the operational amplifier 100 depends on how fast the bias current IT 1 of the amplification stage circuit charges or discharges the compensation capacitor C M , and can be expressed by the following slew rate equation:

S ⁢ ⁢ R = I T ⁢ ⁢ 1 C M = Δ ⁢ ⁢ V t .

Thus, if the amplification stage bias current increases, the compensation capacitor can be charged or discharged much faster, so that the response speed of the operational amplifier 100 can be enhanced.

In the prior art, the internal slew rate of the operational amplifier is generally enhanced by increasing the bias current IT 1 of the amplification stage circuit. However, this not only increases circuit area, e.g. by increasing layout area of the bias transistors, but also causes additional power consumption of the operational amplifier. Thus, an import issue for circuit designers in this art is how to increase the slew rate of the operational amplifier without increasing power consumption.

It is therefore an objective of the present invention to provide an operational amplifier and related method capable of enhancing slew rate.

According to the present invention, an operational amplifier capable of enhancing slew rate is disclosed. The operational amplifier includes a first current generator for generating a first bias current; a second current generator for generating a second bias current; an amplification stage circuit for generating an amplification signal according to an input signal; an output stage circuit, coupled to the amplification stage circuit, for generating an output signal according to the amplification signal, and a bias current allocation unit, coupled to the first current generator, the second current generator, the amplification stage circuit and the output stage circuit, for reallocating current intensities of the first bias current and the second bias current to enhance the slew rate of the operational amplifier according to a control signal.

According to the present invention, a method capable of enhancing slew rate of an operational amplifier is further disclosed. The operational amplifier includes an amplification stage circuit and an output stage circuit. The method includes steps of generating a first bias current to the amplification stage circuit; generating a second bias current to the output stage circuit; and reallocating current intensities of the first bias current and the second bias current for enhancing the slew rate of the operational amplifier according to a control signal.

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

FIG. 1 is a schematic diagram of a conventional operational amplifier.

FIG. 2 illustrates an internal current path of the operational amplifier in FIG. 1 when an input signal is converted from low to high.

FIG. 3 illustrates an internal current path of the operational amplifier in FIG. 1 when an input signal is converted from high to low.

FIG. 4 is a schematic diagram of a process capable of enhancing slew rate of an operational amplifier according to an embodiment of the present invention.

FIG. 5 is a schematic diagram of an operational amplifier capable of enhancing slew rate according to an embodiment of the present invention.

FIG. 6 is a schematic diagram of an operational amplifier according to a first embodiment of the present invention.

FIG. 7 is a schematic diagram of an operational amplifier according to a second embodiment of the present invention.

FIG. 8 is a schematic diagram of an operational amplifier according to a third embodiment of the present invention.

Please refer to FIG. 4 . FIG. 4 is a schematic diagram of a process 40 for enhancing slew rate of an operational amplifier according to an embodiment of the present invention. Generally, the operational amplifier includes an amplification stage circuit and an output stage circuit. The amplification stage circuit is utilized for increasing current or voltage gain of the operational amplifier, and the output stage circuit is utilized for driving capacitive or resistive loads connected to the operational amplifier. The process 40 includes the following steps:

Step 430 : Reallocate current intensities of the first bias current and the second bias current for enhancing the slew rate of the operational amplifier according to a control signal.

According to the process 40 , the first bias current and the second bias current are respectively generated as static currents (or drive currents) of the amplification stage circuit and the output stage circuit. Then, the current intensities of the first bias current and the second bias current are reallocated for enhancing the slew rate of the operational amplifier according to the control signal in the present invention. Preferably, the control signal can be generated before the operational amplifier receives an input signal, before transition of the input signal occurs, or during the transition of the input signal.

In other words, before the operational amplifier receives the input signal or before the transition of the input signal occurs, the current intensities of the amplification stage bias current and the output stage drive current are reallocated to increase the bias current of the amplification stage circuit, so as to enhance internal drive capability of the operational amplifier. In this case, without increasing power consumption, the slew rate of the operational amplifier can be enhanced.

Please refer to FIG. 5 . FIG. 5 is a schematic diagram of an operational amplifier 500 capable of enhancing slew rate according to an embodiment of the present invention. The operational amplifier 500 is utilized for realizing the process 40 of the present invention, and includes an amplification stage circuit 510 , an output stage circuit 520 , a first current generator 515 , a second current generator 525 and a bias current allocation unit 530 . The amplification stage circuit 510 is formed by transistors MP 1 , MP 2 , MN 1 and MN 2 , and is utilized for generating an amplification signal according to an input signal received by an input terminal AVP. The output stage circuit 520 is formed by a transistor MPO that has a common emitter configuration, and is utilized for generating an output signal through an output terminal Vout according to the amplification signal. The first current generator 515 and the second current generator 525 are utilized for generating a first bias current IT 1 and a second bias current IT 2 to drive the amplification stage circuit 510 and the output stage circuit 520 , respectively. Furthermore, the output terminal Vout of the output stage circuit 520 is further coupled to an input terminal AVN of the amplification stage circuit 510 for forming an output buffer with unit gain and negative feedback. In addition, a compensation capacitor C M is coupled between an output terminal of the amplification stage circuit 510 , i.e. a drain of the transistor MN 2 , and the output terminal Vout of the output stage circuit 520 , and is utilized for performing frequency compensation for output signals of the amplification stage circuit 510 and the output stage circuit 520 to enhance loop stability. Detailed operation of the operational amplifier 500 is similar to the operational amplifier 100 in FIG. 1 , and not narrated again herein.

The bias current allocation unit 530 is coupled to the first current generator 515 , the second current generator 525 , the amplification stage circuit 510 and the output stage circuit 520 , and is utilized for reallocating current intensities of the first bias current IT 1 and the second bias current IT 2 to enhance the slew rate of the operational amplifier according to a control signal CTRL. Preferably, the control signal CTRL is generated before the input signal is received by the operational amplifier 500 or before the transition of the input signal occurs.

Therefore, before the operational amplifier 500 receives the input signal or before the transition of the input signal occurs, the current intensities of the amplification stage bias current and the output stage drive current are reallocated to increase the bias current of the amplification stage circuit 510 for increasing charging or discharging speed of the compensation capacitor C M , so that the response speed of the operational amplifier can be enhanced. In this case, without increasing power consumption, the slew rate of the operational amplifier can be enhanced as well.

For example, in the embodiment of the present invention, the bias current of the amplification stage circuit can be increased by switching a ratio of the output stage drive current to the amplification stage circuit, exchanging the amplification stage bias current with the output stage drive current, exchanging the amplification stage bias current with a ratio of the output stage drive current, and so on, for enhancing the slew rate of the operational amplifier.

Please refer to FIG. 6 . FIG. 6 is a schematic diagram of an operational amplifier 600 according to a first embodiment of the present invention. The operational amplifier 600 is an exemplary embodiment of the operational amplifier 500 , and includes an amplification stage circuit 610 , an output stage circuit 620 , a first current generator 615 , a second current generator 625 and a bias current allocation unit 630 . The first current generator 615 includes a bias transistor MS 1 , which is utilized for generating a current intensity I to form the first bias current IT 1 according to a fixed bias voltage V BIAS . The second current generator 625 includes two parallel bias transistors MS 2 and MS 3 , which are utilized for generating current intensities K*I and (N−K)*I to form the second bias current IT 2 according to the fixed bias voltage V BIAS , respectively. Apparently, the second bias current IT 2 is N times the current intensity of the first bias current IT 1 . The bias current allocation unit 630 includes switches S 1 and S 1 B, which are shorted alternatively according to opposite phases of the control signal CTRL, for switching a specific ratio of the second bias current IT 1 , i.e. the current intensity K*I, to the amplification stage circuit 610 , so as to increase the bias current of the amplification stage circuit 610 .

Thus, before the operational amplifier 600 receives the input signal or before the transition of the input signal occurs, some of the output stage drive current is switched to the amplification stage circuit 610 for enhancing drive capability of the amplification stage circuit 610 . Accordingly, when a voltage level of the input signal is converted, the compensation capacitor C M can be rapidly charged or discharged by the increased bias current IT 1 of the amplification stage circuit 610 , so the response speed of the operational amplifier 600 can be enhanced. In this case, the slew rate of the operational amplifier 600 can be expressed by the following equation:

S ⁢ ⁢ R = I T ⁢ ⁢ 1 C M = ( 1 + K ) × I C M = Δ ⁢ ⁢ V ′ t .

Apparently, with the increase of the bias current IT 1 , the slew rate of the operational amplifier is increased as well.

In addition, since the bias current of the amplification stage circuit 610 is increased by drawing some of the output stage bias current, the slew rate of the operational amplifier can thus be enhanced without additional power consumption. Certainly, after the voltage level of the output terminal comes up with that of the input terminal, the amplification stage bias current and the output stage drive current can be restored to their initial states, so that the large output stage bias current can be used to drive external loads of the operational amplifier 600 .

Note that the first current generator 615 and the second current generator 625 can be realized by any current sources capable of generating fixed current intensities, and are not restricted in the above-mentioned transistors.

Please further refer to FIG. 7 . FIG. 7 is a schematic diagram of an operational amplifier 700 according to a second embodiment of the present invention. The operational amplifier 700 is another exemplary embodiment of the operational amplifier 500 , and includes an amplification stage circuit 710 , an output stage circuit 720 , a first current generator 715 , a second current generator 725 and a bias current allocation unit 730 . The first current generator 715 and the second current generator 725 are utilized for generating a first bias current IT 1 and a second bias current IT 2 to drive the amplification stage circuit 710 and the output stage circuit 720 , respectively. The bias current allocation unit 730 includes switches S 1 , S 1 B, S 2 and S 2 B. The switches S 1 , S 2 and the switches S 1 B, S 2 B are shorted alternately according to opposite phases of the control signal CTRL, and are utilized for exchanging to output the first bias current IT 1 and the second bias current IT 2 to the output stage circuit 720 and the amplification stage circuit 710 .

Since the output stage bias current IT 2 is generally greater than the amplification stage bias current IT 1 , the drive capability of the amplification stage circuit 710 can be further increased by exchanging the amplification stage bias current IT 1 and the output stage bias current IT 2 , so as to increase charging or discharging speed of the compensation capacitor C M . Similarly, in this embodiment, the slew rate of the operational amplifier can also be enhanced without increasing power consumption.

Note that the above embodiments are merely exemplary illustrations of the present invention but not limitations, such that those skilled in the art can certainly make appropriate modifications according to practical demands. For example, please refer to FIG. 8 . FIG. 8 is a schematic diagram of an operational amplifier 800 according to a third embodiment of the present invention. The operational amplifier 800 can be considered a combination of the embodiments in FIG. 6 and FIG. 7 , and exchanges a ratio of the output stage bias current, i.e. the current intensity K*I generated by the bias transistor MS 2 , with the amplification stage bias current in order to achieve the purpose of enhancing the slew rate of the operational amplifier. Such corresponding variation also belongs to the scope of the present invention.

Additionally, in other embodiments of the present invention, modifications like realizing the operational amplifier by P-type differential pairs or realizing the bias current allocation unit by a transmission gate can also be made, and are not restricted herein. Preferably, the operational amplifier can be applied in a driving circuit of a liquid crystal display (LCD) for enhancing response speed of the driving circuit, so as to satisfy increasing demands in large size and high resolution LCDs.

As mentioned above, before the operational amplifier receives the input signal, and before or during the transition of the input signal occurs, the current intensities of the amplification stage bias current and the output stage drive current may be reallocated to enhance the drive capability of the amplification stage circuit, so as to increase the response speed of the operational amplifier. In this case, the slew rate of the operational amplifier can be enhanced without increasing power consumption.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.