JP2970688B2 - Buffer amplifier for liquid crystal display - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a buffer amplifier, and more particularly to a buffer amplifier with improved characteristics in a high frequency band.

[Background Art] To drive a liquid crystal panel with an analog LCD driver,
It is necessary that the buffer amplifier used has a flat frequency characteristic from DC to about 100 MHz and a signal amplitude of about 13 Vpp. To create a wideband buffer amplifier that covers such a low frequency band to a high frequency band, high-performance elements are usually required, and commercially available operational amplifiers such as OP160 manufactured by Analog Devices and National Semiconductor manufactured by National Semiconductor Even if LM6181 is used, the above characteristics cannot be achieved.

As described above, it is difficult to make an inexpensive and wideband buffer amplifier, so the drive frequency is lowered.However, lowering the drive frequency complicates signal distribution and reduces the effect of reducing the drive frequency. The problem arises. In analog LCD drivers,
Since a high voltage output is also required, the cost is generally high.

Incidentally, there is a method of using a peaking coil as a method of improving the high frequency characteristics. For example, a circuit as shown in FIG. The circuit is composed of an input resistor Rb, a parasitic capacitance C1 between the base and emitter of the transistor tl, a parasitic capacitance C2 between the collector and emitter of the transistor tl, a peaking coil L, a resistor R1, a coupling capacitor Cc, and an output resistor Ro. It is configured. Here peaking is
It is generated by the parasitic capacitance C2, the peaking coil L, and the resistor Rl. Therefore, since the gain of the circuit increases near the resonance frequency, a flat characteristic can be obtained by adjusting the size of the resistor Rl (see FIG. 12). However, since the emitter is grounded, if the input signal enters the high frequency band due to the parasitic capacitance C1, it escapes to the ground, so that the overall frequency characteristics are not good. In addition, no treatment is given to the portion above the resonance frequency, so that the characteristics suddenly deteriorate above the resonance frequency. In order to obtain sufficient frequency characteristics in a high frequency band, it is necessary to increase the resonance frequency. However, this requires that the characteristics of the transistor tl in the high frequency band be good and that an expensive transistor be used. Such a circuit is also disclosed in JP-A-4-104578.

DISCLOSURE OF THE INVENTION Accordingly, an object of the present invention is to provide a buffer amplifier having a wider frequency characteristic by using inexpensive elements.

Another object of the present invention is to provide a buffer amplifier capable of supporting a high voltage output, such as an analog LCD driver.

In order to achieve such an object, a first aspect of the present invention provides an amplifier, a high-pass filter connecting an input side and an output side of the amplifier, and an input signal in a high frequency band connected to the amplifier. And an impedance adjuster that raises the impedance of the high-pass filter so that the input signal flows through the high-pass filter. In this way, the characteristics of the amplifier can be used in the low-frequency band, and when the limit of the characteristics of the amplifier is reached in the high-frequency band, the signal can be passed through the high-pass filter, so that the buffer amplifier can be used. Is to improve the frequency characteristics of.

In this case, the high-pass filter may be a capacitor. It is also conceivable that the impedance adjuster includes an inductor and a resistor for adjusting characteristics in a high frequency band.

Still another aspect of the present invention provides an inductor, a resistor for adjusting peaking by the inductor, a transistor whose output side is connected to the input terminal, the base of which is grounded, the input side of the inductor, and the output of the transistor. There is a buffer amplifier having a capacitor for connecting terminals. In this way, resonance is generated by using the inductor and the junction capacitance generated between the base and emitter of the base-grounded transistor, and the high frequency characteristics are improved by peaking as a result of the resonance. In particular, it prevents sudden deterioration of characteristics and provides a wideband buffer amplifier as a whole.

In still another embodiment, an inductor having one end grounded, a resistor for adjusting peaking by the inductor, a transistor having the other end connected to the base, and an input terminal and an output terminal of the transistor are connected. And a buffer amplifier having a capacitor. This is a modification from the above-described embodiment, and is configured to provide a peaking coil on the base. Even in this case, the effect of peaking and the effect in the region above the resonance frequency are the same. Such a modification has an effect that the number of elements can be reduced in the case of the following circuit.

That is, a capacitor that connects the first transistor, the second transistor, the input terminal of the first transistor, and the output terminal of the second transistor, one end of which is connected to the bases of the first and second transistors, and the other end Has an inductor grounded, and a resistor for adjusting peaking by the inductor, wherein an output terminal of the first transistor and an input terminal of the second transistor are connected. If you do this,
Modifications can be made so that a signal can be extracted at a desired output voltage.

In general, a plurality of transistors connected in series, an input terminal of the most input side transistor of the plurality of transistors, a capacitor connecting the output terminal of the most output side transistor, and one end having a plurality of An inductor connected to the base of the transistor and having the other end grounded;
And a resistor for adjusting peaking by the inductor.

This can be modified to provide an inductor for each of the plurality of transistors.

In still another aspect, the first transistor and the second transistor
A transistor, a capacitor connecting the input terminal of the first transistor, and the output terminal of the second transistor, one end connected to the bases of the first transistor and the second transistor, and the other end adjusting the inductor and peaking by the inductor; A semiconductor device including a buffer amplifier having an output pin for connecting a resistor for performing the operation is conceivable. In this way, what can be mounted on the semiconductor chip can be mounted, and an inductor that can not be mounted and a resistor for its adjustment are arranged outside the semiconductor chip, and an output pin for that is provided, so that integration with other circuits is possible. Can be achieved.

The above-described buffer amplifier (including a semiconductor device) has various uses, and can be used, for example, in a stage preceding an analog LCD driver. That is, as shown in FIG. 10, a TFT_LCD panel 13 and an analog LCD
A driver 3, a buffer amplifier 5 according to the present invention,
It is also conceivable to configure the liquid crystal display device 1 with the D / A converter (DAC) 7. Here, the signal to the DAC 7 is input from the frame buffer 9, but a circuit for performing gamma correction may be provided before or after the DAC 7, or inside the DAC 7. If you enable high-speed drive in this way,
The liquid crystal display device 1 can cope with an increase in size, and finally a clear image can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a base-grounded transistor, FIG.
The figure shows the equivalent circuit of FIG.

FIG. 3 is a diagram showing an outline of the present invention, and FIG. 4 is a diagram showing an equivalent circuit of FIG.

FIG. 5 is a diagram showing a modification of the present invention, and FIG. 6 is a diagram showing an equivalent circuit of FIG.

FIG. 7 is a diagram showing a modification of the present invention modified to obtain a high voltage output.

 FIG. 8 is a diagram showing the frequency characteristics of FIG.

FIG. 9 is a schematic diagram when the present invention is applied to a semiconductor device.

FIG. 10 is a block diagram of the whole liquid crystal display device including the present invention.

FIG. 11 is a drawing for explaining the prior art, and FIG.
FIG. 12 is a diagram showing the characteristics.

The correspondence between the numbers of the main components and the names is as follows.

1 Liquid crystal display device 3 Analog LCD driver 5 Buffer amplifier 7 D / A converter 9 Frame backup 13 TFT_LCD panel 15 Output pin 17 Resistance 19 Peaking coil 21 Ground 23 Semiconductor device [Best mode for carrying out the invention] First The grounded-base transistor will be described with reference to FIGS. 1 and 2. FIG. As shown in FIG. 1, the base of a PNP transistor is grounded. At this time, a junction capacitance called C _BE occurs between the base and the emitter. FIG. 2 shows the equivalent circuit of FIG. Equivalently, a resistor R _BE is also generated in parallel with the C _BE , so that a low-pass filter (LPF) is formed by these two elements. Therefore, if a transistor having a common base is used as it is, the frequency characteristic is deteriorated at about 30 MHz.

Therefore, consider a circuit as shown in FIG. That is, the resistance adjusting a peaking coil L _P, the peaking
Provided the R _P in parallel, one end to the emitter of the transistor, the other end to the input side. Then, the input side and the collector of the transistor are connected by a capacitor C. Naturally, the base of the transistor is grounded. Even with such a circuit, a junction capacitance C _BE is generated between the base and the emitter, but an equivalent circuit of this circuit (FIG. 4) shows that the above-mentioned LPF is not formed. As will be readily apparent to those skilled in the art, this constitutes a resonant circuit (details are omitted for clarity).

In this circuit, the values of C _BE and R _BE are determined by the transistor used, so the peaking coil
It is necessary to adjust the frequency characteristics of the circuit according to the values of L _P and resistance R _P. In this case, the resonance frequency may be changed according to the value of the coil, and the characteristics may be flattened (Q-dumped) according to the value of the resistor. If the resonance frequency is set near a frequency at which the characteristics of a normal transistor deteriorate, the gain increases due to the effect of peaking, and the entire dynamic range is widened.

Next, the operation of this circuit will be described. When the frequency of the input signal is low, it operates as a normal base-grounded transistor. However, occurs series resonance by frequency normal base the characteristics of the transistor having the ground to enter the high frequency band such as to reduce peaking coil L _P and the junction capacitance C _BE of the input signal, the input signal flows through the resistor R _BE , The operation is the same as that in the low frequency band. Therefore, the gain of the entire circuit is increased, and it is possible to obtain a frequency characteristic as if the capacity of the transistor was apparently increased. If further increased the frequency of the input signal, the impedance of the circuit of peaking coil L _P later rises, the transistor is cut off. Therefore, in a high frequency band exceeding the capability of the transistor, the signal is output bypassed by the capacitor C,
Deterioration of characteristics becomes gentle.

FIG. 5 shows a modification of the circuit of FIG. Looking at this Figure 5, it can be seen that the position of peaking coil L _P and resistor R _P is changed. That is, provided a capacitor C so as to connect between the emitter and the collector of the transistor, it is provided between the base and the ground of the transistor peaking coil L _P and resistor R _P in parallel. By doing so, the equivalent circuit is as shown in FIG. Here, as can be seen, the change in the equivalent circuit the resistance peaking coil L _P
Only the order of the parallel circuit of R _{P and} the parallel circuit of the junction capacitors C _BE and R _BE is different, and a resonance circuit is configured in the same manner as in FIG.

Therefore, the matters described in the description of FIGS. 3 and 4 can be applied as they are. That is, if the frequency of the input signal is
When entering a high frequency band where the characteristics of a normal grounded transistor deteriorate, the input signal is
_{The current} flows to the _BE, which is similar to the operation in the low frequency band. When the frequency band exceeds the capacity of the transistor, an input signal flows to the capacitor C, and the characteristics gradually deteriorate.

FIG. 7 shows an example in which the circuit shown in FIG. 5 is applied to a circuit for amplifying the output from +1 V to +7.5 and from +7.5 to +14 V for an analog LCD driver. The supply of the input signal in FIG. 7 is a D / A converter (DAC). This DAC is of a constant current type, and generates a voltage of about 0 V to 2 V. That DA
C is provided with a circuit to which the above-described circuit of FIG. 5 is applied. The circuit for performing the frequency peaking is a PNP transistor Q1 and a capacitor C connected to its emitter.
When configured with NPN transistor Q2 connected to the collector of the transistor Q1, a peaking coil L _P connected to the base of the transistor Q1 and Q2 by resistor R _P. In FIG. 7, the model numbers of the transistors Q1 to Q3 are described, but this is merely an example and can be replaced with an equivalent one. This frequency peaking circuit uses two transistors, but simply connects the two transistors in series,
The peaking coil is not provided independently at the base of each transistor, but is provided only in common.
The use of the PNP type and the NPN type is related to peripheral circuits and is not special. The operation of this frequency peaking circuit is the same as that of the circuit shown in FIG.

The output section includes constant current sources I1 and I2, a diode D1,
It consists of resistors R1 and R2 and transistor Q3, and the current flowing through the frequency peaking circuit and the constant current sources I1 and I2
The voltage at the connection point A in FIG. 7 is changed by the current from the power supply of 7.5V, and the output of the transistor Q3 is changed from + 1V to + 1V.
Amplify from 7.5V and + 7.5V to 14V. The output section is not the gist of the present invention and will not be described further.

FIG. 8 shows the frequency characteristics of the circuit shown in FIG. Line A shows the case where no peaking coil is used, and the gain is about half at 100 MHz. On the other hand, when the resonance is performed by using the peaking coil, a line C is obtained. In this case, it can be seen that there is a resonance frequency slightly over 100 MHz, but it is not suitable as a buffer amplifier because it does not have a flat characteristic. Line B shows a case where the resistance _RP is appropriately adjusted and Q dump (Q = 1) is performed. As mentioned above, the resonance frequency
Since the frequency slightly exceeds 100 MHz, the characteristics are flat up to approximately 100 MHz when Q dump is performed. 100M
The characteristic after Hz does not drop sharply.

Although the embodiments of the present invention have been described above, these can be variously modified. That is, if the transistor is 1
Although an example in which two or two pieces are provided is shown, the present invention is not limited to this. If necessary in the peripheral circuit, three or more circuits can be considered. At this time, whether the peaking coil is provided on the base or on the input terminal is arbitrary, and if necessary, may be provided for each transistor. When the peaking coil is provided on the base, only one common peaking coil can be used, as described in FIG. Also, as described with reference to FIG. 7, the capacitor C having the effect of the high-pass filter can be substituted by one when there are a plurality of transistors. Whether this is provided for each transistor depends on the relationship with peripheral circuits and the like.

Further, since the inductor cannot be integrated in the semiconductor device, the transistor, the resistor, and the capacitor may be provided in the semiconductor device as an integrated circuit, and an output pin for connecting the inductor may be provided. In this case, the resistance R _P for adjusting the peaking can be provided in the semiconductor device if the determined its value, since it will generally adjustment necessary, connected at the output pin with the inductor Would be appropriate. This example will be described with reference to FIGS. 9 and 10. The block 11 shown in FIG. 9 can be one semiconductor device. However, since the inductor cannot be included as described above, as shown in FIG.
A transistor and a capacitor are included in the semiconductor device 23, and the resistor 17 and the peaking coil 19 are connected from the output pin 15 of the semiconductor device 23 by a line 25 on a substrate (not shown) connected thereto. If the ends of the resistor 17 and the peaking coil 19 opposite to the ends connected to the output pins are connected to the ground 21 via the wires 27 of the substrate, the same effect can be obtained.

[Industrial Applicability] In summary, with the above-described configuration, it was possible to provide a buffer amplifier having a wider frequency characteristic using inexpensive elements.

In addition, it was possible to provide a buffer amplifier that can handle high-voltage output, such as an analog LCD driver.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Mitsuri Suzuki 1623-14 Shimotsuruma, Yamato-shi, Yamato Prefecture IBM Japan Yamato Office (56) References JP-A-5-315865 (JP, A JP-A-64-51704 (JP, A) JP-A-4-40106 (JP, A) JP-A-61-29203 (JP, A) JP-A-6-252668 (JP, A) (58) Field (Int.Cl. ⁶ , DB name) H03F 1/42 G09G 3/36

Claims (2)

(57) [Claims] An inductor having one end grounded; a resistor for adjusting peaking by the inductor; a transistor having the other end connected to a base; and an input terminal and an output terminal of the transistor. A buffer amplifier having a capacitor and. 2. A capacitor for connecting a first transistor, a second transistor, an input terminal of the first transistor, and an output terminal of the second transistor, one end of the first transistor and the second transistor. An inductor connected to the base and having the other end grounded; and a resistor for adjusting peaking by the inductor, wherein an output terminal of the first transistor and an input terminal of the second transistor are connected. A buffer amplifier characterized by the following.