WO2008082059A1

WO2008082059A1 - Amplifier with damping resistor in constant current load

Info

Publication number: WO2008082059A1
Application number: PCT/KR2007/004704
Authority: WO
Inventors: Ok-Sang Jin
Original assignee: Ok-Sang Jin
Priority date: 2006-12-29
Filing date: 2007-09-27
Publication date: 2008-07-10

Abstract

The invention provides an amplifier having a damping resistor provided in a constant current load that is capable of reducing ringing caused by a rapid variation in a signal and providing signals without distortion. The amplifier having a damping resistor provided in a constant current load includes: a drive amplification stage; an output stage that includes complementary transistors, that is, first and second transistors having bases connected to an output terminal of the drive amplification stage; a constant current load stage that is connected to the output terminal of the drive amplification stage; and a damping resistor that is connected in parallel to the constant current load stage. According to the above-mentioned structure, the damping resistor connected in parallel to the constant current load makes it possible to improve an amplification gain and reduce the occurrence of ringing, thereby obtaining output signals without distortion.

Description

AMPLIFIER WITH DAMPING RESISTOR IN CONSTANT

CURRENT LOAD

Technical Field

[1] The present invention relates to an amplifier having a damping resistor provided in a constant current load, and more particularly, to an amplifier having a damping resistor provided in a constant current load that can reduce a high frequency noise generated from a constant current load. Background Art

[2] In a class-A amplifier using a transistor, a current flowing between the collector and the emitter is varied by a signal voltage input to the base, and is then converted into a voltage by a load connected to the collector, thereby obtaining an amplified signal. The load includes a resistor, a coil, a transformer, or a constant current circuit. When the coil or the transformer is used as a load, a direct current is used as an operating current, and the coil or the transformer operates as an AC load in the actual operation. The impedance of the load depends on the design of the AC load. However, in this case, the coil or the transformer is not generally used as a load since a frequency band is narrow due to the coil, a signal is distorted due to the coil, and the coil or the transformer has a large size.

[3] For this reason, a resistor is generally used as the load. The resistor load used in the

A-class amplifier has problems in that, when operating conditions are determined, a characteristic curve becomes fixed, such that it is difficult to adjust an amplification degree, and a voltage gain is low. Particularly, the resistor load used in the A-class amplifier has problems in that the resistance range of the load resistor is narrowed due to the operating current and it is difficult to obtain a high gain.

[4] Fig. 1 is a circuit diagram illustrating a general class-A amplifier including a resistor load. A normal bias is applied to a base of a transistor Q31. A capacitor C31 is a coupling capacitor that separates an input signal source from the base of the transistor Q31 in a DC manner. It is assumed that a collector voltage of the transistor Q31 is half the positive power supply voltage VPP3. Fig. 2 is a characteristic curve of a resistor R31 shown in Fig. 1. In Fig. 2, it is assumed that there is no voltage drop between the collector and the emitter of the transistor Q31 and there is no voltage drop between both ends of a resistor R32.

[5] In Fig. 1, when a signal of 0 is input to an input terminal IN3, a normal bias is applied to the transistor Q31. Therefore, the collector voltage of the transistor Q31 is half the positive power supply voltage VPP3. In this case, a collector current ice_Q31 of the transistor Q31 satisfies the following relationship:

[6] ice_Q31 = VPP3 / 2 / R31, which represented by a point B in the graph of Fig. 2.

[7] When a negative signal having the lowest level is input to the input terminal IN3, a base current of the transistor Q31 does not flow, and no current flows between the collector and the emitter. Therefore, no current flows through the resistor R31 , and the voltage between both ends of the resistor R31 is zero. As a result, no current flows through the resistor R31 (a point A in Fig. 2).

[8] On the other hand, when a positive signal having the highest level is input to the terminal IN3, the maximum amount of base current flows through the transistor Q31, and a current flows between the collector and the emitter, which causes a maximum amount of current to flow through the resistor R31. As a result, the voltage between both ends of the resistor R31 is equal to the positive power supply voltage VPP3, and the maximum amount of current (VPP3 / R31) flows through the resistor R31 (a point C in Fig. 2).

[9] In the circuit shown in Fig. 1 , the impedance of the resistor R31 is calculated from the graph of Fig. 2 by Expression 1 given below:

[10] [Expression 1]

[11] R31 = voltage variation / current variation,

[12] R31 - (VPP3 - 0) / (VPP3/R31 - 0).

[13] The amplification degree A3 of the voltage in Fig. 1 is calculated by Expression 2 given below:

[14] [Expression 2]

[15] A3 = R31 / R32.

[16] In the amplifier shown in Fig. 1, in order to obtain a maximum output signal from an input signal, the collector current of the transistor Q31 needs to vary from 0 to VPP3/R31, which is the maximum value. When the transistor Q31 operates in an overall collector current range of 0 to 100%, the operation of the transistor Q31 becomes unstable in the vicinities of 0% and 100%, which results in the distortion of an amplified output signal. (Description 1)

[17] In the class- A amplifier using a transistor, a voltage gain is calculated by

Expression 2.

[18] Since the resistor R32 connected to the emitter of the transistor Q31 is related to the stability of the class-A amplifier using a transistor, it is not preferable to excessively reduce the resistance of the resistor R32. Therefore, it is necessary to increase the resistance value of the resistor R31 in order to improve the amplification degree. (Description 2)

[19] When the same load resistor is used, it is preferable to narrow the operation range of the collector current in order to stabilize the operation of an amplifier and reduce the distortion of an output waveform as in Description 1. However, in this case, an output voltage is lowered. (Description 3)

[20] Further, in Description 2, it is preferable to increase the resistance value of the load resistor as large as possible in order to improve the amplification degree. However, when the resistance value of the load resistor increases under the condition the same power supply voltage is applied (that is, when the resistance value of the resistor R31 increases), the amount of collector operating current of the transistor Q31 is reduced, resulting in a low load driving force. (Description 4)

[21] For this reason, it is necessary to increase the power supply voltage in order to increase the resistance of a load resistor (Description 5). However, when the power supply voltage is fixed, it is difficult to increase the power supply voltage in a subsequent process. In this case, even though the power supply voltage increases, the impedance of the circuit increases.

[22] From Descriptions 3, 4, and 5, it is difficult to increase the resistance of a load resistor in order to improve the amplification degree of an amplifier while preventing the distortion of an amplified signal. That is, it is difficult to control the characteristic curve of a load resistor to be steep, as represented by a bold dashed line in the graph of Fig. 2.

[23] In general, the following method is used to improve the amplification degree.

[24] As shown in Fig. 3, there is a method of using a constant current circuit as a load.

Fig. 3 is a circuit diagram illustrating an example of a class-A amplifier including a constant current load CC4. In the amplifier, a normal bias is applied to a base of a transistor Q42, a capacitor C41 is a coupling capacitor that separates an input signal source from the base of the transistor Q42 in a DC manner. It is assumed that a collector voltage of the transistor Q42 is half the positive power supply voltage VPP4.

[25] A positive voltage formed between both ends of a Zener diode Z41 by a resistor

R41 is applied to the base of the transistor Q41. A current flowing between the emitter and the collector of the transistor Q41 is kept at a constant value at all times by a resistor R42 connected to the emitter of the transistor Q41.

[26] Fig. 4 is a graph illustrating a characteristic curve of the constant current load CC4 shown in Fig. 3. In Fig. 4, it is assumed that there is no voltage drop between the collector and the emitter of the transistor Q42 and between both ends of the resistor R43.

[27] In Fig. 3, when a signal of 0 is input to an input terminal IN4, a normal bias is applied to the transistor Q42. Therefore, a collector voltage of the transistor Q42 is half the positive power supply voltage VPP4 (a point B in Fig. 4)

[28] In this case, a collector current ice_Q42 is equal to a collector current of the transistor Q41. [29] A current ice_Q41 flowing through the constant current load CC4 is represented as follows:

[30] ice_Q41 = (NJZA 1 - Vbe_Q41 ) / R42.

[31] When a negative signal is input to the input terminal IN4, a base current of the transistor Q42 is reduced, and impedance between the collector and the emitter of the transistor Q42 increases, which causes a constant current to flow through the constant current load CC4. Then, the voltage between the collector and the emitter of the transistor Q41 of the constant current load CC4 becomes the minimum (a point A in Fig. 4).

[32] On the other hand, when a positive signal is input to the input terminal IN4, the base current of the transistor Q42 increases, and the impedance between the collector and the emitter of the transistor Q42 decreases, which causes a constant current to flow through the constant current load CC4. Then, the voltage between the collector and the emitter of the transistor Q41 of the constant current load CC4 becomes the maximum (a point C in Fig. 4).

[33] It has been known that the characteristic curve of a load in the amplifier shown in

Fig. 3 varies suddenly in the vicinity of the constant current load, as shown in Fig. 4. That is, in the amplifier, the constant current load operates a resistor having high resistance. Therefore, it is possible to arbitrarily set the collector current of the transistor Q41 by adjusting the resistance of the resistor R42. As a result, this structure makes it possible to solve the problem of Description 4, and it is unnecessary to increase the power supply voltage, which is explained in Description 5. However, since the characteristic curve is steep as shown in Fig. 4, the operation range of the collector current (Description 3) is excessively narrowed, which makes it difficult to perform a normal amplifying operation.

[34] In the amplifier shown in Fig. 3, an amplification degree A4 is represented as follows:

[35] A4 = impedance of constant current load CC4 / R43,

[36] A4 = infinity / R43, and

[37] A4 = infinity.

[38] An output signal is turned on or off with respect to an input signal.

[39] As described above, a class-A amplifier having a constant current load is not independently used to amplify an analog signal, but is used to stably amplify a signal using a negative feedback. In addition, the amplifier is used as an emitter load of a common collector amplifier (emitter follower).

[40] When the constant current load is provided in the negative feedback amplifier, it is possible to obtain a high voltage gain using a small number of amplifying stages and simplify a circuit structure. In addition, it is possible to freely adjust a voltage gain by controlling a negative feedback amount.

[41] Further, when the constant current load is provided in an emitter load of the common collector amplifier (emitter follower), it is possible to obtain a large amount of operation current and large input impedance, and substantially follow a variation in input signal, thereby reducing distortion.

[42] Fig. 5 is a circuit diagram illustrating a constant current load stage 12 including a transistor Q25 and a resistor R26 that is used as a load of a transistor Q24 in a drive amplification stage 10. Since a constant voltage is applied to a base of the transistor Q25 by the Zener diode ZD21 at all times, the transistor Q25 operates such that a constant voltage is maintained between both ends of the resistor R26 connected to an emitter of the transistor Q25.

[43] When a voltage between both ends of the Zener diode ZD21 is V_ZD21, a voltage between the base and the emitter of the transistor Q25 is Vbe_Q25, and a voltage between both ends of the resistor R26 is V_R26, the following relationship is established:

[44] V_R26 = V_ZD21 - Vbe_Q25.

[45] A current I_R26 flowing through the resistor R26 satisfies the following relationship:

[46] I_R26 - V_R26 / R26.

[47] The transistor Q25 operates such that a constant current I_R26 flows between the collector and the emitter of the transistor Q25 at all times. The voltage between the collector and the emitter of the transistor Q24 in the drive amplification stage 10 varies according to an amplified signal as represented by a solid line in Fig. 4. Since the voltage gain of the drive amplification stage 10 is very high due to the constant current load, the differential amplification stage 14 including transistors Q21 and Q22 and a feedback stage 16 including negative feedback resistors Rf21 and Rf22 feed back an output signal to lower the voltage gain. In Fig. 5, reference numeral 18 indicates a constant current stage for the differential amplification stage 14 that allows a constant voltage to be applied to the differential amplification stage 14, and reference numeral 20 indicates an output stage that outputs a signal output from the drive amplification stage 10.

[48] The constant current load has come into widespread use for various amplifiers.

[49] However, when a constant current circuit is used as a load, the following problems arise, which will be described with reference to the circuit diagram shown in Fig. 5.

[50] First, Fig. 5 shows a common emitter amplifier in which the collector of the transistor Q24 and the collector of the transistor Q25 are connected to each other (since a bias voltage source BV21 is a constant voltage source, a description thereof will be omitted in an AC circuit) to form a constant current load. The common emitter amplifier shown in Fig. 5 operates such that a constant current flows through the collector of the transistor Q24 and the collector of the transistor Q25 at all times. Therefore, according to a variation in the signal input to the base of the transistor Q24, the collector current of the transistor Q24 is kept constant, and impedance between the collector and the emitter of the transistor Q24 varies. For example, when the impedance between the collector and the emitter of the transistor Q24 decreases, impedance between the collector and the emitter of the transistor Q25 increases. In contrast, when the impedance between the collector and the emitter of the transistor Q24 increases, the impedance between the collector and the emitter of the transistor Q25 decreases.

[51] That is, a variation in the impedance of the transistor Q24 and the transistor Q25 according to an input signal is converted into a voltage, and then output. A voltage signal generated due to the variation in the impedance is faster than a voltage signal generated due to a variation in current. The reason is as follows. It is necessary to vary a current flowing between the base and the emitter in order to change the collector current. In addition, it is required to vary the amount of charge stored in a capacitor between the base and the emitter in order to change a base current. In this case, it requires a lot of time to change the amount of charge, and thus the time required to vary the current is lengthened. Since the voltage signal is generated due to a variation in impedance, the time required for the drive amplification stage 10 to generate an output signal is short. Therefore, the following phenomenon occurs at node B between the drive amplification stage 10 and the constant current load stage 12.

[52] In Fig. 5, when a signal voltage input to the input terminal IN2 varies, the differential amplification stage 14 performs primary amplification, and the drive amplification stage 10 performs secondary amplification. Then, the amplified signal is output from an output terminal OUT2 of the output stage 20. That is, the output stage 20 outputs a signal voltage that varies due to the variation in the signal voltage input to the input terminal IN2. (Operation 1)

[53] Then, the voltage output from the output terminal OUT2 is fed back to a base of a transistor Q22 of the differential amplification stage 14 through the feedback stage 16. The differential amplification stage 14 performs a correcting operation to make the voltage input to the input terminal IN2 equal to the feedback voltage. (Operation 2)

[54] Operations 1 and 2 are repeated until the voltage input to the input terminal IN2 is equal to the feedback voltage. (Operation 3)

[55] When the voltage output from the output terminal OUT2 is stabilized, a voltage

V_OUT is output from the output terminal OUT2, which is represented as follows:

[56] V_OUT = V_IN * ( 1 + Rf21 / Rf22).

[57] The voltage amplification degree A2 is represented as follows: [58] A2 = l + Rf21 / Rf22.

[59] In Fig. 5, it is assumed that, as in Operation 1, the time until the output voltage from the output terminal OUT2 varies after the signal voltage input to the input terminal IN2 varies is referred to as an input/output delay time T_IO, and as in Operation 2, the time until the differential amplification stage 14 compares the voltage applied to the input terminal IN2 with the feedback voltage and performs the correcting process after the voltage output from the output terminal OUT2 is varied is referred to as a feedback delay time T_fb.

[60] In Fig. 5, when a rising signal shown at A of Fig. 6 is input to the input terminal

IN2, a voltage is varied by Operation 1 at node B after the input/output delay time T_IO has elapsed. At that time, the rising direction of the voltage at node B is the same as that of the voltage at node A, but the level of the voltage at node B tends to vary at a rate that is higher than an amplification ratio since the voltage has not been corrected by a feedback operation. That is, the voltage at node B increases in a direction in which the level of the input signal increases to be more than a voltage according to the amplification ratio. When the differential amplification stage 14 compares the feedback signal with the input voltage after the feedback delay time T_fb has elapsed, as in Operation 2, the level of the feedback signal is higher than that of the signal input to the input terminal IN2. Therefore, the differential amplification stage 14 lowers an output. (Operation 4)

[61] At that time, the voltage at node B varies in a direction opposite to the direction in which the voltage at node A increases, and the level of the voltage at node B is changed at a rate that is lower than the amplification ratio since the voltage has not been corrected by feedback. That is, the voltage at node B decreases in the opposite direction of the input signal to be lower than a voltage according to the amplification ratio. As in Operation 2, when the differential amplification stage 14 compares the feedback signal with the input voltage after the feedback delay time T_fb has elapsed, the level of the feedback signal is lower than that of the signal input to the input terminal IN2. Therefore, the differential amplification stage 14 increases an output. (Operation 5)

[62] When Operations 4 and 5 are repeated several times, the voltage alternately rises and falls at a predetermined period until it is stabilized to a voltage at the amplification ratio, which results in ringing as shown in Fig. 6.

[63] The frequency of the ringing becomes higher as the collector capacity Cob_Q24 of the transistor Q24 and the collector capacity Cob_Q25 of the transistor Q25 become lower. In addition, the larger the amplitude of the ringing becomes, the longer the feedback delay time T_fb becomes.

[64] Second, as described above, a ringing having a high frequency of several megahertz occurs in a place where the input signal suddenly varies, and the amplitude of the ringing is not negligible. As a result, noise is mixed with an amplified signal. In particular, a sound system outputs harsh sounds, which is uncomfortable for the listener.

Disclosure of Invention

Technical Problem

[65] The invention has been finalized in order to solve the above-described problems.

An object of the invention provides an amplifier having a damping resistor provided in a constant current load that is capable of preventing the occurrence of ringing caused by a sudden change in an input signal and outputting a signal without distortion. Technical Solution

[66] In order to achieve the object, according to an aspect of the invention, an amplifier includes: a constant current load stage; and a damping resistor that is connected in parallel to the constant current load stage.

[67] According to another aspect of the invention, an amplifier includes: a plurality of drive amplification stages; a plurality of constant current load stages whose number is equal to that of drive amplification stages and which are connected to the corresponding drive amplification stages; and a plurality of damping resistor whose number is smaller than that of constant current load stages, and which are connected in parallel to some of the constant current load stages.

[68] According to still another aspect of the invention, an amplifier includes: a drive amplification stage; an output stage that includes complementary transistors, such as first and second transistors having bases connected to an output terminal of the drive amplification stage; a constant current load stage that is connected to the output terminal of the drive amplification stage; and a damping resistor that is connected in parallel to the constant current load stage.

[69] In the above-mentioned aspects, the constant current load stage may include a third transistor having one end connected to the output terminal of the drive amplification stage, and the damping resistor may have one end connected to the one end of the third transistor and the other end connected to the other end of the third transistor.

[70] In the above-mentioned aspects, the drive amplification stage may include a fourth transistor having one end connected to the bases of the first and second transistors. The constant current load stage may include a fifth transistor having one end connected to the one end of the fourth transistor. The damping resistor may have one end connected to the one end of the fourth transistor and the other end connected to the other end of the fifth transistor.

Advantageous Effects [71] As described in detail above, according to the invention, a damping resistor is connected in parallel to a constant current load, which makes it possible to improve an amplification gain and prevent ringing, thereby obtaining an output signal without distortion. [72] Further, an output signal includes little harmonics, which makes it unnecessary to provide an additional filter. [73] Furthermore, when the invention is applied to a sound system, the sound system can reproduce high quality sound without harsh sounds

Brief Description of the Drawings [74] Fig. 1 is a circuit diagram illustrating an example of a general class-A amplifier including a resistor load. [75] Fig. 2 is a graph illustrating operation characteristics of the resistor load shown in

Fig. 1. [76] Fig. 3 is a circuit diagram illustrating an example of a general class-A amplifier including a constant current load. [77] Fig. 4 is a graph illustrating operation characteristics of the constant current load shown in Fig. 3. [78] Fig. 5 is a circuit diagram illustrating an example of a negative feedback amplifier including a constant current load according to the related art. [79] Fig. 6 is a graph illustrating the waveforms of signals passing through both ends of the constant current load shown in Fig. 5. [80] Fig. 7 is a circuit diagram illustrating an amplifier having a damping resistor provided in a constant current load according to a first embodiment of the invention. [81] Fig. 8 is a waveform graph illustrating the effect of the damping resistor shown in

Fig. 7. [82] Fig. 9 is a circuit diagram illustrating an amplifier having a damping resistor provided in a constant current load according to a second embodiment of the invention. [83] Fig. 10 is a circuit diagram illustrating an amplifier having a damping resistor provided in a constant current load according to a third embodiment of the invention. [84] Fig. 11 is a circuit diagram illustrating a modification of the invention.

[85] <Reference Numerals>

[86] 10: drive amplification stage

[87] 12: constant current load stage

[88] 14: differential amplification stage

[89] 16: feedback stage

[90] 18: constant current stage

[91] 20: output stage Best Mode for Carrying Out the Invention

[92] Hereinafter, an amplifier having a damping resistor provided in a constant current load according to an embodiment of the invention will be described with reference to the accompanying drawings.

[93] Fig. 7 is a circuit diagram illustrating an amplifier having a damping resistor provided in a constant current load according to a first embodiment of the invention. Fig. 8 is a waveform graph illustrating the effect of the damping resistor shown in Fig. 7. Fig. 7 shows a negative feedback amplifier. The circuit shown in Fig. 7 is similar to the circuit shown in Fig. 5 except that a damping resistor is connected in parallel to a constant current load. In Fig. 7, the same components as those shown in Fig. 5 may be denoted by the same reference numerals. However, in this embodiment, for the convenience of explanation, the components shown in Fig. 7 and the components shown in Fig. 5 are denoted by different reference numerals.

[94] The amplifier according to the first embodiment includes: an input terminal INl; a differential amplification stage 14 including a transistor QI l and a transistor Q 12; a drive amplification stage 10 including a transistor Q14 and a resistor R15; a constant current load stage 12 including a Zener diode ZDl 1, resistors R14 and R 16, and a transistor Q 15; a feedback stage 16 including a negative feedback resistor RfI 1 and a negative feedback resistor Rf 12; an output stage 20 that includes complementary transistors, such as a transistor Q16 and a transistor Q17, and drives a load; a bias voltage source BVl 1 that applies a bios voltage to the bases of the transistor Q16 and the transistor Q17 in the output stage 20; and a damping resistor R19 connected in parallel to the constant current load stage 12.

[95] In this embodiment, the bias voltage source BVl 1 may be referred to as a bias stage.

[96] The resistor R14 of the constant current load stage 12 may be excluded from the constant current load stage 12. This is also applied to the other embodiments.

[97] The amplifier having a damping resistor provided in a constant current load according to the first embodiment is a negative feedback amplifier in which a damping resistor R19 is connected to the drive amplification stage 10 in parallel to the constant current load stage 12. The bias voltage source BVl 1 is a positive voltage source, and thus a description thereof will be omitted in an AC circuit. Therefore, in this case, it is considered that the collector of the transistor Q 14 in the drive amplification stage 10 is connected to the collector of the transistor Q15 in the constant current load stage 12.

[98] In Fig. 7, in order to connect the damping resistor R19 in parallel to the constant current load stage 12, one end of the damping resistor R 19 is connected to the collector of the transistor Q15, but the invention is not limited thereto. The one end of the damping resistor R19 may be connected to the collector of the transistor Q 14 in the drive amplification stage 10.

[99] Next, the operation of the first embodiment will be described below. The first embodiment provides a common emitter amplifier in which the collector of the transistor Q 14 is connected to the collector of the transistor Q 15, thereby forming a constant current load. The common emitter amplifier is configured such that a constant current flows through the collector of the transistor Q 14 and the collector of the transistor Q15 at all times. Therefore, a collector current of the transistor Q14 is kept constant due to a variation in the signal input to the base of the transistor Q 14, and impedance between the collector and the emitter of the transistor Q14 varies. For example, when the impedance between the collector and the emitter of the transistor Q14 decreases, impedance between the collector and the emitter of the transistor Q15 increases. In contrast, when the impedance between the collector and the emitter of the transistor Q14 increases, the impedance between the collector and the emitter of the transistor Q15 decreases.

[100] That is, a variation in the impedance of the transistor Q14 and the transistor Q15 according to an input signal is converted into a voltage, and then output. A voltage signal generated due to the variation of the impedance (an amplifier operated due to a variation in collector impedance) is faster than a voltage signal generated due to a variation in current (an amplifier operated due to a variation in collector current). The reason is as follows. It is necessary to vary a current flowing between the base and the emitter in order to change the collector current. In addition, it is required to vary the amount of charge stored in a capacitor between the base and the emitter in order to change a base current. In this case, it requires a lot of time to change the amount of charge, and thus it takes a lot of time for the current to vary. (Theorem 11)

[101] Therefore, the drive amplification stage 10 using the constant current load stage 12 shown in Fig. 7 as a load serves as an amplifier that is operated due to a variation in impedance, and the output signal of the drive amplification stage 10 rapidly rises or falls. For this reason, the damping resistor R19 is provided in parallel to the constant current load stage 12. This embodiment is configured to make a portion of the current flowing through the collector of the transistor Q 14 in the drive amplification stage 10 flow through the damping resistor R19 and to operate the transistor Q14 as an amplifier that is operated due to a variation in collector current. In this way, when the signal input to the base of the transistor Q 14 is amplified, the rise and fall times of a signal output to the collector of the transistor Q14 are delayed, thereby reducing the occurrence of ringing.

[102] Next, the operation the circuit structure shown in Fig. 7 will be described in more detail below. [103] When a signal voltage of an input terminal INl varies, primary amplification is performed by the differential amplification stage 14, and secondary amplification is performed by the drive amplification stage 10. Then, the amplified signal is output from an output terminal OUTl of the output stage 20. In this way, the output stage 20 outputs a signal voltage that varies due to a variation in the signal voltage of the input terminal INl. (Operation 11)

[104] Then, the voltage signal from the output terminal OUTl is fed back to the base of the transistor Q12 in the differential amplification stage 14 through the feedback stage 16, and the differential amplification stage 14 performs a correcting process such that the voltage applied to the input terminal INl is equal to the feedback voltage. (Operation 12)

[105] Then, Operations 11 and 12 are repeated until the voltage applied to the input terminal INl is equal to the feedback voltage. (Operation 13)

[106] In Fig. 7, it is assumed that, as in Operation 11, the time until the output voltage from the output terminal OUTl varies after the signal voltage of the input terminal INl varies is referred to as an input/output delay time T_IO1, and as in Operation 12, the time until the differential amplification stage 14 compares the voltage applied to the input terminal INl with the feedback voltage and performs the correcting process after the voltage output from the output terminal OUTl varies is referred to as a feedback delay time T_fb 1.

[107] In Fig. 7, the following phenomenon occurs at node B between the drive amplification stage 10 and the constant current load stage 12.

[108] In Fig. 7, when a rising signal shown at A of Fig. 8 is input to the input terminal

INl, a voltage is varied by Operation 11 at node B after the input/output delay time T_IO1 has elapsed. At that time, the rising direction of the voltage at node B is the same as that of the voltage at node A, but the level of the voltage at node B tends to vary at a rate that is higher than an amplification ratio since the voltage has not been corrected yet by a feedback operation. However, since the damping resistor R19 is connected in parallel to the constant current load stage 12, a current flowing through the damping resistor R19 is changed due to a variation in the voltage at node B. The change of the current causes a variation in the collector current of the transistor Q 14 in the drive amplification stage 10, and It takes a lot of time for the collector current to vary, as in Theorem 11.

[109] Therefore, the transistor Q 14 serves as an amplifier that is operated due to the variation in the collector current, in an amplifying stage that is operated due to the variation in collector impedance. It takes a lot of time for the current flowing through the damping resistor R19 to vary, and the rise time of the voltage at node B is lengthened. When the rise time of the voltage is lengthened, a variation in voltage per unit time is reduced, which results in a low overshoot voltage.

[110] Then, as in Operation 12, after the feedback delay time T_fbl has elapsed, the differential amplification stage 14 compares the feedback signal with the input voltage, and the differential amplification stage 14 lowers an output since the feedback signal has a higher level than the signal input to the input terminal INl. (Operation 14)

[111] In this case, since the rising direction of the voltage at node B is opposite to that of the voltage at node A and the level of the voltage at node B has not been corrected by the feedback yet, the voltage at node B tends to be changed at a rate that is lower than the amplification ratio. However, since the damping resistor R19 is connected in parallel to the constant current load stage 12, the current flowing through the damping resistor R19 varies due to the variation in the voltage at node B. This variation causes a change in the collector current of the transistor Q14 in the drive amplification stage 10, and It takes a lot of time for the collector current to vary, as in Theorem 11.

[112] Therefore, the transistor Q14 serves as an amplifier that is operated due to the variation in the collector current, in an amplifying stage that is operated due to the variation in the collector impedance. It takes a lot of time for the current flowing through the damping resistor R19 to vary, and the fall time of the voltage at node B is lengthened. When the fall time of the voltage is lengthened, a variation in voltage per unit time is reduced, which results in a low overshoot voltage.

[113] Then, as in Operation 12, after the feedback delay time T_fbl has elapsed, the differential amplification stage 14 compares the feedback signal with the input voltage, and the differential amplification stage 14 increases an output since the feedback signal has a lower level than the signal input to the input terminal INl. (Operation 15)

[114] When Operations 14 and 15 are repeated several times, the voltage alternately rises and falls at a predetermined period until it is stabilized to a level at the amplification ratio, which is called ringing. However, as can be seen from Fig. 8, the amplitude of the overshoot voltage is considerably smaller than that in the related art.

[115] That is, the damping resistor R 19 provided in parallel to the constant current load stage 12 enables the transistor Q14 of the drive amplification stage 10 to serve as an amplifier that is operated due to a variation in the collector current, in the amplifying stage that is operated due to the variation in impedance, thereby lengthening the rise and fall times of the output signal. As a result, it is possible to prevent the occurrence of ringing in the output signal.

[116] Fig. 9 is a circuit diagram illustrating an amplifier having a damping resistor provided in a constant current load according to a second embodiment of the invention.

[117] The amplifier according to the second embodiment is a common collector amplifier

(emitter follower). The amplifier according to the second embodiment includes: a drive amplification stage 30 that includes a transistor Q51 having a base that receives an input signal from an input terminal IN5 and an emitter that outputs a buffered signal; a constant current load stage 32 that is connected to the emitter of the transistor Q51 and includes a Zener diode ZD51, resistors R51 and R52, and a transistor Q52; and a damping resistor R53 that is connected in parallel to the constant current load stage 32.

[118] The operation of the amplifier according to the second embodiment will be described below.

[119] The amplifier according to the second embodiment is a common collector amplifier

(emitter follower) in which a signal is input from the input terminal IN5 to a base of the transistor Q51 and a buffered signal is output from an emitter of the transistor Q51 to an output terminal OUT5.

[120] When the signal voltage applied to the input terminal IN5 varies, the transistor Q51 operates such that the voltage between the base and the emitter of the transistor Q51 is 0.6 V (which indicates the voltage between the base and the emitter of the transistor Q51, and is represented by 0.6 V for the convenience of explanation). The operation of the transistor Q51 causes the emitter voltage of the transistor Q51 to vary, and the voltage is output to the output terminal OUT5. (Operation 51)

[121] Then, the variation in the voltage of the output terminal OUT5 is fed back to the transistor Q51 to confirm that the voltage between base and the emitter of the transistor Q51 is 0.6 V (which is referred to as a local feedback that occurs inside the transistor). That is, an operation for correcting the voltage between the base and the emitter of the transistor Q51 to 0.6 V is performed. (Operation 52)

[122] Operations 51 and 52 are repeated until the voltage between the base and the emitter of the transistor Q51 is 0.6 V. (Operation 53)

[123] In Fig. 9, it is assumed that, as in Operation 11, the time until the output voltage from the output terminal OUT5 varies after the signal voltage of the input terminal IN5 varies is referred to as an input/output delay time T_IO5, and as in Operation 52, the time until the operation for correcting the voltage between the base and the emitter of the transistor Q51 to 0.6 V is performed after the voltage output from the output terminal OUT5 varies is referred to as a feedback delay time T_fb5.

[124] The amplifier according to the second embodiment is a common collector amplifier in which the collector of the transistor Q52 is connected to the emitter of the transistor Q51, thereby forming a constant current load. The common collector amplifier is configured such that a constant current flows through the emitter of the transistor Q51 and the collector of the transistor Q52 at all times. Therefore, an emitter current of the transistor Q51 is kept constant according to variation in the signal input to the base of the transistor Q51, and impedance between the collector and the emitter of the transistor Q51 varies. For example, when the impedance between the collector and the emitter of the transistor Q51 decreases, impedance between the collector and the emitter of the transistor Q52 increases. In contrast, when the impedance between the collector and the emitter of the transistor Q51 increases, the impedance between the collector and the emitter of the transistor Q52 decreases.

[125] That is, a variation in the impedance of the transistor Q51 and the transistor Q52 according to an input signal is converted into a voltage, and then output. A voltage signal generated due to the variation in the impedance (an amplifier that is operated due to a variation in the collector impedance) is faster than a voltage signal generated due to a variation in the current (an amplifier that is operated due to a variation in the collector current). The reason is as follows. It is necessary to vary a current flowing through between the base and the emitter in order to change the collector current. In addition, it is required to vary the amount of charge stored in a capacitor between the base and the emitter in order to change a base current. In this case, it requires a lot of time to change the amount of charge, and thus it takes a lot of time for the current to vary. (Theorem 51)

[126] Therefore, since the transistor Q51 shown in Fig. 9 is an amplifier that is operated due to a variation in impedance, the rise and fall times of an output signal are short. For this reason, the damping resistor R53 is provided in parallel to the constant current load stage 32. This embodiment is configured to make a portion of the current flowing through the emitter of the transistor Q51 in the drive amplification stage 30 flow through the damping resistor R63 and to operate the transistor Q51 as an amplifier that is operated due to a variation in the collector current. In this way, the rise and fall times of an amplified signal are delayed, which makes it possible to reduce the occurrence of ringing.

[127] In Fig. 9, the following phenomenon occurs at node B between the emitter of the transistor Q51 and the collector of the transistor Q52.

[128] When a rising signal shown at A of Fig. 8 is input to the input terminal IN5, a voltage is varied by Operation 51 at node B of Fig. 9 after the input/output delay time T_IO5 has elapsed. At that time, the rising direction of the voltage at node B is the same as that of the voltage at node A, but the voltage between the emitter and the base of the transistor Q51 tends to be lower than 0.6 V since the voltage has not been corrected by a feedback operation (the potential at node B increases). However, since the damping resistor R53 is connected in parallel to the constant current load stage 32, a current flowing through the damping resistor R53 is changed due to a variation in the voltage at node B. The change of the current causes a variation in the emitter current of the transistor Q51 , and it takes a lot of time for the collector current to vary, as in Theorem 51.

[129] Therefore, the transistor Q51 serves as an amplifier that is operated due to the variation in the collector current, in an amplifying stage that is operated due to the variation in collector impedance. It takes a lot of time for the current flowing through the damping resistor R43 to vary, and the rise time of the voltage at node B is lengthened. When the rise time of the voltage is lengthened, a variation in voltage per unit time is reduced, which results in a low overshoot voltage.

[130] Then, as in Operation 52, since the voltage between the emitter and the base of the transistor Q51 is lower than 0.6 V by a feedback signal after the feedback delay time T_fb 1 has elapsed, the transistor Q51 operates to increase the voltage between the base and the emitter. (Operation 54)

[131] In this case, since the rising direction of the voltage at node B is opposite to that of the voltage at node A and the level of the voltage at node B has not been corrected by the feedback yet, the voltage at node B tends to be higher than 0.6 V (The potential at node B is lowered). However, since the damping resistor R53 is connected in parallel to the constant current load stage 32, the current flowing through the damping resistor R53 varies due to the variation in the voltage at node B. This variation causes a change in the emitter current of the transistor Q51. It takes a lot of time for the collector current to vary, as in Theorem 51.

[132] Therefore, the transistor Q51 serves as an amplifier that is operated due to the variation in the collector current, in an amplifying stage that is operated due to the variation in the collector impedance. It takes a lot of time for the the current flowing through the damping resistor R53 to vary, and the fall time of the voltage at node B is lengthened. When the fall time of the voltage is lengthened, a variation in voltage per unit time is reduced, which results in a low overshoot voltage.

[133] Then, as in Operation 52, since the voltage between the emitter and the base of the transistor Q51 is lower than 0.6 V by a feedback signal after the feedback delay time T_fb 1 has elapsed, the transistor Q51 operates to decrease the voltage between the base and the emitter (the potential at node B increases). (Operation 55)

[134] When Operations 54 and 55 are repeated several times, the voltage between the base and the emitter alternately rises and falls at a predetermined period until the voltage reaches 0.6 V, which is called ringing. However, as can be seen from Fig. 8, the amplitude of the overshoot voltage is considerably smaller than that in the related art. That is, the damping resistor R53 provided in parallel to the constant current load stage 32 enables the transistor Q51 to serve as amplifier that is operated due to a variation in the collector current, in the amplifying stage that is operated due to the variation in impedance, thereby lengthening the rise and fall times of the output signal. As a result, it is possible to prevent the occurrence of ringing in the output signal.

[135] Fig. 10 is a circuit diagram illustrating an amplifier having a damping resistor provided in a constant current load according to a third embodiment of the invention.

[136] The amplifier according to the third embodiment is a common emitter amplifier. The amplifier according to the third embodiment includes: a drive amplification stage 40 that includes a transistor Q61 having a base that receives a signal input to an input terminal IN6 and a collector that outputs an amplified signal; a constant current load stage 42 that is connected to the collector of the transistor Q61 and includes a Zener diode ZD61, resistors R61 and R62, and a transistor Q62; and a damping resistor R63 connected in parallel to the constant current load stage 42.

[137] Next, the operation of the amplifier according to the third embodiment will be described below.

[138] The amplifier according to the third embodiment is a common emitter amplifier in which a signal is input from the input terminal IN6 to the base of the transistor Q61 and an amplified signal is output from the collector of the transistor Q61 to the output terminal OUT6.

[139] The amplifier is configured such that a constant current flows between the collector of the transistor Q61 and the collector of the transistor Q62 at all times. Therefore, a collector current of the transistor Q61 is kept constant according to a variation in the signal input to the base of the transistor Q61, and impedance between the collector and the emitter of the transistor Q61 varies. For example, when the impedance between the collector and the emitter of the transistor Q61 decreases, impedance between the collector and the emitter of the transistor Q62 increases. In contrast, when the impedance between the collector and the emitter of the transistor Q61 increases, the impedance between the collector and the emitter of the transistor Q62 decreases.

[140] That is, a variation in the impedance of the transistor Q61 and the transistor Q62 according to an input signal is converted into a voltage, and then output. A voltage signal generated due to the variation in the impedance (an amplifier that is operated due to a variation in collector impedance) is faster than a voltage signal generated due to a variation in the current (an amplifier that is operated due to a variation in collector current). The reason is as follows. It is necessary to vary a current flowing through between the base and the emitter in order to change the collector current. In addition, it is necessary to vary the amount of charge stored in a capacitor between the base and the emitter in order to change a base current. In this case, it takes a lot of time to vary the amount of charge, and thus it takes a lot of time for the current to vary.

[141] The operation of the circuit structure shown in Fig. 10 is the same as that between the drive amplification stage and the constant current load stage according to the first embodiment. Therefore, a detailed description thereof will be omitted since those skilled in the art can easily understand the operation of the circuit structure shown in Fig. 10 from the operations of the drive amplification stage and the constant current load stage according to the first embodiment.

[142] In the third embodiment, the damping resistor R63 provided in parallel to the constant current load stage 42 enables the transistor Q61 to serve as an amplifier that is operate due to a variation in the collector current, in the amplifying stage that is operated due to the variation in impedance, thereby lengthening the rise and fall times of the output signal. As a result, it is possible to prevent the occurrence of ringing in the output signal.

[143] Fig. 11 is a circuit diagram illustrating a common collector amplifier having a constant current circuit according to a modification of the invention. In the circuit diagram, a collector and a base of a transistor Q73 are connected to each other, and a base of a transistor Q72 is connected to a node between the collector and the base of the transistor Q73. One end of a resistor R71 is connected to a node between the transistors Q72 and Q73, and the other end of the resistor R71 is connected to a power supply terminal VCC7.

[144] The circuit having the connection structure between the transistors Q72 and Q73 shown in Fig. 11 is called a current mirror. That is, when a current Ice_Q73 flows to the node betweende the transistors Q72 and Q73 through the resistor R71, the amount of current Ice_Q72 flowing through the collector of the transistor Q72 is equal to the amount of current Ice_Q73 flowing through the collector of the transistor Q73.

[145] The constant current circuit including the transistors Q72 and Q73 and the resistor

R71 serves as a constant current load stage. The structure having the damping resistor R72 connected in parallel to the constant current load stage serves as the common collector amplifier according to the second embodiment. Those skilled in the art can easily understand the operation of the structure shown in Fig. 11 from the structure according to the second embodiment.

[146] Meanwhile, although not shown in the drawings, in a negative feedback amplifier or a non-negative feedback amplifier, a constant current load may be used as a load in each drive amplification stage. In this case, in general, a damping resistor is selectively provided in the constant current load of the drive amplification stage in order to prevent the occurrence of ringing. That is, in an amplifier including a plurality of drive amplification stages, a constant current load may be connected to each drive amplification stage, and a damping resistor may be connected in parallel to each constant current load. Alternatively, the damping resistor may be connected in parallel to only the constant current load in order to reduce ringing and optimize the circuit structure. As described above, a resistor provided in parallel to a constant current load is called a damping resistor. In the amplifier using constant current load according to the above- described embodiment, the damping resistor connected in parallel to the constant current load makes it possible to lower a ringing voltage at a position where an input signal rapidly varies.

[147] It will be apparent to those skilled in the art that various modifications and changes may be made without departing from the scope and spirit of the present invention. Therefore, it should be understood that the above embodiments are not limitative, but illustrative in all aspects. The scope of the present invention is defined by the appended claims rather than by the description preceding them, and therefore all changes and modifications that fall within metes and bounds of the claims, or equivalents of such metes and bounds are therefore intended to be embraced by the claims.

Claims

[ 1 ] An amplifier comprising : a constant current load stage; and a damping resistor that is connected in parallel to the constant current load stage.

[2] An amplifier comprising: a plurality of drive amplification stages; a plurality of constant current load stages whose number is equal to that of drive amplification stages and connected to each of the corresponding drive amplification stages; and a plurality of damping resistor whose number is smaller than that of constant current load stages and connected in parallel to some of the constant current load stages.

[3] An amplifier comprising: a drive amplification stage; a output stage that includes complementary transistors, such as first and second transistors having bases connected to an output terminal of the drive amplification stage; a constant current load stage that is connected to the output terminal of the drive amplification stage; and a damping resistor that is connected in parallel to the constant current load stage.

[4] The amplifier according to claim 3, wherein the constant current load stage includes a third transistor having one end connected to the output terminal of the drive amplification stage, and the damping resistor has one end connected to the one end of the third transistor and the other end connected to the other end of the third transistor.

[5] The amplifier according to claim 3, wherein the drive amplification stage includes a fourth transistor having one end connected to the bases of the first and second transistors, the constant current load stage includes a fifth transistor having one end connected to the one end of the fourth transistor, and the damping resistor has one end connected to the one end of the fourth transistor and the other end connected to the other end of the fifth transistor.

[6] A negative feedback amplifier comprising the amplifier according to any one of claims 1 to 5.