Abstract:
A high output amplifier includes a comparison amplifier having a first input, a second input, and an output, wherein a set voltage is applied to the first input, a voltage of the output is coupled to the second input, and the output is generated in response to a difference between the voltage applied to the first input and the voltage coupled to the second input. The high output amplifier also includes a low-pass filtering device that receives and performs low-pass filtering on the output of the comparison amplifier, a conversion device that converts the output of the low-pass filtering device to complementary signals, and a push-pull output device, driven by the complementary signals, that supplies electrical current to a load, wherein an increase in the electrical current supplied by the push-pull output device is decreased by changes in the load due to the low-pass filtering device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an amplifier for driving a load, and, in particular, it relates to an amplifier that is high-speed, of high output power and stable in varying load. 
     2. Background Arts 
     There are cases in which the necessity arises for high-speed and high-electric-power driving of circuit elements that serve as loads in various types of electrical and electronic devices such as circuit element testers. In such cases, push-pull amplifiers, complementary amplifiers or combinations thereof are frequently used as the high output amplifiers which are the final stages in which such driving is performed. 
     A push-pull amplifier that is constructed using bipolar transistors is described in Japanese Laid-open Publication JP1996-32367A. This push-pull amplifier could show a lower crossover distortion without increasing an idling current (through current) in the output stage and high efficiency. Because any part of the circuit is not cut off there is not accumulation of minority carriers, which may be a problem with bipolar transistors, then, higher speed and broader bands can be achieved. 
     FIG. 1 shows high output amplifier  10  based on a conventional technology similar to that of the aforementioned push-pull amplifier. The high output amplifier  10  is constructed of circuit elements arranged between supply voltages VD and VS. Load voltage V 2 , which is essentially equal to control voltage V 1  that has been input into terminal  1 , is output to terminal  2 . Transistors Q 1  and Q 2  are biased by a series of bias circuits comprised of resistances R 1  and R 2  and diodes D 1  and D 2 . Terminal  1  is connected to a common connecting point of diodes D 1  and D 2  so that control voltage Vc is input into the control terminals (bases) of transistors Q 1  and Q 2  respectively. Transistors Q 1  and Q 2  are, respectively, NPN and PNP transistors and their emitters are connected to terminal  2  through resistance R 4  or resistance R 5 . Transistors Q 1  and Q 2  act as a complementary buffer. The respective collectors of transistors Q 1  and Q 2  are connected to supply voltages VD and VS through resistances R 3  and R 6  respectively and are also connected to the non-inverting input terminals of amplifiers A 1  and A 2 . Outputs of amplifiers A 1  and A 2  are connected to the gates of respective transistors Q 3  and Q 4  and the inverting input terminals of amplifiers A 1  and A 2  are connected to the sources of the respective transistors Q 3  and Q 4 . Further, the sources of transistors Q 3  and Q 4 , which are field effect transistors (FET), are connected to the respective supply voltages VD and VS through resistances R 7  and R 8 . The drains of transistors Q 3  and Q 4  are both connected to terminal  2 . Load LD is connected to terminal  2 . Load LD may generally be passive or active. 
     First, we shall consider this amplifier  10  by ignoring transistors Q 3  and Q 4 . Transistors Q 1  and Q 2  act in response to control voltage Vc so as to output voltage V 2 , which is close to control voltage Vc, to load LD. However, when load LD is heavy (when load current  12  is high such as in the case of low load impedance), load voltage V 2  does not follow control voltage Vc. At a setting at which load current I 2  flows out to load LD, transistor Q 1  approaches saturation due to an increase of load current I 2  and transistor Q 2  is cut off. However, load current I 2  also flows to resistance R 3  and voltage is generated across resistance R 3 . Amplifier A 1  controls the gate voltage of transistor Q 3  so that the voltage applied across resistance R 3  and the voltage applied across resistance R 7  become equal to each other. Current of a value obtained by dividing the value of voltage across the resistance R 3  by the value of resistance R 7  is supplied from the drain of transistor Q 3  to the terminal  2 . When this occurs, the current flowing through transistor Q 1  is decreased and the voltage across resistance R 3  is also decreased. By means of this negative feedback, high output amplifier  10  is stabilized when the voltage applied across resistance R 3  reaches a certain level. Similar stabilization also occurs at a setting at which load current I 2  flows in from load LD. Similar actions are also effected on the sides of transistors Q 2  and Q 4 . Consequently, there are two feedback loops of electrical signals. In one loop the signal is fed back from terminal  2  to terminal  2  via the transistor Q 1 , amplifier A 1  and transistor Q 3  and in another loop the signal is fed back from terminal  2  to terminal  2  via the transistor Q 2 , amplifier A 2  and transistor Q 4 . So that any electrical signal on terminal  2 , input to or generated at terminal  2 , which may be a change in voltage or current through originated from a change in voltage, current or circuit parameter, is fed back negatively. 
     In the amplifier  10  of FIG. 1, as indicated above, we have described transistors Q 3  and Q 4  as field effect transistors for high power use. However, they may also be bipolar transistors for high power use and switching-mode power sources. Further, field effect transistors or amplifiers may be used in place of transistors Q 1  and Q 2 . However, transistors Q 1  and Q 2  are generally of comparatively high speed and low power and transistors Q 3  and Q 4  are of comparatively low speed and high power. 
     When transistors Q 3  and Q 4  are field effect transistors or bipolar transistors as described above, and, in particular, the circuit parameters are set so that the output currents of the transistors do not become completely zero even if the load current is zero. This is performed for the purpose of fast response when the load has suddenly changed. Accordingly, when the load current is zero in this way, the through current that flows through the two transistors is called the design through current. 
     Where load LD may have various type of impedance (passive or active) the aforementioned negative feedback loop may possibly be changed to a positive feedback loop to cause oscillation. Therefore, capacitors C 1  and C 2  are connected in parallel to resistances R 3  and R 4  and roll-off of the negative feedback loop is effected. When transistors Q 3  and Q 4  are power field effect transistors, the frequency characteristics of these power field effect transistors do not extend over a wide range. Then without capacitors C 1  and C 2 , the actual power field effect transistors can simulate equivalents of wide-band field effect transistors with capacitors C 1  and C 2 . 
     SUMMARY OF THE INVENTION 
     Although a structure like that of high output amplifier  10  described above is stable when the load current changes slowly, there is the possibility that a large through current that exceeds the expected design through current will be generated in transistors Q 3  and Q 4  when the load current changes suddenly (quickly). For example, there are cases in which a suitable load resistance is connected and in which the input voltage changes rapidly, i.e., more rapidly than the response speed of the aforementioned negative feedback loop. Further, when high output amplifier  10  is used in the device power source of an integrated circuit (IC) tester, there are cases in which the operating state of an IC that is connected as the load will change due to signals from the outside. For example, when there has been a change from the usual operating state to a standby state, the load current changes more rapidly than the response speed of the aforementioned negative feedback loop of high output amplifier  10 . 
     In order to clearly understand problems this invention intends to solve, we should consider the case in which load current I 2  flowing into the current source load LD changes alternately and rapidly between 0 and IL. When load current I 2  is IL, said load current I 2  flows out from terminal  2  to the load (current source load). 
     (a) A steady state in which the load current I 2  is 0: The voltages of terminal  1  and terminal  2 , although this is not essential, are supposed to be both zero(0) for the sake of convenience. The gate voltages of transistors Q 3  and Q 4  are close to the respective threshold voltages and the respective drain currents are at low levels to give a design through current. The voltages applied on resistances R 7  and R 8  are controlled according to the non-inverting input terminal voltages of amplifiers A 1  and A 2  that are set depending on resistances R 4  and R 5  which have low resistance levels, and resistances R 3  and R 6  which are the collector resistances to the transistors Q 1  and Q 2 . 
     (b) A state in which load current I 2  changes rapidly from 0 to IL: At first, transistors Q 3  and Q 4  are not essentially operated, the emitter voltage of transistor Q 1  is decreased and load current I 2  is supplied from transistor Q 1 . The voltage across resistance R 3  begins to rise accompanying the charging of capacitor C 1  by the collector current of transistor Q 1 . The emitter voltage of transistor Q 2  falls and transistor is essentially cut off, the voltage across resistance R 6  begins to decrease at a time constant of R 6 *C 2  then transistor Q 4  goes off. 
     (c-1) A case in which load current I 2  abruptly changes instantaneously from 0 to IL, after that the constant value IL of I 2  is maintained: The current from transistor Q 3  to terminal  2  is increased as the voltage across resistance R 3  rises. Consequently, the current that is supplied from transistor Q 1  to the load LD decreases and the rise of the voltage across resistance R 3  is slowed down. The voltage of terminal  2  gradually rises, then the sum of the emitter current of transistor Q 1  and the drain current of transistor Q 3  becomes equal to load current I 2 (IL) to be stabilized When the level IL of load current I 2  is high, it is designed so that the drain current of transistor Q 3  accounts for most of load current I 2 . 
     (c-2) A case in which load current I 2  changes instantaneously from 0 to IL, maintains a constant value of IL for a short time then returns to 0: This case differs from the case (c-1) and a through current flows through transistors Q 3  and Q 4  as below described. 
     The current from transistor Q 3  to terminal  2  is increased as the voltage across resistance R 3  rises. Consequently, the current from transistor Q 1  to load LD decreases and the rise in the voltage across resistance R 3  is slowed down. The voltage of the terminal  2  gradually rises and the sum of the emitter current of transistor Q 1  and the drain current of transistor Q 3 , as load transistor Q 2  remains off, becomes equal to load current I 2  and stabilized. When the IL level of load current I 2  is high, it is designed so that the drain current of transistor Q 3  accounts for most of load current I 2 . 
     Then, when load current I 2  returns to 0, the current flowing out from transistor Q 3  causes the voltage of the terminal  2  to rise, transistor Q 1  is cut off, transistor Q 2  is turned on, current flows into the transistor Q 2  and the voltage of the non-inverting input terminal of amplifier A 2  is raised. As the voltage of said non-inverting input terminal rises, transistor Q 4  is turned on and current is drawn from the terminal  2 . Consequently, a through current that passes through transistor Q 3  and transistor Q 4  is generated. Because discharge of capacitor C 1  does not occur rapidly, the through current persists for some time. 
     (d) From the state (c-2) described above up to the state in which load current I 2  again assumes the level IL after a short time and then returns to 0: The voltages across capacitors C 1  and C 2  are greater than those in the case (a), a larger through current flows into transistors Q 3  and Q 4 . Consequently, when the load current again assumes the level IL, the current from transistor Q 3  becomes approximately the sum of the through current and load current I 2 . There is very slight discharge of capacitor C 2  during the period that load current I 2  assumes the value IL. Thus, when load current I 2  goes to 0, increase of the voltage across capacitor C 2  causes a further increase in the through current. 
     As should be clear from the foregoing description, when process (d) described above is repeated, the through current gradually increases. This through current is not extracted to the outside as load current I 2 , causing an increase in the internally power consumption of transistors Q 3  and Q 4 . When the internally consumed power is low, no particular problems occur. However, in high power devices, it reaches several tens of W to several hundred of W. In extreme cases, transistors Q 3  and Q 4  themselves may be destroyed. Even when this does not happen, operation of transistors Q 3  and Q 4  becomes unstable since the operation is not performed at the operating point initially designed. 
     Consequently, the object of this invention is to provide a stable high output amplifier with which increases exceeding the design through current of the through current are controlled and which results in low internally power consumption. 
     The high output amplifier of this invention for the purpose of solving the aforementioned problems is equipped with a comparison amplifier in which the set voltage is received in the input of one side, a voltage of the output side is returned to the input of the other side and that generates output in response to the difference between the voltage of the aforementioned voltage of one side and the voltage of the aforementioned voltage of the other side, a low-pass filtering device that receives the output of the comparison amplifier, and that performs low-pass filtering of the output of the aforementioned comparison amplifier and outputs it, conversion devices that convert the output of said low-pass filtering device to complementary signals and push-pull output devices that supply electrical current for loading and that are driven by said complementary signals for outputting the voltage of the aforementioned output side, an increase in the through current of the aforementioned push-pull output device being decreased by changes in the aforementioned load due to the aforementioned low-pass filtering device. Consequently, a high output amplifier that is stable and consumes low power internally. 
     In the high output amplifier of this invention, the aforementioned comparison amplifier may be a complementary amplifier that generates complementary output and in that the aforementioned low-pass filtering device connects the complementary output of the aforementioned comparison amplifier and subjects it to low-pass filtering. When this is done, both of the complementary outputs of the complementary amplifier are used. Therefore, the response of the push-pull output stage is further accelerated when there is an abrupt change in load current. 
     The high output amplifier of this invention may be equipped with a buffer that receives the voltage set by the aforementioned comparison amplifier in the input of one side and with a resistance element that connects the output of said buffer with the input of the aforementioned other side so that the aforementioned voltage output can be obtained from the aforementioned resistance element. When this is done, a simple structure can be achieved whereby changes in the load current are detected directly and high-speed response can be effected. 
     In the high output amplifier of this invention, an integrating-type adder amplifier may be used as the low-pass filtering device. When it is constructed in this way, the frequency characteristics of the feedback circuit can be determined solely by adjustment of the integrating adder-type amplifier. Consequently, the driving of the push-pull output stage can be set so that the through current is increased. 
     Further, the aforementioned conversion device can be constructed so that it performs a low-pass filtering action for differing complementary signals that are output. By this means, variations in the response characteristics of the elements of the push-pull output stage can be regulated. Consequently, a high output amplifier with which there is little increase in through current at higher speeds can be obtained. 
     As described above, because the high output amplifier of this invention is stable in the presence of variations of load current, it is particularly suited for use with an IC tester so that the aforementioned push-pull output stage supplies current to the device to be tested by the IC tester. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic circuit diagram of a high output circuit based on the prior art; 
     FIG. 2 is a schematic diagram of the high output amplifier according to the first embodiment of this invention; 
     FIG. 3 is a schematic diagram of the high output amplifier according to the second embodiment of this invention; and 
     FIG. 4 is a schematic circuit diagram of modified embodiments of the output stage of the high output amplifiers according to this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     High output amplifiers  20  and  30  as shown in FIG.  2  and FIG. 3 are presented for the purpose of explanation of this invention. In FIG.  2  and FIG. 3, the same reference numbers as in FIG. 1 are used for equivalent circuit elements in explaining the circuit elements and this invention. Further, explanations of bipolar transistors, which are used instead of field effect transistors (FET) and amplifiers (OPAMP), are omitted as they are well-known to persons of average skill in the art. Nevertheless, they are not excluded from the scope of this invention. 
     This invention is based on a structure whereby the structure that surpresses the increase of the through current of the push-pull output stage and the structure which increases the stability of the high output circuit do not readily interfere with each other. 
     FIG. 2 shows an example of a schematic circuit diagram of high output amplifier  20 , which is the first embodiment of this invention. Supply voltages VD and VS are explained as being of equal magnitude and of opposite polarity. However, as those persons skilled in the art should easily understand, other supply voltages can be selected. For example, the VS can be 0V (ground) and the ground in FIG. 2 can be a voltage of one-half the VD or it can be a voltage that is higher or lower than one-half of it. Terminal  1 , resistances R 1  to R 6 , diodes D 1  and D 2  and transistors Q 1  and Q 2  are connected as described in FIG.  1  and form a comparison amplifier constructed as a complementary amplifier. A set voltage is input into terminal  1 , which is the input on one side, the voltage of terminal  2  on the output side is fed back to the junction of resistances R 4  and R 5 , which is the input on the other side, and the voltage outputs, which corresponds to the difference between the aforementioned voltage of the input on one side and the aforementioned voltage of the input on the other side, are generated in the collectors of NPN transistor Q 1  and PNP transistor Q 2 . These voltage outputs are subjected to voltage current conversion by resistance R 21  and PNP transistor Q 1 A, or, by resistance R 22  and NPN transistor Q 2 A, then are added and combined to become input into the non-inverting input terminal of amplifier A 20 . Capacitor C 20  and resistance R 23  are also connected to the non-inverting input terminal of amplifier A 20 , and the current signals, which have been added and combined, are converted to voltage signals and subjected to low-pass filtering, and are then input into the non-inverting input terminal of amplifier A 20 . The inverting input terminal and the output terminal of amplifier A 20  are directly connected or are connected by resistances and amplifier A 20  functions as a buffer of amplification factor 1. 
     The output of amplifier A 20  is input into the succeeding conversion device. The conversion device has a comparable structure to that of the comparison amplifier. The junctions of diodes D 3  and D 4 , resistances R 9  to R 14 , diodes D 3  and D 4  and transistors Q 5  and Q 6  correspond respectively to terminal  1 , resistances R 1  to R 6 , diodes D 1  and D 2  and transistors Q 1  and Q 2  of the comparison amplifier. The junctions of resistances R 12  and R 13 , which correspond to resistances R 4  and R 5 , are grounded. The output of amplifier A 20  is input into the junction of diodes D 3  and D 4  and provides complementary output to the collectors of NPN transistor Q 5  and PNP transistor Q 6 . Each of said complementary outputs is input into the non-inverting input terminals of amplifiers A 1  and A 2  and the respective drain currents of P channel transistor Q 3  and N channel transistor Q 4  that are supplied to terminal  2  are controlled. Resistances R 7  and R 8  are, while depending on the design of the drain currents, often of values of 1 ohm or less. 
     Load current I 2  that flows into load LD is increased, the emitter current of transistor Q 1  is increased and the emitter current of transistor Q 2  is decreased. Consequently, the collector voltages of both transistors fall. The emitter voltage of transistors Q 1 A and Q 2 A also falls accompanying the fall in the collector voltages of two transistors Q 1  and Q 2 . The collector current of transistor Q 1 A increases, the collector current of transistor Q 2 A decreases, the added collector currents of the two transistors are converted to voltages that are raised by parallel connection of resistance R 23  and capacitor C 20  and become the input voltage of amplifier A 20  to the non-inverting input terminal. Consequently, the output of amplifier A 20  is input into the junction of diodes D 3  and D 4  which connects the bases of NPN transistor Q 5  and PNP transistor Q 6 . When this is done, the drain current of transistor Q 3  is increased and the drain current of transistor Q 4  is decreased. The current through transistor Q 4  is a through current, and, in the operation described above, undergoes essentially no increase from the design through current. Even if the load current undergoes further fluctuation, the through current undergoes essentially no increase from the initial value at the load current 0 due to the presence of the load current. 
     For the foregoing operation, capacitor C 20  is selected appropriately and the stability of the feedback operation is assured so that the intrinsic time constants (which are transmission properties from gate to drain) of transistors Q 3  and Q 4  do not affect the stability of the feedback loop. Specifically, R 23 ×C 20  should be selected as a value greater than several times the intrinsic time constants of Q 3  and Q 4 . Further, when there is a considerable difference between the properties of transistors Q 3  and Q 4 , values of resistances R 11  to R 14  are adjusted, or a capacitor and/or a low-pass filtering element are connected in parallel to either the resistance R 11  or R 14 , so that a difference in drain currents are controlled, an increase in through current is inhibited. As a result the stability and response to load current fluctuations may be improved in high output amplifier  20 . 
     The comparison amplifier of high output amplifier  20  shown in FIG. 2 can also be replaced functionally by an operational amplifier if control capacity of the load currents due to transistors Q 1  and Q 2  is not desired. It is also possible to use field effect transistors as transistors Q 1  and Q 2  to harmonize their manufacturing process with the processes for other elements. 
     In addition, transistors Q 3  and Q 4  can be bipolar transistors depending on the operating speed and the output power and can also be switching-type voltage to current converters. 
     It is also possible to by-pass amplifier A 20  and the structure of high output amplifier  20  is simplified. The gates of transistors Q 3  and Q 4  can be controlled so that the voltages across resistances R 11  and R 12  of amplifiers A 1  and A 2  become precisely the voltages across resistances R 7  and R 8 . However, simple buffers and/or direct connections are also possible as is described below further. 
     FIG. 3 shows an example of a schematic circuit diagram of high output amplifier  30  which is the second embodiment of this invention. Terminal  1 , resistances R 1  to R 6 , diodes D 1  and D 2  and transistors Q 1  and Q 2  are connected to form a complementary amplifier (buffer) as is shown in FIG.  1 . The complementary amplifier with resistance R 30  connected to its output forms a comparison amplifier. The input of one side of the comparison amplifier is terminal  1  and the input of the other side is considered to be a point X 1  which is the point at which the resistance R 30  is connected to terminal  2 . Amplifier A 30  amplifies differentially the voltages on the respective terminals of resistance  30 . The circuit components subsequent to the output terminal of amplifier A 30  are connected to form the same structure and to perform the same function as in the first embodiment of this invention to which FIG. 2 pertains. However, the output of amplifier A 30  is fed to the junction of resistances R 12  and R 13 , which differs from high output amplifier  20  where the output of amplifier A 20  is fed to the junction of diodes D 3  and D 4 . 
     The output of the complementary amplifier (buffer) is produced at the junction of resistances R 4  and R 5  and is input into the inverting input terminal of amplifier A 30  via resistance R 31  and the load voltage that is produced on terminal  2  is input from point X 1  into the non-inverting input terminal of amplifier A 30  via buffer A 31  and resistance R 32 . In addition, the non-inverting input terminal of amplifier A 30  is grounded via parallel combination of resistance R 34  and capacitor C 31 . Further, resistance R 33  and capacitor C 30  are connected in parallel between the inverting input terminal and the output terminal of amplifier A 30 . Because it is constructed in this way, the voltage produced across resistance R 30  is differentially amplified and is subjected to low-pass filtering, and is then transmitted from the output terminal of amplifier A 30  to the junction of resistances R 12  and R 13 . Resistances R 31  and R 32  may be selected so that they are equal in value, resistance R 33  and resistance R 34  may be selected so that they are equal in value and capacitor C 30  and capacitor C 32  may be selected so that they are equal in value. A value of R 33 ×C 30  should be selected at a level several times the inherent time constants of Q 3  and Q 4  so that the inherent time constants of Q 3  and Q 4  have essentially no effect on the stability of the (negative) feedback operation. 
     When resistance R 30  is several ohms and resistances R 31  and R 32  are several kilo-ohms, there may be essentially no deterioration of performance even when buffer A 31  is shorted out, the circuit is simplified and resistance R 32  is directly connected to resistance R 30  and terminal  2 . This is because buffer A 31  is connected to the terminal  2  only to prevent loading of resistance R 32  and etc for improving precision in detecting an increase of the load current. 
     As is described in the first embodiment of this invention shown in FIG. 2, when the load current changes, the voltage on terminal  2  changes along with it and this is detected directly by resistance R 30 . The fluctuating voltage that has been detected is subjected to low-pass filtering and is supplied to the conversion device from the junction of resistances R 12  and R 13 . As a result, the drain currents of transistors Q 3  and Q 4  are controlled. Resistances R 3  to R 6  can be omitted to enhance the buffering capacity of the complementary amplifier (buffer). In the second embodiment as well, the stability of the circuits of high output amplifier  30  is substantially controlled by the effect of the low-pass filtering when the voltage signals pass through amplifier A 30  and the through current is controlled by the succeeding conversion device as is controlled in the first embodiment. 
     Next, referring to FIG. 4, we shall describe the structure of a modified embodiment of the output parts of high-output amplifiers of this invention which are connected to the output of transistors Q 5  and Q 6 . The structures described below may be conveniently used in both of high output amplifiers  20  and  30 . 
     As shown in FIG.  4 (A), the inverting input terminal of amplifier A 1  is connected to supply voltage VD via resistance R 41  and the output terminal of amplifier A 1  via resistance R 42 , then connecting to the gate of transistor Q 3 . In addition, the inverting input terminal of amplifier A 2  is connected to supply voltage VS via resistance R 43  and to the output terminal of the amplifier A 2  via resistance R 44 , then connecting to the gate of transistor Q 4 . A difference in conductance between transistors Q 3  and Q 4  can be cancelled by adjusting resistances R 41 -R 44  to changing the gains of amplifiers A 1  and A 2 . Resistances R 41  and R 43  may be omitted and amplifiers A 1  and A 2  act as buffers which have good driving capability with unity gain. 
     As shown in FIG.  4 (B), in further modified embodiment of the invention, resistances R 11  and R 12  can be replaced by resistances R 11 A and R 12 A, respectively. Next, amplifiers A 1  and A 2  are shorted out and the collectors of transistors Q 5  and Q 6  are directly connected to the gates of transistors Q 3  and Q 4 . Direct-current sources  40  and  41  are also connected to the collectors of transistors Q 5  and Q 6 . 
     In the structure as described, a mismatching in conductance between transistors Q 3  and Q 4  can be cancelled by adjusting resistances R 11 A and Rl 2 A and a mismatching in the gate threshold voltage between transistors Q 3  and Q 4  can be cancelled by adjusting direct-current sources  40  and  41 . The design through current can also be adjusted by resistances R 11 A and R 12 A and direct-current sources  40  and  41 . 
     Further, in the circuit shown in FIG.  4 (A), direct-current sources  40  and  41  can be respectively connected to the respective collectors of transistors Q 5  and Q 6  or to the respective inverting input terminals of amplifiers A 1  and A 2 , so that a mismatching in the gate threshold voltage between transistors Q 3  and Q 4  can be cancelled. In addition, a voltage sources can be inserted in series between resistances R 41  and R 43  and a mismatching in the gate threshold voltage between transistors Q 3  and Q 4  can be cancelled by adjusting said voltage source(s). 
     As above, we have described the first and second embodiments of this invention as well as modifications of these. However, these descriptions do not exclude other modified embodiments and/or application examples nor include all aspects of the invention. 
     By embodiment of the invention, oscillation, instability and breakdown due to a sudden change of the load in the high output amplifiers can be effectively prevented. 
     The use of FETs(Field Effect Transistors) without minority carrier accumulation effect as transistors Q 3  and Q 4  of the output push-pull stage is desirable because it contributes to enhance speed of the high output amplifier of this invention. 
     When the high output amplifiers of this invention are used for applications such as device power sources for IC testers in which sudden changes of load tend to occur, it is beneficial and very advantageous.