JP2782350B2 - Low distortion current source - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a low-distortion current source having a small distortion and a small parasitic capacitance.

[Conventional technology]

When the output terminal voltage of a conventional current source composed of a transistor operates with a large amplitude of about several _VPP , the collector or drain terminal of the current source transistor connected to the output terminal is fixed to a fixed potential. Since the transistor operates with a large amplitude of several _VPP between the base and the gate terminal of the transistor, harmonic distortion occurs due to the voltage dependence of the junction capacitance between the base and collector or between the gate and drain, There is a problem that the signal band is limited by the capacitance value. These problems are caused in the technical field where the distortion generated by the current source affects the performance of the entire circuit, that is, a sample / hold circuit or a current source in which a switching diode bridge is driven by the current source is used as an active load. This was particularly remarkable in an amplifier circuit that performs the operation.

FIG. 9 shows a conventional first current source composed of an NPN transistor and a resistor. When the output terminal 4, that is, the collector terminal voltage of the transistor 1 operates with a large amplitude in this current source, the voltage between the base and the collector of the transistor 1 changes with a large amplitude. There is a problem of generating wave distortion. In FIG. 9, 2 is a resistor, and 3 is a voltage source.

FIG. 10 shows a second conventional current source composed of a PNP transistor and a resistor. In the case of this current source as well, there is a problem that harmonic distortion is generated due to the voltage dependence of the junction capacitance between the base and the collector of the transistor 5, as in the current source of FIG. In FIG. 10, 6 is a resistor, 7 is an output terminal, and 8, 9 are voltage sources.

FIG. 11 shows a conventional circuit in which a diode bridge is driven by a current source in a sample-and-hold circuit using a diode bridge as a switch. In the figure, 50
Is an input signal terminal, 51 is an output signal terminal, 52 to 55 are diodes constituting a diode bridge, 56 and 57 are diodes for turning off the diode bridge, 58 is a buffer amplifier for bootstrap, 59 is a hold capacitance, and 60 and 61
Is a clock signal input terminal. When the transistor 63 is on, current flows through the diodes 52 to 55, and the input signal terminal
The output signal terminal 51 and the output signal terminal 51 are in the ON state, that is, the sample mode. On the other hand, when transistor 62 is on,
Since current flows through 56 and 57, the input signal terminal 50 and the output signal terminal 51 are in the off state, that is, the hold mode.

In the case of this circuit, since the input signal amplitude is usually as large as about 2 V _PP , in the sample mode, the transistors 62, 6
Due to the voltage dependence of 3, 66, 67 base-collector junction capacitance values, there is a problem that harmonic distortion is generated and the signal band is limited by these capacitances. Also,
Also when switching from the sample mode to the hold mode, there is a problem that harmonic distortion is generated due to the signal level dependency of the transient current flowing through the junction capacitance. Note that
In FIG. 11, 64, 69, 70 are current sources, and 65, 68, 71 are voltage sources.

FIG. 12 shows a load of a constant current circuit composed of current sources 110 and 111 and transistors 107 and 108 (so-called active load).
This is a conventional differential amplifier used as a conventional differential amplifier. In this circuit, when the signal amplitude of the output signal terminals 102 and 103 is large, there is a problem that harmonic distortion occurs in the transistors 107, 108, 104 and 105 due to the voltage dependence of the junction capacitance between the base and the collector. In FIG. 12, 100 and 101 are input signal terminals, 106 is a current source, and 109 and 112 are voltage sources.

FIG. 13 shows a cascode-connected conventional differential amplifier circuit. In this circuit, when the signal amplitude of the output signal terminals 132 and 133 is large, there is a problem that harmonic distortion occurs due to the voltage dependence of the base-collector junction capacitance value of the transistors 138 and 139. In FIG. 13, 130, 13
1 is an input signal terminal, 134 and 135 are transistors, 136 is a current source, 137 and 142 are voltage sources, and 140 and 141 are resistors.

[Problems to be solved by the invention]

The present invention has been made in view of such a point,
An object of the present invention is to greatly reduce harmonic distortion due to the voltage dependency of the junction capacitance value and improve the operation speed.

[Means for solving the problem]

In order to solve such a problem, the present invention detects a voltage change at an output terminal of a current source, and detects a current flowing through a junction capacitance between a base and a collector or a gate and a drain of a transistor connected to the output terminal. A compensation current having the same magnitude is generated in all bias states and supplied to the output terminal.

[Action]

In the low distortion current source according to the present invention, the junction capacitance between the base and collector or the gate and drain of the transistor connected to the output terminal of the current source is equivalently zero.

〔Example〕

FIG. 1 is a circuit diagram showing a first embodiment in which the present invention is applied to the conventional current source shown in FIG. The current source includes a common-base transistor 10 between the transistor 1 and the output terminal 11. In this case, the voltage dependency of the capacitance value between the base and the collector of the transistor 10 is a cause of distortion. In FIG. 1, the collector terminal voltage and the base terminal voltage of the transistor 13 are values that are one stage lower than the collector terminal voltage and the base terminal voltage of the transistor 10 by the base-emitter voltage of the transistor 10, respectively. • The collector-to-collector voltage always has the same value, independent of the signal amplitude. Therefore, the transistor
By making the sizes of 10 and 13 equal, a current of the same magnitude flows in each base-collector junction capacitance, and a current flowing in the base-collector junction capacitance of the transistor 10 is supplied by the transistor 13, The base-collector junction capacitance value of the transistor 10 viewed from the output terminal 11 of the current source is equivalently zero, and harmonic distortion can be reduced. The size of transistor 14 is also transistor 10.
Small enough compared to the size of. Therefore, transistor 14
The harmonic distortion generated due to the voltage dependence of the base-collector junction capacitance of the transistor 10 is smaller than that of the transistor 10. In FIG. 1, 2 and 15 are resistors, 3, 16 and 17 are voltage sources, 1
2 is a transistor.

In FIG. 2, the emitter of the transistor 13 in FIG. 1 is connected to the collector terminal of the transistor 12. Since the transistor 12 is separated from the transistor 14, the load on the transistor 14 is reduced. Other operations are the same as those in FIG.

FIGS. 3 and 4 show an embodiment in which the present invention is applied to the second conventional circuit shown in FIG. 10, and the respective operating principles are substantially the same as those in FIGS. 1 and 2. The third
In FIG. 4 and FIG. 4, 5,31,32,33,35 are transistors,
6, 30 are resistors, 8, 9, and 34 are voltage sources, and 36 is an output terminal.

FIG. 5 shows an embodiment in which the present invention is applied to improve the harmonic distortion and the signal band in the conventional sample and hold circuit shown in FIG. Based on the same principle as described with reference to FIGS. 2 and 4, the distortion of the transistor 73 is reduced by the transistors 79 and 81, and the distortion of the transistor 74 is
0,82, the distortion of transistor 83 is reduced by transistors 86,9
Due to 2, the distortion of transistor 84 is compensated by transistors 87 and 93, respectively. With this configuration, the transistors 73 and 79, 74 and 80, 83 and 86, and 84 and 87 always operate with the same bias. Transient current flowing through the combined capacitor can be compensated, and harmonic distortion caused by the transient current can also be compensated.
It has been confirmed by circuit simulation that the application of the present invention can reduce harmonic distortion to about 1/30 or less as compared with the conventional sample and hold circuit shown in FIG. In FIG. 5, the same or corresponding parts as those in FIG. 11 are denoted by the same reference numerals, 75 to 78, 88 to 91 are current sources, and 72, 85
Is a voltage source.

FIG. 6 shows an embodiment in which the present invention is applied in the conventional circuit shown in FIG. 12 in order to reduce the distortion caused by the voltage dependence of the base-collector junction capacitance of the transistors 107 and 108. Based on the operation principle described with reference to FIG.
Also, the distortion of the transistor 107 is reduced by the transistors 115 and 116.
And compensated. In FIG. 6, the same or corresponding parts as those in FIG. 12 are denoted by the same reference numerals, and 117 and 118 are current sources.

FIG. 7 shows an embodiment in which the present invention is applied in the conventional cascode-connected differential amplifier circuit shown in FIG. 13 to compensate for distortion caused by the voltage dependence of the base-collector capacitance of the transistors 138 and 139. It is. Based on the operation principle described in FIG. 2, the distortion of the transistor 138 is compensated by the transistors 143 and 144, and the distortion of the transistor 139 is compensated by the transistors 146 and 147. In FIG. 7, the same or corresponding parts as those in FIG. 13 are denoted by the same reference numerals, and 145 and 148 are current sources.

FIG. 8 shows an embodiment in which the embodiment of FIG. 6 and the embodiment of FIG. 7 are combined. Based on the operation principle described above, the distortion of the transistor 108 is reduced by the transistors 113 and 114.
Then, the distortion of the transistor 107 is changed by the transistors 115 and 116.
The distortion of the transistor 138 is compensated by the transistors 143 and 144, and the distortion of the transistor 139 is compensated by the transistors 146 and 147, respectively. In FIG. 8, the same or corresponding parts as in FIGS. 6 and 7 are denoted by the same reference numerals.
9,150 is an output signal terminal.

〔The invention's effect〕

As described above, the present invention provides an emitter follower circuit or a source follower circuit that detects a potential change of an output terminal,
A current generating circuit for generating a compensation current having the same magnitude as a change in the collector current or the drain current of the transistor whose collector or drain is connected to the output terminal in all bias states, and providing the compensation current to the output terminal. By supplying, the junction capacitance between the base and the collector or the gate and the drain of the transistor connected to the output terminal can be equivalently reduced to zero, so that the harmonic distortion and the operation speed can be greatly improved.

[Brief description of the drawings]

1 to 8 are circuit diagrams showing an embodiment of a low-distortion current source according to the present invention, and FIGS. 9 to 13 are circuit diagrams showing a conventional current source. 1,10,12,13,14 …… Transistor, 2,15 …… Resistance, 3,16,
17 ... Voltage source.

Claims (1)

(57) [Claims] 1. A current source comprising a transistor,
An emitter-follower circuit or a source-follower circuit for detecting a potential change of an output terminal, and a compensation current having the same magnitude as a change in a collector current or a drain current of a transistor having a collector terminal or a drain terminal connected to the output terminal in all bias states. And a current generation circuit for generating the compensation current, and supplying the compensation current to the output terminal.