JPH09219366A - Amplifier circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the reliability by inserting a current path which includes a MESFET for protection which is applied at its gate with such a voltage that may turn the device ON when the potential difference between the gate and the drain of a MESFET for amplification becomes a specified value which is lower than the withholding strength, between the gate and the drain of the MESFET for amplification which receives an input signal at its gate. SOLUTION: When the level of an input signal which is input into the gate of a differential MESFET 9 becomes too low, a MESFET for protection 10 comes to an on-state and current starts flowing. The current flowing in the MESFET 10 causes a voltage drop in a load resistor 7 and prevents the increase in the drain voltage of the MESFET 9 and it also supplies current to a MESFET 4, a source of constant current, of an input-step circuit. As a result, in the input-step circuit, the currents in a diode 3 which conducts a level shift operation and in a source follower MESFET 1 decrease by the amount of current which flows in the MESFET 10 and thereby decrease the amount of level shift and prevent the level of an input node 13 from becoming too low.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an amplifier circuit, and more particularly to a GaAs (gallium arsenide) MESFET (Meta
(1 Semiconductor FET) and is effectively applied to a differential amplifier circuit or an inverting amplifier circuit (inverter circuit) that drives a laser diode or an external modulator type optical modulator.

[0002]

2. Description of the Related Art For example, a differential amplifier circuit or an inverter circuit as a drive circuit of the above optical modulator is described in 1986.
, "Compound Semiconductor Device Handbook", published by Sunence Forum, Inc., page 253. FIG.
Shows an example of such a differential amplifier circuit.
This circuit has the same structure as the ECL including silicon bipolar transistors and is composed of a current switching type circuit. That is, the common source 23 of the differential MESFETs 9 and 16 is provided with the MESFET 22 that performs constant current operation,
This constant current is switched by the differential MESFETs 9 and 16 to obtain an output signal corresponding to it from the drains 8 and 15. A source follower circuit including a source follower amplification MESFET 1, a level shift diode 3 and a MESFET 5 as a constant current load is provided as an input stage circuit.
1 through the diode 3 through the differential amplification MESFET9
An input signal is supplied to the gate of.

[0003]

DISCLOSURE OF THE INVENTION Differential amplification MESF
In relation to the other differential MESFET 16, the ET 9 is turned off when an input signal below the threshold voltage is supplied to the gate, and the drain output 8 has the ground potential G
It becomes a high level like ND. However, due to the voltage adjustment of the gate terminal 5 of the constant current MESFET 4 of the level shift circuit, the manufacturing variation of the threshold voltage, etc., the output signal 13 of the preceding circuit, in other words, the differential MESFET 9
If the level of the input signal supplied to the gate of the MESFET 9 drops too much, the voltage between the gate and the drain of the MESFET 9 exceeds its withstand voltage, and the FET characteristics may be deteriorated or destroyed.

An object of the present invention is to provide an amplifier circuit in which a large-amplitude output signal is obtained by an amplifying MESFET and deterioration or destruction of characteristics is prevented to improve reliability. The above and other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.

[0005]

The following is a brief description of an outline of a typical invention among the inventions disclosed in the present application. That is, between the gate and the drain of the amplification MESFET whose input signal is supplied to the gate, a voltage that is turned on when the gate-drain voltage of the amplification MESFET becomes a predetermined potential difference below the breakdown voltage is applied to the gate. A current path including a protective MESFET applied to the circuit is provided.

[0006]

1 is a circuit diagram of an embodiment of an amplifier circuit according to the present invention. Each circuit element is
It is formed on one semiconductor substrate by a known manufacturing technique of a compound semiconductor integrated circuit. Although not particularly limited, the amplifier circuit shown in the figure is directed to a differential amplifier circuit that drives a laser diode or an external modulation type optical modulator. The MESFETs in the figure are all depletion type.

The sources of the differential amplification MESFETs 9 and 16 are connected to the common source 23. This common source 23
The MESFET 21 that operates as a constant current source is provided between the negative voltage terminal Vss and the negative voltage terminal Vss. This MESFET2
A constant voltage for operating the MESFET 21 with a constant current is applied to the first gate terminal 22. Above differential M
The drains of ESFETs 9 and 16 and the ground potential GND of the circuit
Load resistors 7 and 14 are provided between and. One of the differential MESFETs 9 and 16 described above
An input signal to be amplified is supplied to the gate of 9 via an input stage circuit described later. The other differential MESFET 16
A reference voltage (or a bias voltage) set to a substantially midpoint potential between the high level and the low level of the input signal is applied to the gate terminal 20 of the.

In this embodiment, a source follower circuit is provided as an input stage circuit. In other words, source follower M
An input signal is supplied to the gate terminal 2 of the ESFET 1. The source of the MESFET 1 is provided with a level shifting diode 3 and a MESFET 4 as a constant current load. The MESFET 4 is operated as a constant current source by applying a constant voltage to the gate terminal 5. The input signal supplied to the gate terminal 2 is the MES
The gate and source of FET1, the anode of diode 3,
The differential MESF is level-shifted through the cathode
It is transmitted to the input node 13 to which the gate of ET9 is connected.

In the above differential amplifier circuit, the constant current source MES is used.
The constant current is undesirably increased due to the process variation of the constant voltage setting or the threshold voltage supplied to the gate terminal 5 of the FET 4, or the source follower MESFET 1 described above.
It is expected that the threshold voltage of will increase due to process variations. As a result, the differential MESF
The low-level input signal transmitted to the input node 13 to which the gate of ET9 is coupled becomes too low, and the differential MESFET 9 is turned off to raise the drain to the ground potential GND of the circuit, thereby applying a gate-source voltage to the MESFET 9 If the withstand voltage between the gate and the drain is exceeded, or if the withstand voltage is increased to the vicinity of the withstand voltage between the gate and the drain, breakdown of the withstand voltage between the gate and the source or deterioration of characteristics will occur.

In this embodiment, as described above, the differential MES is
Even when the input signal applied to the gate of the FET 9 becomes undesirably low, the following protection circuit is provided in order to prevent breakdown of the breakdown voltage between the gate and drain and deterioration of characteristics. Between the gate of the MESFET 9 and the output terminal 8 which is the drain, the MESFET 10 in which level shifting diodes 10 and 12 are respectively inserted in the drain and the source is provided. That is,
A series circuit including a diode 11, a MESFET 10 and a diode 12 is provided between the drain and the gate of the MESFET 9. Although not particularly limited, the above M
The gate of the ESFET 10 is connected to the common source 23 in which the sources of the differential MESFETs 9 and 16 are shared. The other differential MESFET 16 is not particularly limited.
Between the drain and the gate of the diode 1
8, a protection circuit composed of the MESFET 17 and the diode 19 is provided. The gate of the MESFET 17 is
Similar to the above, it is connected to the common source 23.

The diode 11 of the protection circuit is a MES.
MESFET applied between the gate and drain of FET 10
The level is shifted so that a voltage higher than the gate-drain breakdown voltage of 10 is not applied. The diode 12 is a MES
By increasing the effective threshold voltage of the FET 10, the MES
When the low level of the input signal supplied to the gate of the FET 9 is sufficiently small with respect to the breakdown voltage between the gate and the drain, almost no current flows through the MESFET 10, and a desired amplification gain is secured. Further, these diodes 10 and 12 prevent the current from flowing from the gate side to the drain even when the gate voltage is made higher than the drain of the MESFET 9 such as when the input signal is set to the high level. It is for

In this embodiment, the differential MESFET 9
When the level of the input signal that is transmitted to the gate of the gate is too low, the protective MESFET 10 is turned on and a current flows. The current flowing through the MESFET 10 causes a voltage drop in the load resistance 7 of the differential amplifier circuit and causes the MES.
The drain voltage of the FET 9 is prevented from rising and current is supplied to the constant current source MESFET 4 of the input stage circuit. As a result, in the input stage circuit, the current flowing through the diode 3 that performs the level shift operation and the source follower MESFET 1 is reduced by the amount of the current flowing through the MESFET 10, and the level shift amount is reduced so that the level of the input node 13 does not drop too much. To work. The protection MESFET 10
It is possible to prevent an undesired increase in the gate-drain voltage of the differential MESFET 9 due to the above-described two effects of the current flowing through. Above differential ME
Also in the differential MESFET 16 that operates differentially with the SFET 9, the differential MESF generated by the same protection circuit as above.
This prevents an undesirably large voltage from being applied between the gate and drain of ET16.

FIG. 2 is a circuit diagram of another embodiment of the amplifier circuit according to the present invention. In this embodiment, a modification of the amplifier circuit is shown. First, in the source follower circuit, a source follower MESFE similar to the above is used.
The level-shifting diode provided at the source of T31 is replaced with the MESFET 33 in the form of a diode in which the gate and the drain are connected. In the protection circuit, a protection MESFET 40 and a level shift diode 41 are provided between the drain of the differential MESFET 39 and the source of the source follower MESFET 31. However, the differential amplification MESFET39
Between the gate and drain of the protection MESFET
40, the level shifting diode 41, and the diode-type MESFET 33 are connected in series, which is substantially the same as the embodiment of FIG. That is, the level shift MESFET 33 of the source follower circuit is shared with the level shift circuit of the protection circuit.

On the other side of the differential MESFET 46, a protective ME is provided so as to correspond to the side of the differential MESFET 39.
A level shift diode 48 is provided between the SFET 47 and the terminal 49. The reference voltage or the bias voltage is applied to the gate terminal 50 of the differential MESFET 46, and a voltage having a potential difference corresponding to the level shift amount of the level shift MESFET 33 with respect to the reference voltage or the bias voltage is applied to the terminal 46. Is applied. The terminals 49 and 50 may be provided with a diode-formed MESFET similar to the MESFET 33, and a bias current similar to that of the MESFET 33 may flow through the MESFET.

Also in this embodiment, the differential MESFET is also used.
When the level of the input signal transmitted to the gate of 39 is too low, the protective MESFET 40 is turned on and a current flows. The current flowing through the MESFET 40 causes a voltage drop in the load resistor 37 of the differential amplifier circuit to prevent the drain voltage of the MESFET 39 from rising, and also supplies a current to the constant current source MESFET 34 of the input stage circuit. As a result, in the input stage circuit, the source follower ME
The current flowing through the SFET 31 is reduced by the amount of the current flowing through the MESFET 40, and the level shift amount is reduced to prevent the level of the input node 43 from falling too low, thereby undesirably increasing the gate-drain voltage of the differential MESFET 39. You can prevent it. Above differential M
Differential MESFET 4 that operates differentially with ESFET 39
In the case of 6 as well, the differential ME applied by the same protection circuit
This prevents an undesirably large voltage from being applied between the gate and drain of the SFET 46.

FIG. 3 shows a circuit diagram of another embodiment of the amplifier circuit according to the present invention. In this embodiment, a unique constant voltage is applied to the gate of the protective MESFET 10 unlike the embodiments of FIGS. 1 and 2. That is, MES
A constant voltage formed by the diodes 56, 57 and 58 in series is applied to the gate of the FET 10. These diodes 56, 57 and 58 include a load MESFET 55 and a level shift diode 5 respectively.
A bias current is supplied by 4. By setting the constant voltage low, the level shifting diode provided in the source side of the protective MESFET 10 as in the embodiment circuit of FIG. 1 is omitted. The other differential MESFE
The protection circuit provided in T16 is similar to that of the embodiment shown in FIG. Since the operation of this embodiment is substantially the same as that of the embodiment of FIG. 1, the description thereof will be omitted.

FIG. 4 shows a circuit diagram of still another embodiment of the amplifier circuit according to the present invention. In this embodiment, it is directed to an inverting amplifier circuit (inverter circuit). As an input stage circuit, a source follower MESFET 60 having an input signal supplied to a gate terminal 61, a diode type MESFET 62 and a constant current source MESFET 63.
A source follower circuit similar to that described above is provided.

The source of the amplifier circuit is the ground potential GN of the circuit.
An amplifying ME connected to D and having a gate connected to an input node 71 through which the output signal of the source follower circuit is transmitted.
It is composed of an SFET 67 and a MESFET 65 which is provided between the drain of the SFET 67 and the power supply voltage Vdd and is operated as a constant current load by connecting the gate and the source thereof. Amplification MES in such an inverting amplifier circuit
In order to protect the breakdown voltage between the gate and drain of the FET 67, M
A protection circuit including the ESFET 68 and the diode 69 is provided. This protection circuit is substantially the same as the protection circuit shown in FIG. That is, the amplification MESFET 67 has substantially the same relationship with the protection circuit as the differential MESFET 39 of FIG.

FIG. 5 is a circuit diagram of a further embodiment of the amplifier circuit according to the present invention. In this embodiment, the inverting amplifier circuit (inverter circuit) is of enhancement type. That is, the amplification MESFET 72
Is an enhancement type, and a depletion type MESFET 65 is used as a load. In such an inverting amplifier circuit, an enhancement MESFET 73 having a gate to which a ground potential is applied is provided as a protection circuit between the gate and drain of the amplification MESFET 72.
In this amplification circuit, when a negative voltage such as undershoot is applied to the input signal input to the gate of the amplification MESFET 72, the protection MESFET 73 is turned on, and the drain voltage is lowered to reduce the gate-drain voltage. It is intended to prevent the breakdown voltage from being exceeded.

The functions and effects obtained from the above embodiment are as follows. (1) Amplified MESFE in which the input signal is supplied to the gate
A current including a protective MESFET that is applied between the gate and drain of T such that a voltage that is turned on when the gate-drain voltage of the amplification MESFET becomes a predetermined potential difference equal to or lower than the withstand voltage is applied to the gate. By providing the path and lowering the drain voltage of the amplification MESFET, it is possible to obtain the effect that breakdown breakdown and characteristic deterioration can be prevented.

(2) The input signal is transmitted to the gate of the amplification MESFET via the source follower amplification MESFET and the MESFET in the form of a diode for level shifting, and the current flowing through the protection MESFET is transmitted to the gate of the amplification MESFET. By causing the level shift MESFET to flow between the drain and the gate, the number of elements of the protection circuit is reduced, and at the same time, the level of the input signal is prevented from being lowered by the above current, and the drain voltage is lowered. Therefore, it is possible to effectively prevent the breakdown of the breakdown voltage and the deterioration of the characteristics.

Although the invention made by the inventor has been specifically described based on the embodiment, the invention of the present application is not limited to the embodiment, and various modifications can be made without departing from the gist of the invention. Needless to say. For example, FIG.
To the load MESF in the embodiment of FIG.
Can be replaced with ET. It is not a condition to provide a source follower circuit as an input stage circuit in the differential amplifier circuit. INDUSTRIAL APPLICABILITY The present invention can be widely used for an amplifier circuit using an amplifier element having a relatively small breakdown voltage between source and drain.

[0023]

The effects obtained by typical ones of the inventions disclosed in the present application will be briefly described as follows. That is, a constant voltage that is turned on when the gate-drain voltage of the amplified MESFET becomes a predetermined potential difference below the breakdown voltage between the gate and drain of the amplified MESFET whose input signal is supplied to the gate. By providing a current path including the protective MESFET applied to the gate and lowering the drain voltage of the amplification MESFET, breakdown voltage breakdown and characteristic deterioration can be prevented.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of an amplifier circuit according to the present invention.

FIG. 2 is a circuit diagram showing another embodiment of the amplifier circuit according to the present invention.

FIG. 3 is a circuit diagram showing another embodiment of the amplifier circuit according to the present invention.

FIG. 4 is a circuit diagram showing still another embodiment of the amplifier circuit according to the present invention.

FIG. 5 is a circuit diagram showing still another embodiment of the amplifier circuit according to the present invention.

FIG. 6 is a circuit showing an example of a conventional amplifier circuit.

[Explanation of symbols]

1, 31, 60 ... Source follower MESFET, 2, 3
2, 61 ... Gate terminal, 3, 33 ... Diode, 4, 3
4, 63 ... Constant current MESFETs, 5, 35, 64 ... Gate terminals, 7, 37 ... Resistors, 8, 38 ... Output terminals, 9, 3
9 ... Differential MESFET, 10, 40, 68 ... Protection MES
FET, 11, 12, 41, 69 ... Diode, 13,
43, 71 ... Input node, 14, 44 ... Resistor, 15, 4
5 ... Output terminal, 16, 46 ... Differential MESFET, 17,
47 ... Protection MESFET, 18, 19, 48 ... Diode, 20, 50 ... Gate terminal, 21, 51 ... Constant current ME
SFET, 22, 52 ... Gate terminal.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Mitsuru Arai Mitsuru Arai 5-20-1 Kamimizuhoncho, Kodaira-shi, Tokyo Inside Hiritsu Cho-LS Engineering Co., Ltd. (72) Inventor Yasushi Hatta Ome, Tokyo 2326 Imai, Ichi, Hitachi, Ltd. Device Development Center

Claims (3)

[Claims] 1. An amplifying ME in which an input signal is supplied to a gate.
The amplification MESFET is provided so as to form a current path between the gate and the drain of the amplification MESFET.
An amplifier circuit comprising: a protection MESFET having a gate applied with a voltage that is turned on when the gate-drain voltage of the ESFET reaches a predetermined potential difference equal to or lower than the withstand voltage. 2. The amplification MESFET is a pair of amplification MESFETs connected in a differential form, and the protection MESFET is provided.
The ESFET is a pair of MESFETs connected between the respective gates and drains of the differential amplification MESFETs via level shifting diodes, and each gate is a common source of the pair of amplification MESFETs. The amplifier circuit according to claim 1, wherein the amplifier circuit is connected to. 3. The input signal is a source follower amplifier M.
It is transmitted to the gate of the amplification MESFET through the ESFET and the MESFET in the form of a diode for level shift, and the protection MESFET is provided between the drain and gate of the amplification MESFET together with the level shift MESFET. The amplifier circuit according to claim 1, wherein the amplifier circuit constitutes a current path.