JPH0246092Y2 - - Google Patents

Description

[Detailed description of the invention] [Technical field of the invention] The present invention is a bias circuit for a high-frequency FET amplifier that operates by supplying bias voltage to the drain terminal and gate terminal of an FET (field effect transistor) from independent power supplies. Regarding.

[Technical background of the invention and its problems]

GaAs as an amplification element in microwave amplifiers
FETs are already widely used. This GaAs
To operate the FET, it is necessary to apply an appropriate DC bias, and as shown in Figure 5, the method for doing this is to apply the power supply voltage +V only to the drain of the GaAs FET TR, and then apply a resistor RS to the source electrode.
is connected, and the voltage -
A self-bias method in which VG = -ID・RS is added, and a GaAs FET TR as shown in Figure 6.
The drain voltage +VD, -VG is supplied to the drain electrode and gate electrode from separate power supplies.2
There is a power supply method.

Although the self-bias method requires only one power supply, it is not possible to directly connect or fix the source electrode to a case or the like. For this reason, in microwave amplifiers with medium or higher power, where a large amount of heat is generated from the GaAs FET TR, a two-power supply system is mainly used, which allows the source electrode to be directly connected and fixed to the case, etc., and has good heat dissipation.

And when operating a GaAs FET TR,
In particular, when it is desired to obtain sufficient output power, a bias voltage is often applied to the gate electrode so that a drain current that is approximately 1/2 of the drain saturation current IDSS flows. Also, GaAs FET TR
The same conditions are set taking into account the allowable power loss.

By the way, if we consider the case where bias voltage is supplied to the GaAs FET TR to operate it as a microwave amplifier, the bias voltage -
If the bias voltage +VD is supplied to the drain electrode before VG is supplied, the drain current becomes IDSS, excessive DC power is supplied to the GaAs FET TR, exceeding the allowable power loss of the GaAs FET TR, and finally will damage the GaAs FET TR.
For this reason, a two-power supply FET amplifier requires its bias circuit to have a sequence control function.
That is, it is necessary to prevent the bias voltage +VD from being supplied to the drain before the bias voltage -VG is supplied to the gate.

A bias circuit conventionally used for this purpose will be explained with reference to FIG. In FIG. 7, a drain voltage input terminal 1 is connected to a drain electrode terminal 3 via a constant voltage circuit 8, this drain electrode terminal 3 is connected to the drain of a high frequency amplification FET 5, and the source of this FET 5 is grounded. be done. It is connected to the gate electrode terminal 4 of the FET 5, and this terminal 4 is connected to the gate voltage input terminal 2 and to the anode of the Zener diode 6. The cathode of this Zener diode 6 is connected to the gate of switching FET 7.
The drain of the FET 7 is connected to the reference voltage terminal 10 of the constant voltage circuit 8. The constant voltage circuit 8 may use an IC having the same function.

In other words, this circuit uses the fact that in a constant voltage circuit, when the reference voltage decreases, the output voltage also decreases, and the voltage -VI at the gate voltage input terminal 2 becomes 0.
At this time, the FET 7 is turned on, the voltage at the reference voltage terminal 10 in the constant voltage circuit 8 becomes 0, and the voltage at the drain electrode terminal 3 also becomes 0 due to the function of the constant voltage circuit 8. When the voltage -VI at the terminal 2 falls, the voltage -VO at the gate electrode terminal 4 also falls. Similarly, if the sum of the Zener voltage of Zener diode 6 and the pinch-off voltage of FET 7 is exceeded, the FET
7 is turned off, and a voltage determined by the reference voltage element 9 is applied to the reference voltage terminal 10.
The same voltage +VO is output to terminal 3, and GaAs
A bias voltage is supplied to the drain electrode of FET5. Although this circuit thus operates as a sequencer, it has the drawbacks described below.

This circuit is basically a constant voltage circuit, so
A voltage drop above a certain level occurs between terminals 1 and 3. For this reason, in microwave amplifiers that use many GaAs FETs, power consumption increases and the overall efficiency decreases.In addition, since the increased power becomes heat, a heat sink is required, resulting in a complex structure and weight. It also gets heavier. Furthermore, since the two power supplies are connected in a direct current manner, if a failure occurs in this circuit, it is possible that not only the expensive GaAs FET but also the power supply will be damaged, so a new protection circuit will be required.

[Purpose of invention]

The present invention eliminates the above-mentioned drawbacks, and aims to provide a bias circuit for a high-frequency FET amplifier that is simple in structure and capable of sequence control without increasing power consumption.

[Summary of the idea]

The present invention includes a photocoupler whose light-emitting side is connected to the gate of a high-frequency amplification FET, and a switching element controlled by the light-receiving side output current of this photocoupler. A high frequency device characterized in that the supplied drain bias voltage is switched by the gate bias voltage of the FET.
This is a bias circuit for FET amplifiers.

[Example of idea]

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, in which a drain voltage input terminal 1 is connected to a switching transistor 12.
and the base of the transistor 12 via a resistor R1. The collector of this transistor 12 is connected to the drain electrode terminal 3, and this terminal 3 is connected to the FET for high frequency amplification.
The source of this FET 5 is grounded. The gate of the FET 5 is connected to a gate electrode terminal 4, and this terminal 4 is connected to the gate voltage input terminal 2. This terminal 2 is grounded via the primary side (light emitting side) of the photocoupler 11 and a resistor 13. The secondary side (light receiving side) of the photocoupler 11 is connected to the base of the transistor 12 via a resistor R2.

That is, the feature of this circuit is that when the voltage at the gate voltage input terminal 2 is 0, no current flows to the primary side of the photocoupler 11, so no current flows to the secondary side, and as a result, the transistor 12 turns off. The voltage at terminal 3 becomes 0, and no bias voltage is supplied to the drain electrode. When the voltage at terminal 2 falls, the voltage at terminal 4 also falls, and at the same time, when the forward voltage of photocoupler 11 is exceeded, a current flows to the primary side of photocoupler 11. Correspondingly, current also flows to the secondary side, so transistor 12 is turned on, and the voltage at terminal 1 is output as voltage 3. The resistor 13 is a resistor that controls the current flowing to the primary side of the photocoupler 11. In this way, this circuit operates as a sequencer, but since the transistor 12 only performs an off/on switching operation, the voltage drop and power consumption when it is on can be almost ignored. This simplifies the structure because there is no need to use a heat sink, etc., and since a photocoupler is used as the voltage detection element, there is no direct current connection between the two voltage sources, so if a failure occurs in this circuit, However, it does not damage the GaAs FET or other circuits and is highly reliable.

2 to 4 are connection diagrams showing other embodiments of this invention.

In the example shown in FIG. 2, a Zener diode 14 is used as a level shift element between the terminal 2 and the photocoupler 11.
When the voltage at terminal 2 exceeds the sum of the Zener voltage of Zener diode 14 and the forward voltage of photocoupler 11, transistor 11 is turned on and a bias voltage is applied to the drain electrode. be.

In the example shown in FIG. 3, a capacitor 15 is connected in parallel to the primary side of the photocoupler 11, and the time constant with the resistor 13 delays the voltage applied to the primary side of the photocoupler 11 when the power is turned on.As a result, the voltage applied to the primary side of the photocoupler 11 is delayed. The bias voltage is delayed for a certain period of time.

Figure 4 shows diode 1 between terminal 2 and terminal 4.
6 is connected to a capacitor 17, and the voltage of the gate electrode is held for a certain period of time by the voltage charged in the capacitor 17 after the power supply is cut off.

In addition, although the above embodiment has been described using a transistor as a switching element, the present invention is not limited to this, and for example, a MOS type FET or the like may be used.

[Effect of idea]

As described above, according to the present invention, by using a switch using a photocoupler for voltage detection, it has the advantages of low power consumption, no need to use a heat sink, etc., and a simple configuration. Since a photocoupler is used as an element, 2
There is no direct current connection between the two voltage sources, so even if a failure occurs in this circuit, the FET will not be damaged, which has the advantage of increasing reliability.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing one embodiment of the present invention;
4 to 4 are circuit diagrams showing other embodiments of the present invention, FIGS. 5 and 6 are circuit diagrams explaining a bias supply method for GaAs FET, and FIG.
FIG. 2 is a circuit diagram of a bias circuit for a GaAs FET amplifier. 1...Drain voltage input terminal, 2...Gate voltage input terminal, 3...Drain electrode terminal, 4...
Terminal for gate electrode, 5... FET for high frequency amplification,
6, 14... Zener diode, 7... FET,
8... Constant voltage circuit, 9... Reference voltage element, 10...
...Reference voltage terminal, 11...Photocoupler, 12...
...Transistor, 13...Resistor, 15, 17...
Capacitor, 16...diode.

Claims (1)

[Claims for Utility Model Registration] (1) For high frequency amplification where the drain is connected to the drain voltage input terminal via a switching element
A high frequency device comprising a FET, a gate voltage input terminal connected to the gate of the FET, and a photocoupler whose light emitting side is connected to the gate voltage input terminal and whose light receiving side is connected to the control section of the switching element. Bias circuit for FET amplifier. (2) The bias circuit for a high frequency FET amplifier according to claim 1, wherein the light emitting side of the photocoupler is connected to the gate voltage input terminal via a level shift element.