High-frequency Circuit

Details High-frequency Circuit High-frequency Circuit

1998-10-28

2003-05-13

Le, Dinh T. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Gating

Signal transmission integrity or spurious noise override

C327S389000, C327S427000, C327S566000, C257S275000

Reexamination Certificate

active

06563366

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high-frequency circuit having a high-frequency switch capable of being driven by a low voltage which is built into in a portable device such as a cellular phone.
2. Description of the Related Art
Dazzling developments have been made in mobile communications such as cellular telephones and personal communications in recent years. In Japan, for example, in addition to the conventional 800 MHz band analog cellular phones, 800 MHz and 1.5 GHz band digital cellular phones (PDC) have been newly commercialized and, several years ago, the “Personal Handiphone System” (PHS) started service. Recently, in particular, there have further been active developmental efforts on a global scale of the next generation digital communications using the latest digital modulation technologies. The field of mobile communications is becoming increasingly active.
Such mobile communications, especially digital communications systems, often make use of the quasi-microwave band. Therefore, there have been strong demands for a switching circuit (high-frequency switching circuit) for switching high-frequency signals used in the portable terminals of these systems which is provided with not only superior high-frequency characteristics, but also the ability to be driven with a low voltage.
Since portable terminals handle signals of as high as the GHz band, switching circuits using GaAs FETs exhibiting excellent high-frequency characteristics are starting to be used for switching high-frequency signals in portable terminals.
FIG. 1
shows a switching FET constituting a basic unit of a high-frequency switching circuit. The switching FET shown in
FIG. 1
is connected at its gate to a resistance element Rg having a high resistance value. As a result, the equivalent circuit of the switching FET can be represented as an on-resistance Ron of several ohms in an on-state and of a cut-off capacitance of several hundreds of fF in an off-state. The cut-off capacitance in an off-state is the combined capacitance of the series capacitance between a gate and a source or drain (both referred to as Cg in this example) and a capacitance Cds between the source and drain connected in parallel. Since the FET with a gate connected to the high resistance element Rg clearly exhibits a resistance characteristic and a capacitance characteristic in the on-state and the off-state in this way, it has excellent characteristics as a basic unit of a quasi-microwave band switching circuit.
FIG. 2
is a diagram of impedance changes in a gate bias state of a switching FET.
An impedance Zds between a drain and a source of the switching FET becomes sufficiently large when the gate bias voltage Vg is less than a pinch-off voltage Vp, while conversely becomes sufficiently low when the gate bias voltage Vg is close to the gate voltage Vf at which the FET turns on (referred to as a “turn-on voltage” hereinafter). Accordingly, when using this FET for switching, the gate bias voltage Vg(on) when the FET is turned on is set to be larger than the turn-on voltage Vf, while the gate bias voltage Vg(off) when the FET is turned off is set to be sufficiently lower than the pinch-off voltage Vp.
When such a switching FET handles a large power (large amplitude) RF signal, distortion and other disadvantages with signal deterioration occur. This distortion disadvantage at the time of large power input is related to the fact that it is impossible to obtain a large voltage difference between the gate bias voltage Vg(on) and Vg(off) when having to make the drive voltage as small as possible such as in a portable terminal. Namely, to ensure that Vg(on) does not fall below the turn-on voltage Vf despite the trend toward a small voltage difference between the gate bias voltages Vg(on) and Vg(off) due to the low voltage driving function, it is necessary to narrow the margin between the voltage Vg(off) and the pinch-off voltage Vp and, as a result, signal distortion easily arises in the off-state.
As shown in
FIG. 2
, when an RF signal is applied to the FET in the off-state, the gate bias voltage is subjected to modulation by the RF signal around Vg(off). When the RF signal has a large amplitude, the degree of modulation becomes large. When it exceeds a certain limit, the modulated gate bias voltage Vg becomes larger than the pinch-off voltage Vp as shown in FIG.
2
. Finally, the FET is no longer in the pinch-off state, that is, an off-state, and as a result the waveform of the output voltage becomes distorted.
In order to reduce the signal distortion at the time of a large power input, a high-frequency switching circuit having a multi-stage configuration with a plurality of FETs connected in series is ordinarily used.
FIG. 3
shows an example of an FET switching circuit of a three-stage configuration.
This FET switching circuit
100
comprises a switching FET portion
101
and a short-circuiting FET portion
102
which is connected between the output of the switching FET portion
101
and a supply line (Vss line
103
) of a common voltage and holds an output node of the switching FET portion
101
in the off-state at the common voltage. Each of the portions has a multi-stage configuration.
In the switching FET portion
101
, the three FET
1
-
1
to FET
1
-
3
are connected in series between an input terminal Tin and an output terminal Tout of the high frequency signal, and gates of the FETs are connected to a common control signal input terminal Tc
1
via high resistance elements Rg. Similarly, in the short-circuiting FET portion
102
, the three FET
2
-
1
to FET
2
-
3
are connected in series between the output terminal Tout of the high frequency signal and the Vss line
103
, and gates of the FETs are connected to a common control signal input terminal Tc
2
via high resistance elements Rg.
In the high-frequency switching circuit
100
having the above configuration, in an on-state, all of the switching FET
1
-
1
to FET
1
-
3
are on and all of the short-circuiting FET
2
-
1
to FET
2
-
3
are off. When the high frequency switching circuit
100
shifts to an off-state, all of the switching FET
1
-
1
to FET
1
-
3
turn off and all of the short-circuiting FET
2
-
1
to FET
2
-
3
shift to an on-state. Even if there is a slight leakage of signal components in the switching FET
1
-
1
to FET
1
-
3
in an off-state, this is relieved to the common potential and therefore high reliable insulator at a high-frequency between the input and output can be achieved. That is, by being combined with the short-circuiting FET
2
-
1
to FET
2
-
3
, the switching FET
1
-
1
to FET
1
-
3
can obtain excellent isolation characteristics when turned off without any accompanying signal loss at the time of an on-state.
By making each of the FET portions a multi-stage configuration, the RF signal voltage input is divided in accordance with the number of stages. While each of the stages of the FETs is subjected to RF modulation, though to a different degree, distortion is more difficult than in the case of a single-stage configuration since the input signal voltage is divided. Accordingly, a switching circuit with an FET portion having a multi-stage configuration features a higher maximum power handled and improved distortion tolerance at the time of large power input.
Next, the maximum power handled by this multi-stage configuration high-frequency switching circuit will be explained in more detail with reference to the equivalent circuit in the off-state.
FIG. 4
is a view of an equivalent circuit of the high-frequency switching circuit
100
shown in
FIG. 3
in the on-state, that is, the state with the switching FET portion
101
in the on-state and the cut-off FET portion
102
in the off-state. Note that when the switching circuit
100
is in the off-state, for which the equivalent circuit is not shown, the state (on/off) of the switching FET portion
101
and the short-circuiting FET portion
102
becomes reverse to the case in FIG.
4
. In both cases, the distortion of the ou

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