RF variable gain amplifying device

Details RF variable gain amplifying device RF variable gain amplifying device

2002-02-14

2003-12-23

Nguyen, Khanh Van (Department: 2817)

Amplifiers

Combined with automatic amplifier disabling switch means

C330S053000, C330S286000

Reexamination Certificate

active

06667657

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an RF variable gain amplifying device having a switch element for switching gain between different levels.
In a radio communication system represented by a mobile phone, a low noise characteristic and a high gain characteristic are required of an amplifying circuit in the initial stage of a receiving device when a small signal is received. In addition, a low distortion characteristic and a low gain characteristic are also required when a large signal is received. In recent mobile communication, in particular, the intensity of an electric field during reception greatly changes depending on the distance between a base station and a mobile station so that the receiving device requires a wide dynamic range. As a result, the low-noise amplifying circuit of a receiving front end unit is required to have a gain control function.
As an example of such an amplifying circuit having the gain control function, an RF variable gain amplifying device is disclosed in Japanese Laid-Open Patent Publication No. 10-173453.
FIG. 9
shows the RF variable gain amplifying device disclosed in the publication, which is an amplifying circuit in one of plural stages of amplifying circuits.
As shown in
FIG. 9
, an RF variable gain amplifying circuit
300
comprises: an amplifying circuit
310
: and an RF switch element
320
Q composed of a field effect transistor (FET) for opening and closing the signal bypass circuit of the amplifying circuit
310
.
Between the amplifying circuit
310
and input and output nodes Ti and To, respective dc blocking capacitors Ci and Co are connected in series.
The amplifying circuit
310
includes an amplifying element
311
composed of a FET in a source-grounded configuration. To the gate of the amplifying element
311
, an RF signal is supplied from an input node Ti via an input matching circuit
312
and a specified gate bias voltage Vgg is supplied through a resistor
313
. The drain of the amplifying element
311
outputs an amplified signal to an output node To via an output matching circuit
315
, while it is connected to a selection switch circuit
305
via a drain bias supply circuit
316
.
The selection switch circuit
305
has a first input node a connected to a supply line for a power supply voltage Vdd and a second input node g which is grounded. Switching between the nodes a and g is performed by a power supply control circuit
307
which operates in accordance with power transmission control data or on the level of a received signal.
The RF switch element
320
Q uses a depletion-type FET having a drain connected to the input node Ti via a dc blocking capacitor
303
c
, a source connected to the drain of the amplifying element
311
, and a gate connected to the ground via the resistor
306
.
In the conventional RF variable gain amplifying device
300
thus constructed, the power supply voltage Vdd is supplied to the amplifying circuit
310
via the selection switch circuit
305
under the control of the power supply control circuit
307
, while the selection switch circuit
305
is connected to the first input node a, whereby the amplifying circuit
310
is brought into an operating state. The power supply voltage Vdd applied via the selection switch circuit
305
is supplied as a control signal to the RF switch element
320
Q so that a gate-source voltage in the RF switch element
320
Q is lowered to bring the RF switch element
320
Q into an OFF state and bring the connection of the signal bypass circuit into an open state. In the open state, a high-gain operating mode is established in which the level of an output RF signal at the output node To is higher than the level of an input RF signal at the input node Ti by the gain of the amplifying circuit
310
.
If the selection switch circuit
305
is switched to the second input node g, on the other hand, the supply of the power supply voltage Vdd to the amplifying circuit
310
is cut off so that the operation of the amplifying circuit
310
halts, while a ground potential is supplied as a control signal via the selection switch circuit
305
to bring the RF switch element
320
Q into an ON state and bring the signal bypass circuit into a connected state. When the signal bypass circuit is in the connected state, a low-gain operating mode is established in which the level of the output RF signal at the output node To is lower than the level of the input RF signal at the input node Ti by the sum of a loss resulting from the insertion of the RF switch element
320
Q and a loss resulting from mismatched impedances at the input node Ti.
By thus switching the RF amplifying device to the high-gain operating mode if the level of the RF signal inputted to the input node Ti is lower than a specified level and switching the RF amplifying device to the low-gain operating mode if the level of the RF signal inputted to the input node Ti is higher than the specified level, the RF amplifying device capable of handling an RF signal with a wide dynamic range can be implemented.
When the conventional RF variable gain amplifying device is in the high-gain operating mode in which the selection switch circuit
305
is switched to the first input node a, however, the drain of the RF switch element
320
Q is brought into a floating state. This causes the problem that a residual charge in the floating state varies a drain potential and the varied drain potential varies isolation when the RF switch element
320
Q is in the OFF state.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to prevent varying isolation when an RF switch element provided in a signal bypass circuit is in the OFF state by solving the foregoing conventional problems.
To attain the object, the present invention constructs an RF variable gain amplifying device such that the electrode of the switch element for signal bypass circuit is not brought into the floating state.
Specifically, a first RF variable gain amplifying device according to the present invention comprises: an amplifying circuit; a switch element connected in parallel with the amplifying circuit; and a resistor connected in parallel with the amplifying circuit and with the switch element, the amplifying circuit not operating when the switch element is in an ON state but operating when the switch element is in an OFF state, a potential at each of input and output terminals of the switch element being lower when the switch element is in the ON state than when the switch element is in the OFF state.
Since the first RF variable gain amplifying device comprises the resistor connected in parallel with the amplifying circuit and with the switch element, the electrode of the switch element is not brought into the floating state even if the switch element is in the OFF state and the amplifying circuit is in the ON state, i.e., in a high gain state (in the high-gain operating mode). Accordingly, isolation does not vary and isolation provided by the switch element is increased. If the switch element is in the ON state and the amplifying circuit is in the OFF state, i.e., in a low gain state (in the low-gain operating mode), on the other hand, an insertion loss when the switch element is in the ON state is reduced.
In the first RF variable gain amplifying device, the amplifying circuit preferably has a bipolar transistor, an input signal to the amplifying circuit is preferably inputted to a base of the bipolar transistor, and an output signal from the amplifying circuit is preferably outputted from a collector of the bipolar transistor. In the arrangement, if a base potential is adjusted to approximately zero so that a collector current becomes approximately zero when the amplifying circuit is in the OFF state, a potential at each of the input and output terminals of the switch element can be lowered more reliably when the amplifying circuit is in the OFF state (when the switch element is in the ON state) than when the amplifying circuit is in the ON state (when the switch element is in the OFF state).
In the first variable ga

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