Amplifiers – With semiconductor amplifying device – Including gain control means
Reexamination Certificate
2002-05-15
2004-04-20
Nguyen, Khanh Van (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including gain control means
C330S051000, C330S278000, C330S311000
Reexamination Certificate
active
06724259
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a variable gain amplifier for radio-frequency signals.
BACKGROUND OF THE INVENTION
FIGS. 6 and 7
show prior-art variable gain amplifiers.
In the variable gain amplifier shown in
FIG. 6
, an input signal, RF
in
, to be amplified is applied to the base of an common emitter transistor
101
through an input terminal
200
and, in a high-gain state, a bias voltage is applied to a common base transistor
102
from a bias power supply
204
through a contact
202
of a selector switch
201
. As a result, a power supply voltage is applied to the collector of the transistor
101
from a power supply
205
through load Z and the collector-emitter of the transistor
102
. Reference number
206
indicates a bias power supply, which generates a bias voltage determining the operating point of the transistor
101
.
The signal, which is amplified by amplification factor found by transconductance gm of the transistor
101
that determines current flowing in load Z and magnitude of the load Z, is provided from an output terminal
208
connected to the connecting point
207
between load Z and the transistor
102
.
In a low-gain state, a bias voltage is applied to the transistor
102
from the power supply
204
through a contact
203
of the selector switch
201
. As a result, current between the collector and emitter of the transistor
102
is turned off and current between the collector and emitter of the common base transistor
103
is turned on and thereby a power supply voltage is applied to the collector of the transistor
101
from the power supply
205
through load Z, a resistor
104
, and the collector-emitter of the transistor
103
. The signal, which is amplified by amplification factor found by transconductance gm of the transistor
101
that determines current flowing in load Z and magnitude of the load Z, is provided from the output terminal
208
, as in the high-gain state. A gain lower than that in the high-gain state is set by the resistor
104
.
In the variable gain amplifier shown in
FIG. 7
, the gain is changed as follows.
Operation of this variable gain amplifier in a high-gain state is the same as that of the variable gain amplifier shown in FIG.
6
. In a low-gain state, a bias voltage is applied to the base of a common base transistor
105
from a bias power supply
204
through a contact of a selector switch
201
, current between the collector and emitter of the transistor
102
is turned off, and current between the collector and emitter of the transistor
105
is turned on and thereby a power supply voltage is applied to the collector of a transistor
101
from the power supply
205
through the collector-emitter of the transistor
105
. A signal, which is amplified by amplification factor found by transconductance gm of the transistor
101
that determines current flowing in load Z and magnitude of the load Z, is coupled to the connecting point
207
at radio frequency through junction capacitance Cj, which is determined by the isolation property when current is turned off between the collector and emitter of the transistor
102
, and provided through an output terminal
208
in a low-gain state.
An example of circuits in which a variable gain amplifier is used can be a first amplifier provided at the front end of a radio-frequency receiver. As shown in
FIG. 8
, bandpass filters
209
and
210
are connected to the input and output stages, respectively, of first amplifier A in order to improve selectivity. Reference number
211
indicates a mixer circuit.
Because the band-pass filters
209
,
210
are designed to satisfy an required pass frequency property at certain input and output impedances, the gain of first amplifier A is switched between high-gain and low-gain states in accordance with electric field strength in a area in which the radio-frequency receiver is used.
Therefore, it is required that the certain input and output impedances of first amplifier A be maintained independently of the switching between high and low gain states.
However, in the prior-art variable gain amplifier shown in
FIG. 6
, the input and output impedances vary depending on the high-gain or low-gain states.
In particular, the output impedance at 1 GHz is not significantly changed on switching between the high-gain and low-gain state as indicated by symbols Z
H
and Z
L
on a Smith chart in FIG.
9
. On the other hand, the output impedance at 5 GHz is significantly changed on switching between the high-gain and low-gain state as indicated by symbols Z
HH
and Z
LL
.
In the prior-art variable gain amplifier shown in
FIG. 7
, the input and output impedances are changed on switching between the high-gain state and low-gain state, and in addition, the low gain is determined by the isolation property of the transistor
102
in the off-state and therefore cannot be controlled.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a variable gain circuit whose impedance does not significantly vary depending on a high- or low-gain state and whose gain can be freely set in the low-gain state.
According to aspect
1
of the present invention, there is provided a variable gain amplifier in which the output circuit of a second transistor
2
is provided between the output circuit of a first transistor and a load, and an input signal provided to the input of the first transistor is amplified, the amplified signal is taken from a connecting point between the load and the second transistor, and the second transistor is turned on and off to switch between gains, wherein: a series circuit of an attenuator means and an output circuit of a third transistor is connected in parallel with an output circuit of the second transistor; the output circuit of a fourth transistor is connected in parallel with a series circuit of the output circuit of the second transistor and the load; selector means is provided for switching between a high gain state and a low gain state; and the selector means turns off the third and fourth transistors and turns on the second transistor to take the signal, which is amplified by amplification factor found by transconductance gm of the first transistor that determines current flowing in load Z and magnitude of the load Z, from the connecting point in the high-gain state, and turns off the second transistor and turns on the third and fourth transistors to take the signal, which is amplified by amplification factor found by transconductance gm of the first transistor that determines current flowing in load Z and magnitude of the load Z, from the connecting point in the low-gain state.
According to aspect
2
of the present invention, the attenuator means in the variable gain amplifier according to aspect
1
is formed with a resistor or a parallel circuit of a resistor and a capacitor.
According to aspect
3
of the present invention, the attenuator means in the variable gain amplifier according to aspect
1
is formed with a field effect transistor.
According to aspect
4
of the present invention, there is provided a variable gain amplifier in which the output circuit of a second transistor
2
is provided between the output circuit of a first transistor and a load, and an input signal provided to the input of the first transistor is amplified, the amplified signal is taken from a connecting point between the load and the second transistor, and the second transistor is turned on and off to switch between gains, wherein: a series circuit of an attenuator means and an output circuit of a third transistor is connected in parallel with an output circuit of the second transistor; the output circuit of a fourth transistor is connected in parallel with a series circuit of the output circuit of the second transistor and the load; selector means is provided for switching a high gain state, a medium gain state, and a low gain state; the third, fourth, and fifth transistors are turned off and the second transistor is turned on to take the signal, which is amplified by amplification factor found by tran
Matsushita Electric - Industrial Co., Ltd.
Nguyen Khanh Van
Parkhurst & Wendel L.L.P.
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