Variable gain amplifier

Details Variable gain amplifier Variable gain amplifier

2001-06-22

2003-01-21

Pascal, Robert (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including gain control means

C330S254000

Reexamination Certificate

active

06509798

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to a variable gain amplifier and particularly to a variable gain amplifier which includes a parallel connection of a resistance and a capacitor, or a parallel connection of an inductance and a capacitor.
2. Description of the Prior Art
Cascade amplifiers are employed for amplifying radio frequency signals for the pocket telephones and information terminals. Further, the cascade amplifiers are often provided with means for obtaining variable gain.
FIG. 4
is a block diagram of a conventional variable gain cascade amplifier. The amplifier as shown in
FIG. 4
comprises a cascade amplifier including common-emitter transistor Q
0
and common-base transistor Q
1
, a variable gain unit including transistor Q
2
and Q
3
and resistance R
2
, resistance R
0
for applying bias V
3
to the base of transistor Q
0
, resistance R
1
for the load resistance of the cascade amplifier, voltage supply V
2
for the amplifier as a whole, voltage supply V
0
(voltage supply
50
) for controlling the gain of the variable gain unit, signal input terminal
10
, signal output terminal
40
, power supplies and coupling capacitors connected with signal input terminal
10
and signal output terminal
40
, respectively.
The variable gain cascade amplifier as shown in
FIG. 4
obtains the variable gain by varying V
0
so as to change the route of current which flows into transistor Q
0
. Concretely, the amplification gain becomes low, when V
0
is greater than V
1
, because the current flows via transistors Q
2
and Q
3
into transistor Q
0
, while the amplification gain becomes high, when V
0
is smaller than V
1
, because the current flows via transistor Q
1
into transistor Q
0
.
If the current of Q
2
is smaller than the current of Q
1
, The gain becomes low, when V
0
is greater than V
1
, while the gain becomes high, when V
0
is smaller than V
1
.
FIG. 5
is a graph of a frequency characteristics of the conventional variable gain cascade amplifier as shown in FIG.
4
. The cut off frequency at the high gain state is about 4 GHz, while the cut off frequency at the low gain state is about 1.5 GHz, as shown in FIG.
5
.
As shown in
FIG. 5
, the cut off frequency at the high gain state is different from that at the low gain state, because the impedance of the base of transistor Q
2
is high due to resistance R
2
at higher frequency, and CR circuit by resistance R
2
and parasitic capacitance C between the base and the collector of transistor Q
2
limits the frequency characteristics. Therefore, the frequency characteristics varies, as the gain varies, in the conventional variable gain amplifier.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a variable gain amplifier wherein the frequency characteristics is unchanged, when the gain is changed.
The variable gain amplifier of the present invention comprises an amplifying unit for amplifying an inputted signal and a variable gain unit for varying the gain of the amplifying unit. The variable gain unit includes a transistor and a parallel connection of a resistor and a capacitor connected with the transistor. The variable gain unit may includes a transistor and a parallel connection of a resistor and an inductance.
The impedance of the base of the transistor is lowered by the parallel connection of a resistance and a capacitor in higher frequency range.
The variable gain amplifier of the present invention are applied for various electronic appliances such as pocket telephones.
According to the variable gain amplifier of present invention, the frequency characteristics remain unchanged, because the impedance of the control electrode such as a base or a gate of amplifying transistor are made low in higher frequency range, by using a parallel connection of a resistance (or inductance) and a capacitance connected with the control electrode of the amplifying transistor.


REFERENCES:
patent: 4344043 (1982-08-01), Harford
patent: 5999058 (1999-12-01), Fong
patent: 6043710 (2000-03-01), Smith et al.
patent: 6046641 (2000-04-01), Chawla et al.
patent: 6181206 (2001-01-01), Palmisano et al.
patent: 6249153 (2001-06-01), Moraveji
patent: 4-74010 (1992-03-01), None
patent: WO 95/07574 (1995-03-01), None

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