Variable gain amplifier circuit and gain control method

Amplifiers – With semiconductor amplifying device – Including gain control means

Reexamination Certificate

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C330S285000

Reexamination Certificate

active

06437646

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a variable gain amplifier circuit and gain control method and, more particularly, to a variable gain amplifier circuit and gain control method which are used for the receiving section of a portable telephone terminal.
2. Description of the Prior Art
Recently, the portable telephone market has rapidly developed. In the receiving section of a portable telephone terminal, a weak RF signal received by an antenna is input first to a low-noise amplifier circuit to be amplified to a desired power, and then, only a desired frequency band is selected by a filter. The resultant signal is frequency-converted into an IF signal.
In a currently used digital portable telephone, to ensure a predetermined bit error rate, a circuit used for a receiving section is required to have a wide range of linearity because signals having various strengths can be received depending on the distance between the receiving section and a base station.
In order to ensure the linear operation of the circuit connected to the subsequent stage of the reception circuit, the above low-noise amplifier circuit is required to have a gain changing function by operating at a high gain with respect to a weak signal and decreasing the gain upon reception of a strong signal.
When a silicon bipolar transistor is used for this variable gain type low-noise amplifier circuit as in conventional terminals, the circuit uses two or more amplifiers and has a gain changing function after the second amplifier.
A heterojunction bipolar transistor (to be referred to as an HBT hereinafter), using a compound semiconductor such as GaAs, can be used for a portable telephone terminal as well.
This compound HBT has excellent RF characteristics, and hence a portable telephone low-noise amplifier circuit can be formed using only one amplifier. This component characteristic can decrease the number of elements used in constructing a portable telephone terminal. It is therefore expected that the reduction in number of components permits reduction in the area occupied by the circuit and power consumption.
Consider the circuit shown in
FIG. 1
as a single-stage variable gain amplifier circuit using a bipolar transistor such as the compound HBT.
FIG. 1
is a circuit diagram showing a conventional variable gain amplifier circuit. This variable gain amplifier circuit is comprised of an npn transistor
51
, a capacitor
52
and inductor
53
which are connected to the collector of the npn transistor
51
, a capacitor
54
and inductor
55
which are connected to the base of the npn transistor
51
, bias application terminals
56
and
57
each connected to the other terminal of a corresponding one of the inductors
53
and
55
, an output terminal
58
connected to the other terminal of the capacitor
52
, and an input terminal
59
connected to the other terminal of the capacitor
54
.
This conventional circuit is an emitter-grounded amplifier circuit using the transistor
51
, in which DC biases are applied to the base and collector of the transistor through the bias application terminals
56
and
57
and inductors
53
and
55
, an input signal input through the input terminal
59
is input to the base of the transistor
51
through the DC blocking capacitor
54
current, and an output signal output from the collector of the transistor
51
is output from the output terminal
58
through the DC blocking capacitor
52
.
In general, an LC impedance matching circuit is often connected to the input/output section, although this circuit is omitted in this case.
In a bipolar transistor, the transconductance (gm) of the elements changes in accordance with the base bias, and the gain approximates gm·ZL where ZL is the load impedance. Therefore, by using the bias application terminal
57
as a gain control terminal and changing the voltage to be applied, this circuit can be used as a variable gain amplifier circuit.
Other variable gain amplifier circuits of this type are disclosed in Japanese Unexamined Patent Publication Nos. 10-93390, 62-188411, 57-147309, and 57-132410 (to be referred to as references
1
to
4
, respectively, hereinafter).
The problem in the conventional variable gain amplifier circuit shown in
FIG. 1
is that a third-order input intercept point (to be referred to as an IIP
3
hereinafter) decreases with a decrease in gain, as indicated by the graph shown in
FIG. 2
which shows the gain dependence of the IIP
3
of the conventional circuit.
This is because, when a low base bias is set for a low gain, the difference between the base bias and the base ON voltage becomes small which increases the nonlinearity of the current/voltage characteristics between the base and the emitter, thereby readily distorting the input waveform.
In the receiving section of a portable telephone, low-gain operation is performed upon reception of a strong signal, and hence a larger IIP
3
is required in low-gain operation than in high-gain operation. For this reason, in order to apply this circuit to a portable telephone; the IIP
3
must be increased in low-gain operation.
Japanese Unexamined Patent Publication No. 6-78241 (to be referred to as reference
5
hereinafter) discloses an automatic gain control circuit for controlling the gain of a variable gain amplifier
5
by using an emitter follower
20
.
In this circuit, however, since the base bias voltage of the variable gain amplifier
5
is controlled by the emitter follower
20
, the IIP
3
undesirably decreases as in the conventional circuit described above.
The automatic gain control circuit in reference
5
is used for a satellite broadcasting receiver. In this regard as well, this conventional circuit differs from the present invention.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above drawbacks of the prior art, and has as its object to provide a variable amplifier circuit and gain control method which can increase IIP
3
in low-gain operation.
In order to achieve the above object, according to the first aspect of the present invention, there is provided a variable amplifier circuit comprising signal amplification means and variable impedance means connected to an input section of the signal amplification means and having a variable output impedance.
According to the first aspect, gain control is performed by the fact that the variable impedance means changes the input impedance of the signal amplification means. That is, gain control is performed without changing the input-side bias voltage of the signal amplification means, and hence the IIP
3
can be increased in low-gain operation. In other words, linearity is maintained even in low-gain operation.
This arrangement permits linearity to be maintained because an emitter follower is connected in parallel with the input terminal of an emitter-grounded amplifier, and the emitter follower is used to control the power input to the emitter-grounded amplifier circuit so as to prevent a deterioration in the distortion characteristics of the emitter-grounded amplifier circuit.
According to the second aspect of the present invention, there is provided a gain control method for a variable gain amplifier circuit including signal amplification means and variable impedance means connected to an input section of the signal amplification means and having a variable output impedance, comprising the first step of keeping a bias voltage applied to the input section of the signal amplification means constant, and the second step of changing an output impedance of the variable impedance means. in a direction in which a, gain of the signal amplification means decreases.
The second aspect has the same effects as those of the first aspect.
In the present invention, an emitter-grounded amplifier circuit is used as the, signal amplification means, and an emitter follower is used as the variable impedance means.
The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon m

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