Transistor circuit with direct-coupled stages

Details Transistor circuit with direct-coupled stages Transistor circuit with direct-coupled stages

1999-09-23

2002-05-28

Tran, Toan (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Specific identifiable device, circuit, or system

Nonlinear amplifying circuit

C327S112000, C327S333000

Reexamination Certificate

active

06396337

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a transistor circuit and more particularly, to a transistor circuit equipped with direct-coupled stages that compensates the effect caused by the shift of the operating point of a transistor or transistors due to high-level input, which is preferably applied to radio communication systems such as portable or mobile telephones operable at a low supply voltage of approximately 1 V.
2. Description of the Prior Art
In recent years, to downsize radio communication systems such as mobile telephones, more and more Integrated Circuits (ICs) have been incorporated into the systems. Under the circumstances, the ICs need to be formed as small as possible, because the chip size of the ICs affects directly the size of the systems.
As known well, generally, the chip area of a capacitor is comparatively large with respect to other electronic elements such as a resistor and a transistor and at the same time, it increases proportionally with the increasing capacitance value. This means that capacitors form a bottleneck in decreasing the chip size of ICs.
Discrete capacitor components may be prepared outside ICs and connected to the ICs, reducing the number of capacitors in ICs. In this case, however, there arises a disadvantage that discrete capacitor components increase the overall mounting area of parts or subsystems of radio communication systems, which is contrary to the downsizing of radio communication systems. Accordingly, it is necessary that the number itself of capacitors in the systems is reduced.
As a result, to reduce the number itself of capacitors in the ICs designed for the radio communication systems of this sort, the direct-coupling configuration has been usually used to interconnect adjoining stages of transistor circuits such as amplifiers, frequency mixers, and so on in the ICs.
FIG. 1
shows the circuit configuration of a prior-art transistor amplifier circuit incorporated into an IC for a mobile telephone. In
FIG. 1
, the input of the prior-art transistor amplifier circuit
100
is connected to an antenna
101
through a tuning circuit
102
and the output thereof is connected to the input of a demodulator
105
. The output of the demodulator
10
; is connected to the input of a data processor
106
. The output of the data processor
106
is connected to the input of a reception indicator
107
.
The antenna
101
receives radio wave including transmitted Radio frequency (RF) signals. The tuning circuit
102
selects a desired one of the RF signals thus received by the antenna
101
and outputs the desired, selected RF signal to the amplifier circuit
100
. The RP signal thus inputted into the amplifier circuit
100
is referred as an input voltage V
IN
.
The amplifier circuit
100
amplifies the input voltage V
IN
and outputs an amplified RF signal to the demodulator
105
. The amplified RF signal thus outputted from the amplifier circuit
100
is referred as an output voltage V
OUT
.
The demodulator
105
demodulates the RE signal (i.e., the output voltage V
OUT
) outputted from the amplifier circuit
100
and outputs a demodulated signal to the data processor
106
. The data processor
106
performs predetermined data processing operations with respect to the demodulated signal. If the identification number (ID No.) included in the demodulated signal accords with the ID No. of the user or holder, the data processor
106
sends a specific signal to the reception indicator
107
, notifying the user of the reception of communications or messages. Thereafter, the user accesses the received communications or messages as necessary.
As seen from
FIG. 1
, the prior-art amplifier circuit
100
has two amplifier stages
103
and
104
directly-coupled together without using any coupling capacitors.
The first amplifier stage
103
has an npn-type bipolar transistor TR
101
whose emitter is connected to the ground and a load resistor R
101
(resistance: R
L
) connected to the collector of the transistor TR
101
. The input voltage V
IN
from the tuning circuit
102
is applied to the base of the transistor TR
101
. The collector of the transistor TR
101
is connected through the load resistor R
101
to a power supply and is applied with a constant supply voltage V
CC
. Here, V
CC
=1 V. An amplified voltage of V
IN
, i.e, an output voltage V
1
of the first amplifier stage
103
, is derived from the collector of the transistor TR
101
.
The second amplifier stage
104
has an npn-type bipolar transistor TR
102
whose emitter is connected to the ground and a load resistor R
104
connected to the collector of the transistor TR
102
. The output voltage V
1
of the first amplifier stage
103
is applied to the base of the transistor TR
102
through a coupling resistor R
103
. The collector of the transistor TR
102
is connected through the load resistor R
104
to the power supply of V
CC
and is applied with the constant supply voltage V
CC
. An amplified voltage of V
1
, i.e., the output voltage V
OUT
of the second amplifier stage
104
, is derived from the collector of the transistor TR
102
and is applied to the demodulator
105
.
The strength of the transmitted radio wave fluctuates in the air due to the change of the propagation and/or reflection conditions. If the antenna
101
is located in a place where the strength of the radio wave is in a high level, the input level for the first amplifier stage
103
of the amplifier circuit
100
(i.e., the input voltage V
IN
, to the first amplifier stage
103
) has a large magnitude. Obviously, due to the amplification operation in the first amplifier stage
103
, the output voltage V
OUT
of the amplifier circuit
100
has a larger magnitude than that of the input voltage V
IN
. This means that the output voltage V
OUT
of the amplifier circuit
100
has an enough magnitude for receiving the transmitted communications or messages. In spite of this fact, there is a possibility that the telephone shown in
FIG. 1
is unable to perform its reception operation, the reason of which is as follows.
Here, as shown in
FIG. 1
, the base-to-emitter voltage, the collector-to-emitter voltage, the base current, and the collector current of the transistor TR
101
are defined as V
BE101
, V
CE101
, I
B101
, and I
C101
, respectively. Similarly, the base-to-emitter voltage, the collector-to-emitter voltage, the base current, and the collector current of the transistor TR
102
are defined as V
BE102
, V
CE102
, I
B102
, and I
C102
, respectively.
The input voltage V
IN
is expressed as the sum of the bias (dc) component V
BB
and the signal (ac) component V
IN
. Then, the base-to-emitter voltage V
BE101
is equal to the input voltage V
IN
and therefore, the following equation (1) is established.
V
IN
=V
BE101
=V
BB
+V
IN
  (1)
The base current I
B101
, of the transistor TR
101
is expressed as the sum of the bias (dc) component I
B101
and the signal (ac) component i
B101
. Thus, the following equation (2) is established.
I
B101
=I
BB101
+I
B101
  (2)
FIG. 2
shows the I
BE101
−V
BE101
characteristic of the transistor TR
101
in the first amplifier stage
103
, in which the reference character P
1
denotes the operating point of the transistor TR
101
located on the curve
52
of the I
BE101
−V
BE101
characteristic. As seen from the equations (1) and (2), the operating point P
1
has an abscissa value of V
BB
and an ordinate value of I
BB101
.
The input voltage V
IN
(i.e., the base-to-emitter voltage V
BE101
of the transistor TR
101
) varies with time as schematically shown by periodic waveforms
51
a
and
51
b
in FIG.
2
. The waveform
51
a
having a small magnitude indicates the change of V
IN
and V
BE101
at the received strength of the radio wave being in a low level. The waveform
51
b
having a large magnitude indicates the change of V
IN
and V
BE101
at the received strength of the radio wave being in a very high level.
Due to the change of V
IN
or V
BE101
, the base current I

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