Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
2000-02-17
2001-08-07
Mis, David (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S1160FE, C331S1160FE, C331S158000, C331S175000, C331S183000, C331S186000
Reexamination Certificate
active
06271734
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric oscillator, and in particular to a piezoelectric oscillator which is excellent in noise characteristic and aging characteristic.
2. Description of the Related Art
As a crystal oscillator used in reference signal sources of communication devices, one having a circuit configuration as shown in
FIG. 3
is known.
In
FIG. 3
, a circuit
101
surrounded by a broken line is a typical Colpitts oscillation circuit. A crystal resonator
102
is connected to base of a transistor
103
. The base of the transistor
103
is connected to its emitter via a capacitor
104
. The emitter of the transistor
103
is connected to ground via a parallel circuit of a capacitor
105
and a resistor
106
. Collector of the transistor
103
is connected to a power supply Vcc. Furthermore, one terminal of the crystal resonator
102
is connected to ground via a series circuit of a capacitor
107
and a trimmer capacitor
108
.
Incidentally, oscillation output is taken out from ends of the trimmer capacitor
108
.
Further, an amplification circuit
109
surrounded by a broken line in
FIG. 3
forms a part of an AGC circuit in conjunction with a rectification circuit described later. Emitter of a transistor
110
is connected to ground. Collector of the transistor
110
is connected to the power supply Vcc via a resistor
111
. The collector of the transistor
110
is connected to its base via a resistor
112
. Furthermore, the base of the transistor
110
is connected to the emitter of the transistor
103
via a capacitor
113
.
Furthermore, a circuit
114
surrounded by a broken line in
FIG. 3
is the above described rectification circuit. Anode of a diode
115
is connected to cathode of a diode
116
. Cathode of the diode
115
is connected to ground. Anode of the diode
116
is connected to the power supply Vcc via a resistor
117
. The anode of the diode
116
is connected to ground via a capacitor
118
. The cathode of the diode
116
is connected to the collector of the transistor
110
via a capacitor
119
. The anode of the diode
116
is connected to the base of the transistor
103
via a resistor
120
in a feedback form.
Incidentally, a capacitor
122
shown in
FIG. 3
is a bypass capacitor.
Operation of the crystal oscillator having such a configuration will now be described.
Since the oscillation circuit
101
is a typical Colpitts oscillation circuit as described above, description thereof will be omitted.
An output signal produced from the emitter of the transistor
103
on the basis of parameters of the oscillation circuit
101
is supplied to the amplification circuit
109
, amplified therein to a required level, and then generated from the collector of the transistor
110
.
The output signal amplified in the amplification circuit
109
is supplied to the diode
115
and the diode
116
of the next stage via the capacitor
119
.
A signal component of half a cycle of plus side of the output signal supplied to the rectification circuit
114
flows through the diode
115
, and the capacitor
119
is charged with electric charge so that the anode side of the diode
115
will have a lower potential. A signal component of half a cycle of minus side of the output signal supplied to the rectification circuit
114
flows through the diode
116
. At the anode of the diode
116
, therefore, there is generated a potential equivalent to the sum of the potential generated by the signal component in the half cycle of minus side and the potential generated by the charged electric charge of the capacitor
119
. The resultant potential is applied to the base of the transistor
103
included in the oscillation circuit
101
.
In this way, the base potential is changed by the control of the AGC circuit. As a result, the collector output of the transistor
103
changes.
When the level of the output signal of the oscillation circuit
101
becomes high, the difference (p—p value) between a minimum value and a maximum value of a signal supplied from the amplification circuit
109
to the rectification circuit
114
increases, and the absolute value of the potential of negative polarity generated on output of the rectification circuit
114
increases. Therefore, the base bias voltage of the transistor
103
falls, and the level of the output signal of the oscillation circuit
101
falls.
On the other hand, in the case where the level of the output signal of the oscillation circuit
101
has fallen, operation opposite to the above-described operation is caused. Therefore, description of its operation will be omitted.
By conducting the above-described operation repetitively, therefore, the crystal oscillator outputs a stable level signal on the basis of the set conditions thereof.
In such a highly stable crystal oscillator as to use an AGC circuit, the aging characteristic especially poses a problem. In order to obtain a favorable aging characteristic, circuit constants are set so as to keep the level of the excitation current of the crystal resonator at a low level.
That is, the excitation current of the crystal resonator is kept at a low level by making the base bias current of the transistor
103
small and holding down the gain of the transistor
103
to a low value.
If the transistor
103
is driven with a low base bias current, however, then the collector current necessarily becomes low. As evident from a relationship between the collector current and the noise figure shown in
FIG. 4
, therefore, there is a problem that the noise figure becomes high.
In other words, it is necessary to set the collector current to a value of a point B for making the noise figure low. For increasing the collector current, however, the base bias current must be increased. As a result, the gain of the transistor
103
becomes larger than need be. The excitation current level of the crystal resonator thus becomes high. Accordingly, a favorable aging characteristic cannot be obtained.
In the conventional crystal oscillator, therefore, the aging characteristic is given priority and the collector current is set to a value of a point A having an extremely low collector current. Thus, the rise of the noise figure is ignored.
An object of the present invention is to provide a crystal oscillator capable of having a favorable aging characteristic and improved in noise characteristic.
SUMMARY OF THE INVENTION
In order to solve the problems, in accordance with a first aspect of the present invention, the above described object is achieved by a piezoelectric oscillator comprising: a Colpitts oscillator including a piezoelectric resonator and an oscillation transistor; amplification means for amplifying an output of the Colpitts oscillator; and rectification means for rectifying an output of the amplification means, an output of the rectification means being fed back to base of the oscillation transistor to keep an oscillation output level constant, in which a base bias is set so as to provide the oscillation transistor with an operation point located in the vicinity of a saturation region, and in which a feedback current from the rectification means is supplied to the base of the oscillation resistor so as to make the operation point approach to the saturation region when the saturation output level has become high and so as to make the operation point go away from the saturation region when the oscillation output level has become low.
In accordance with a second aspect of the present invention, the above described object is achieved by a piezoelectric oscillator comprising: a Colpitts oscillator including a piezoelectric resonator and an oscillation transistor; amplification means for amplifying an output of the Colpitts oscillator; and rectification means for rectifying an output of the amplification means, an output of the rectification means being fed back to base of the oscillation transistor to keep an oscillation output level constant, in which base of a second transistor included in the amplification means is connected to groun
Koda & Androlia
Mis David
Toyo Communication Equipment Co., Ltd.
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