Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
1998-05-18
2001-03-06
Kinkead, Arnold (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S144000, C331S17700V
Reexamination Certificate
active
06198358
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to oscillator circuits, i.e. oscillators, and specifically to controllable oscillators based on multivibrators.
BACKGROUND OF THE INVENTION
Current- and voltage-controlled oscillators (ICO and VCO) are important components in the structures of transmitters and receivers. When applications to portable wireless communications systems are concerned, the main requirements for VCO/ICOs are: an operational frequency range of 1 to 20 GHz, a very low phase noise and the lowest possible operating voltage and power consumption. Depending on the structure, a communications device may comprise several VCO/ICOs needed for different purposes, e.g. frequency conversion, synthetization, modulation, etc. Their performance affects strongly the performance of the entire communications unit. On the other hand, the demand to implement these oscillators for silicon technologies faces several problems.
During the last few years several research projects have focused on finding optimal solutions. Two types of oscillators are mainly used as the cores of VCO/ICOs: sinusoidal oscillators and relaxation oscillators. Sinusoidal oscillators usually produce the best parameters as far as high frequency and low phase noise are concerned, but they can be easily implemented mostly on GaAS technologies only. A transition to bipolar, CMOS or BiCMOS technologies causes several problems mainly due to the highly conductive substrate. On the other hand, the speed of such available technologies is a challenge to researchers, as at present transient frequencies of 10 to 40 GHz are reached, which was previously considered to be a transient range possible to be covered only by materials based on GaAS. The speed of silicon-based technologies is sufficient enough for mobile communication in the frequency range of 1 to 20 GHz, used by most mobile stations and wireless LANs. An additional driving factor in the design of portable equipments has always been a high demand for as low an operating voltage as possible and a very low power consumption.
In oscillators of LC type, the active circuit components are kept out of the non-linear operation range, whereas in relaxation oscillators, the sinusoidal signal is the result of the incapability of the pulse circuit to switch fast enough at very high frequencies.
Oscillator circuits, i.e. oscillators, can be implemented by many different circuit structures. One of them is an astable (free-running) multivibrator.
FIG. 1
shows a conventional emitter-coupled multivibrator circuit, which has been used for implementing voltage-controlled oscillators (VCO). The circuit comprises two transistors Q
1
and Q
2
, between which is provided a positive feedback by connecting each transistor collector via a buffer transistor Q
3
, Q
4
to control the base of the other transistor. The collectors of Q
1
and Q
2
are connected via resistors Rc
1
and Rc
2
, respectively, to one potential of an operating voltage source
1
and the emitters are connected via current sources
3
and
4
, respectively, to the lower potential of the operating voltage source. Correspondingly, the emitters of the buffer transistors Q
3
and Q
4
are connected via current sources
5
and
6
to the lower potential. Additionally, a reference capacitor C is connected between the emitters of Ql and Q
2
. The positive feedback and series resonance circuits Rc
1
-C and Rc
2
-C constituted by the resistors RC
1
and RC
2
and the capacitance C lead to that the output of the multivibrator oscillates continuously between-two states, after the oscillation once has been trigged. The oscillation frequency is determined by the component values of the RC series resonance circuits. The oscillation frequency can be controlled by changing some of these component values, typically the capacitance C.
In the following, the operation of the multivibrator will be examined closer. To begin with, it is assumed that Q
1
and Q
3
are off (non-conduction state). When Q
1
is off, the collector of Q
1
and the base of Q
2
are generally at the operating voltage potential. Then Q
2
is on (conducting state), and its emitter current is I
1
+I
2
. The buffer transistor Q
4
is likewise on and feeds base current to Q
2
. When Q
2
is conductive, the current I
1
flows from the emitter of Q
2
via the capacitance C to the emitter of Q
1
. Then the current I
1
charges/discharges the charge of the capacitance C, whereby the emitter potential of Q
1
falls at a predetermined speed until Q
1
becomes conductive when the base emitter voltage of Q
1
exceeds about 0.6 V. When Q
1
becomes conductive, its collector voltage begins to fall, which leads to that the buffer transistor Q
3
begins to close. On account of a positive feedback via Q
4
, the base voltage of Q
2
falls as well and Q
2
closes. Q
2
closing makes the collector voltage of Q
2
rise, which accelerates the opening of Q
3
. Q
3
opening increases the base current of Q
1
via a positive feedback. A higher base current discharges parasitic capacitances of the base circuit of Q
1
faster and accelerates thus the opening of Q
1
. When Q
2
is off and Q
1
is on, the current I
2
flows from the emitter of Q
1
via the capacitance C to the emitter of Q
2
, where the emitter voltage begins to fall until it makes Q
2
open again and Q
1
close via Q
3
.
The speed of such a multivibrator circuit (maximum resonance frequency) depends primarily on the properties of the transistors Q
1
and Q
2
. The buffer transistors Q
3
and Q
4
increase the speed of the multivibrator circuit, because they make a higher base current possible, which again discharges the parasitic capacitances of the base circuit of the transistors Q
1
and Q
2
faster and accelerates thus the switching of the transistor from one state to another.
The lowest possible operating voltage Vcc will be achieved when it is assumed that the current sources
3
and
4
are ideal, i.e. no voltage loss is provided in them. When the ideal current sources are replaced by some practical circuit structure, such as current mirrors, Vcc increases. Returning to the operating principle of the circuit, it can be stated that current paths are either Q
1
-C-current mirror
4
or Q
2
-C-current mirror
3
and that the current mirrors produce a stable current through the reference capacitor C, which is the main reason for the typical low phase noise. In search of a new way of increasing the speed, the reference capacitor cannot be decreased much more, because it will be of the order of parasitic capacitances, which leads to the fact that a controlled planning of the circuit is not possible.
Nowadays there is, however, a need of ever-increasing speeds while an operating voltage as low as possible is desired, especially in electronic equipments using battery power supplies.
For an implementation of a voltage- or current-controlled oscillator by means of a multivibrator circuit, the circuit requires a suitable supplementary control. Such a control should be as simple as possible.
In the circuit of
FIG. 1
, the pulse amplitude is determined by the sum of the currents I
1
+I
2
multiplied by the value of the collector resistor Rc
1
or Rc
2
of the corresponding cycle. The pulse width is determined by the value of the current which is supplied by I
1
or I
2
via the reference capacitor C during its recharge cycles. Accordingly, either the capacitance of the reference capacitor C or the current flowing through it has to be changed for the frequency control.
The capacitance may be changed if a varactor is used as reference capacitor C. A problem is, however, that varactor technologies are not generally compatible with BiCMOS technologies, for instance. In the BICMOS technology, a PN junction can be used instead. But then the capacitor works in the circuit of FIG.
1
and changes continuously the polarity of the voltage. In this case, a serial connection of two varactors, opposite to each other, may be some sort of solution, but the operation of the forward voltage region of one diode shows certain
Jarske Petri
Tchamov Nikolay
Kinkead Arnold
Scully Scott Murphy & Presser
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