Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Nonlinear amplifying circuit
Reexamination Certificate
1999-03-09
2001-07-10
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Nonlinear amplifying circuit
C327S308000, C330S252000
Reexamination Certificate
active
06259312
ABSTRACT:
TECHNOLOGICAL FIELD
The invention relates to the adjustment of the input impedance of such active components which have a differential input. Typical such active components are some amplifiers and mixers which are used in the radio frequency and intermediate frequency sections of a radio device.
BACKGROUND OF THE INVENTION
Particularly in integrated circuit structures the active components of radio devices, such as amplifiers and mixers, are often realized as differential components, which means that the active component has two inputs and two outputs, whereby the input signal is a variable voltage between the two inputs and the output signal is a variable voltage between the two outputs. An alternative to the differential component is a single input and single output component where the input signal is a variable voltage between the input and a fixed ground potential, and the output signal is a variable voltage between the output and a fixed ground potential. An advantage of the differential structure is particularly that there is less variations in the component's performance caused by the manufacturing process. In the signal propagation direction there can be for instance a balun circuit, an amplifier, a filter or a mixer in front of the differential active component.
FIG. 1
shows a typical prior art differential amplifier
100
, which has inputs RF+ and RF− provided with decoupling capacitors, and outputs OUT
1
and OUT
2
. Two transistors Q
1
and Q
2
act as the amplifying components; the positive operating voltage Vcc is supplied to their collectors via the collector resistors RC and their emitters are connected via the emitter resistors RE and a constant current source Idiff to the ground potential. In addition to the input signal also a bias voltage Vb is supplied to both transistor bases via the biasing resistors Rb in order to bias the transistors Q
1
and Q
2
to the correct operating point. The output signal is taken at the collectors of the transistors Q
1
and Q
2
.
In the operation of an amplifier according to the
FIG. 1
a problem is created by its input impedance. In the signal propagation direction there is most commonly a filter (not shown in the figure) in front of the amplifier, whereby the filter can directly have a differential output, or its non-differential output can be duplicated with a so called balun before it is connected to the inputs of the amplifier. The frequency response of the filter depends on the input impedance of that component to which the signal is supplied from the filter. The input impedance of the amplifier shown in figure is as such very high, i.e. of the order of megaohms. In order to have a correct function of the circuit arrangement formed by the filter and the amplifier in series with it the input impedance of the amplifier must be adjusted to a value, which is a few tens or at most hundreds of ohms. 50 ohm has become a kind of a standard value for the impedances between RF components, but depending on the details of the filter structure a suitable value of the input impedance can also be for instance 100 or 200 ohms.
A simple way to adjust the input impedance of the amplifier according to
FIG. 1
as 200 ohms is to select 100 ohms as the value of both biasing resistors Rb. Another common way to arrange the input impedance of the amplifier according to the
FIG. 1
is that a resistor with a resistance equalling the desired input impedance is connected between the inputs RF+ and RF−. A disadvantage of these solutions is that they impair the noise characteristics of the amplifier.
FIG. 2
shows a more advanced solution, the so called collector feedback. In this solution the signal taken from the collectors of both amplifier transistors Q
1
and Q
2
is supplied to the bases of the additional transistors Q
3
and Q
4
, and the signal to the amplifier outputs OUT
1
and OUT
2
is taken at the emitters of the transistors Q
3
and Q
4
. The transistors Q
3
and Q
4
are supplied with the operating voltage from the common operating voltage source Vcc, and the emitters of both transistors are connected via an own constant current source Idiff
3
and Idiff
4
to the ground potential. From the emitter of the transistor Q
3
there is a connection via the feedback resistor Rfb and a decoupling capacitor to the base of the transistor Q
2
, and from the emitter of the transistor Q
4
there is a similar connection to the base of the transistor Q
1
. The input impedance can be affected by selecting the values of the feedback resistors Rfb in a suitable way. The circuit arrangement according to the
FIG. 2
has generally a high gain and relatively good noise characteristics, but a poor reverse isolation. The last mentioned disadvantageous characteristic means that if undesired oscillations are coupled to the output of the amplifier, for instance from a mixer (not shown in the figure) in series with the amplifier, these oscillations will propagate relatively easily through the amplifier in a direction opposite to that of the signal, and thus these oscillations can be coupled from the input of the amplifier to other parts of the radio device causing interference there (for instance in the antenna).
FIG. 3
shows another prior art way to adjust the input impedance of an amplifier. The circuit arrangement is in other respects similar to that of
FIG. 1
, but the input signal is not supplied from the inputs RF+ and RF− to the bases of the transistors Q
1
and Q
2
but to the emitters, and the bases of the transistors Q
1
and Q
2
are interconnected, whereby the biasing can be made with one biasing resistor Rb. Regarding the radio frequencies the bases of the transistors Q
1
and Q
2
are connected via the decoupling capacitor to the ground potential. The input impedance is mainly determined by the value of the series resistances Rin. The circuit according to the
FIG. 3
is suited only for very low values of the input impedances, because its gain G will always be lower than the ratio of the collector resistances RC to the serial resistances Rin, or G<RC/Rin. At greater values of the input impedance the circuit arrangement according to the
FIG. 3
does not sufficiently amplify the signal.
SUMMARY OF THE INVENTION
The object of the invention is to present a circuit arrangement with which a desired input impedance of a differential active component is obtained without simultaneously impairing other characteristics, such as the reverse isolation, the gain, the noise characteristics and the power consumption.
The objects of the invention are attained by forming a cross-feedback between the transistors of the differential transistor pair.
The differential active component according to the invention, which comprises a first transistor and a second transistor and whose differential input comprises a first input and a second input, of which the first input is connected to the base of the first transistor and the second input is connected to the base of the second transistor, is characterized in that there is a cross-feedback between the first transistor and the second transistor.
The invention relates also to a radio device which comprises at least one differential active component and a second component in front of it as seen in the signal propagation direction, whereby the differential output of the second component has a certain output impedance. The radio device according to the invention is characterized in that in order to match the output impedance of the differential active component so that it corresponds to the output impedance of the component in front of it, as seen in the signal propagation direction, there is a cross-feedback between the first transistor and the second transistor contained in the differential active component.
The invention utilizes the fact that at the first transistor of the differential transistor pair the signal is in the opposite phase compared to that in the second transistor. The base of the first transistor in the differential transistor pair is connected via a feedback res
Nguyen Hai L.
Nokia Mobile Phones Limited
Perman & Green LLP
Wells Kenneth B.
LandOfFree
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