Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – Combining of plural signals
Reexamination Certificate
2001-08-10
2003-09-09
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
Combining of plural signals
C327S358000
Reexamination Certificate
active
06617910
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to circuits in the field of analog multipliers and, more specifically, to those types of multiplier circuits whose operation generally does not involve switching on and off of elements such as transistors and/or diodes. In the current technology, each transistor in an analog multiplier's differential pair where the output is taken, contributes random noise from internal base series resistance, and also each transistor contributes random noise from its collector-to-emitter shot noise current generator.
In the case of a balanced multiplier, such as an LM 1496 or a mixer circuit such as in an LM 1863, both manufactured by National Semiconductor in Santa Clara, Calif., the noise in question comes from the two upper differential pair amplifiers where their bases and collectors are coupled to each other. These upper two differential amplifiers suffer from noise due to their internal base series resistance and collector current shot noise sources. In addition, these “output” amplifiers are sensitive to external noise from one of their inputs, that is, the bases of the upper differential pair amplifiers.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improved signal to noise ratio for a gain control amplifier, analog mixer or multiplier, to lower output noise caused by internal noise and/or to desensitize external noise to an input.
Another object of the invention is to increase the dynamic range of multiplier or mixer circuits known as “Gilbert Quads,” double balanced analog multiplier circuits, single balanced multiplier circuits, and/or voltage controlled amplifier circuits.
It is another object to maintain or reduce low distortion at the multiplier's output, or at the voltage controlled amplifier's output, as the control voltage is varied.
It is yet another object of the invention to not only maintain low distortion, but to further improve on linearity at the output of the voltage controlled amplifier or mixer as the control voltage is varied. This improvement involves paralleled diodes, paralleled diodes in series, serial strings of diodes in parallel, or the like as local feedback elements.
It is also another object of the invention to use similar principles and apply them to single ended amplifiers so that there is an increase in dynamic range with lower distortion when gain control is applied. The single ended controlled amplifier can be used in for example, intermediate frequency (IF) amplifiers in radio receivers.
It is yet another object of the invention to disclose various embodiments of predistortion circuits for improved linearity of multiplier or voltage controlled amplifiers. These predistortion circuits when coupled to a preferred embodiment of this invention comprise a true two port analog multiplier with improved characteristics (i.e., lower noise and/or distortion) over a prior art two port multiplier such as an MC 1495, manufactured by Motorola.
A first embodiment of the invention involves adding a nonlinear element or elements in the local feedback path of at least one differential amplifier to lower noise in a mixer circuit thereof and/or to increase linearity in the mixer circuit. This nonlinear element may be a transistor connected as a diode in series with each differential amplifier's emitter. In general, it is preferred that the series diodes are connected in a pair or in multiple pairs. Because each of the diodes does have a nominal small signal resistance for a given particular emitter current, the diode pair or pairs serve as a means for reducing the output noise of the differential amplifier(s). By using a series diode or multiple diodes connected to each differential pair emitter, noise caused by series base resistors and/or collector shot noise sources from emitter to collector of each differential pair transistor is reduced.
A diode pair or pairs preferably are used, and output distortion is substantially equal to that of a prior art multiplier circuit when the control voltage at the bases is varied. The reason is that the upper differential pair (output) transistors and the added diodes each have essentially the same (equivalent) bulk emitter series resistance.
It is known that for varying control voltages, there exists a small amount of output distortion in conventional differential pair voltage controlled amplifiers such as the LM 1496 of previous mention. This distortion is caused by equivalent series emitter resistors and/or the internal base series resistors in the upper differential pair transistors. However, as is disclosed by the invention, this distortion can be reduced by adding series-parallel diodes to the emitters. The reason for distortion reduction is because the added series-parallel combinations of diodes dilute the non linearity effect of the series emitter resistors of the upper differential pair transistors.
That is, noise and/or distortion in differential pair voltage controlled amplifiers may be reduced by adding “N” sets of “M” paralleled diodes in series with the amplifier's emitters, or by paralleling “M” sets of “N” serial diode strings in series with the emitters.
The invention contemplates that more noise reduction is achieved by using more than one diode in series with each emitter, as illustrated in
FIG. 2A
below. In general, use of these diodes allows for the reduction of internal transistor noise at the output of the collector(s), but also for reduction of external noise that may be part of the control voltage, Vcontrol. For example, external noise added to Vcontrol (i.e., via crosstalk of other signals on the circuit board) results in spurious noise and/or modulation noise at the output. Use of the diode feedback elements of the invention changes the transfer function of the control voltage input port and causes a shallower slope in the transfer function of the Vcontrol input versus output signal. Thus, when compared to prior art circuits, the invention provides a circuit of larger control voltage range along with noise reduction, as is illustrated in the discussion of
FIG. 2A
below.
By using multiple diodes for local feedback, and feeding the signal current through the diodes, a lower noise voltage controlled amplifier is achieved while maintaining low (or original) distortion, as the control voltage is varied. The use of diodes in series with the emitter of the differential amplifier also results in a larger control range and/or more linear voltage range than is provided by prior art circuits.
In a prior art circuit such as shown in
FIG. 1B
, if perfect transistors were used, there would be no distortion of the signal current (i.e., Isig) when the control voltage is varied. A perfect transistor would have zero ohm internal series base resistance and zero ohm internal series (bulk) emitter resistance. But as previously mentioned, most transistors have finite series internal base and/or internal emitter resistance, which then will cause distortion in a voltage control amplifier when the control voltage is varied.
One distortion component in a typical voltage controlled amplifier occurs when the input signal, part of the “tail” current, is set via the control voltage for other than 50% output (i.e., 0 volt across the bases of the upper transistors) or 100% output. This output distortion (i.e., harmonic distortion) arises from the differential amplifier's transistor internal equivalent emitter series resistance, EESR. Resistance EESR is also determined here by the emitter series bulk resistance plus the total series base resistance divided by the current gain, beta, of the transistor, where beta is considered large. The larger the EESR, the greater the distortion when a control voltage is set for other than 50% or 100% output, for example, when Vcontrol is set for 30% output of the signal from the tail current. Thus the invention described below provides a reduction in the output distortion of a voltage control amplifier which is caused by the resistance EESR. It should be noted that this type of
Almeida George
Callahan Timothy P.
Cox Cassandra
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