Amplifiers – Sum and difference amplifiers
Reexamination Certificate
2001-10-30
2003-09-16
Mottola, Steven J. (Department: 2817)
Amplifiers
Sum and difference amplifiers
C330S12400D
Reexamination Certificate
active
06621337
ABSTRACT:
An amplifier is a circuit that has many uses in electronics. The most important application of an amplifier is to change, and particularly to increase, the voltage level or the current intensity level of an electrical signal. Occasionally an amplifier is also used to isolate an amplifier input signal from the output, e.g. to prevent sources of noise from feeding back into the signal source.
Under most circumstances, it is desirable that the amplifier degrades the signal as little as possible. Undesirable degradation particularly includes noise and distortion.
At the same time, it is important for many applications that the amplifier's power consumption be as small as possible. A compromise must be found between these requirements when designing an amplifier, because they cannot be optimized mutually. Non-linear distortions particularly increase significantly as the amplifier design tends to favor reduced power consumption. This relationship is true in general, but is particularly marked in amplifiers that are manufactured in CMOS technology. As a result, many products can only be produced in CMOS technology if the performance is reduced or the power consumption is increased.
In applications for which the prime concern is to achieve a signal as free from distortion as possible, countercoupled amplifier circuits are used.
FIG. 7
shows an example of a countercoupled operational amplifier OPV, which amplifies a circuit input signal Vin in conventional manner to create a circuit output signal Vout.
As is illustrated in
FIG. 8
, a good approximation of the operational amplifier OPV can be achieved with a serial connection from an ideal amplifier having frequency-independent amplification V, a low-pass filter TP(f) and a non-linear voltage transmission function NL(Vm), where f is the signal frequency and Vm is the voltage supplied to the output stage of the operational amplifier. This model is predicated on the assumption that the output stage of amplifier OPV is the dominant source of the non-linear distortion. A qualitative representation of the undesirable non-linearity NL(Vm) is shown in FIG.
9
.
The non-linearity NL(Vm) is determined by the non-linear voltage-to-current characteristic of the amplifier transistors and the current that is flowing through these transistors (“transistor non-linearity”). The smaller this current is, the more marked is the non-linearity. This is the case, for instance, when the amplifier is rated to operate with a relatively small current.
The effect of the non-linearity of amplifier OPV on the signal can be reduced by countercoupling amplifier OPV, as is shown in FIG.
7
. This reduction is proportional to the loop amplification A
loop
(f):
A
loop
(
f
)=
V*TP
(
f
)*
R
1
/(
R
1
+R
2
)
In order to obtain a high degree of linearity of the signal, it is necessary to aim for a high loop amplification A
loop
(f) in the frequency range under consideration. To this end, the following two methods are used:
1. Use of a high transit frequency of the amplifier loop
The transit frequency f
transit
is the frequency at which the loop amplification A
loop
(f) is reduced to value 1 due to the effect of the low pass filter TP(f), i.e. A
loop
(f
transit
)=1. The higher the transit frequency f
transit
is in comparison to the signal frequencies f, the lower the reduction of the loop amplification A
loop
that is caused by low-pass filter TP. Amplifiers that use a current negative feedback (“current feedback amplifier”) achieve a high transit frequency.
If the desired high linearity is obtained by means of a high transit frequency f
transit
, the power consumption of the amplifier is also high. It is not possible to achieve low power consumption and high transit frequency using the transistors that are currently available. The present method is therefore appropriate only for applications in which power consumption is of lesser importance.
2. Use of multiple amplification stages connected in series
In this method, the amplification at frequencies f below the transit frequency f
transit
is raised instead of the transit frequency f
transit
. This is achieved with the use of a low pass filter of a higher order. This method allows for low power consumption with high linearity of the amplifier circuit, provided signal frequencies f are sufficiently low. Amplifiers of this kind are designated, for example, by the names “Nested Miller” and “Double Nested Miller”.
The advantage of having amplification stages connected in series becomes less evident as the signal frequencies f approach the transit frequency f
transit
, as is shown in the following example:
With the architecture shown in
FIG. 7
, a sinusoidal signal having frequency f=10 MHz is to be amplified by 20 decibels (dB). Let the operational amplifier have a transit frequency of f
transit,amp
=990 MHz and let the feedback have an impedance ratio R
1
/R
2
={fraction (1/10)}. The transit frequency of the loop amplification f
transit,loop
is then:
f
transit
,
loop
=
f
transit
,
amp
*
R1
/
(
R1
+
R2
)
=
f
transit
,
amp
*
R1
/
(
R1
+
10
*
R1
)
=
990
MHz
*
1
/
11
=
90
MHz
In a two-stage operational amplifier OPV with a first order low-pass filter, the amplification in this frequency range is then close to:
A
loop
(
f
)=
f
transit,loop
/f
It follows that, for the third harmonic (f3) of signal frequency f, which has a frequency of 30 MHz, the loop amplification A
loop
(f3) is:
A
loop
(
f
3)=
f
transit,loop
/f
3=
f
transit,loop
/(3*10 MHz)=90 MHz/30 MHz=3=9.54 dB
The non-linearity of the output stage at this frequency f3 is therefore reduced by 9.54 dB. If the application requires, for instance, a signal-to-distortion ratio (S/D) of 70 dB up to 30 MHz, the base linearity of the output stage must be at least 60.46 dB. This can only be achieved with very high currents (class A output stages).
If a three-stage operational amplifier (“Nested Miller” amplifier) is used, loop amplification A
loop
is increased by about 3 dB at 30 MHz. The requirement for linearity of the output stage is therefore reduced to 57.46 dB. If more amplification stages are added, the effect is negligible (<1 dB). This is because, for reasons of frequency compensation, each additional amplification stage must be slower by one third than the stage to which it is connected. If signals in the MHz frequency range with high linearity requirements have to be amplified, up to now this can only be achieved with a correspondingly high transit frequency and high output stage linearity. However, high output stage linearity and high transit frequency both lead to high power consumption.
The present invention treats in the first instance of an amplification circuit having a circuit input for a circuit input signal and an amplification zone for amplifying the circuit input signals. The present invention treats in the second instance of a process for amplifying a signal.
The object of the present invention is to provide an amplification circuit and a process for signal amplification entailing reduced power consumption, with which signal distortions can largely be precluded.
The amplification circuit according to the invention is characterized in that the amplification zone includes two amplifiers, each of which is countercoupled, and to which the circuit input signal is fed in parallel, and the amplifier outputs of which are or can be connected with a circuit output to produce a circuit output signal, wherein the amplification input zone of one of the two amplifiers is connected with the amplification input zone of the other of the two amplifiers by means of a further amplifier.
The process for signal amplification according to the invention is characterized in that the signal is amplified in parallel through two countercoupled amplifiers and the two amplifier output signals are or can be combined to produce the amplified signal, wherein a signal that has been split from the amplification input
Mottola Steven J.
Pearne & Gordon LLP
Xignal Technologies AG
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