Nonlinearity compensation in an electric signal processing circuit

An electric signal processing circuit with an operational amplifier having a signal input, a feedback input and a signal output, and a nonlinear circuit device having a characteristic with a distortion-producing nonlinearity and located in the input signal circuit or in the feedback circuit of the operational amplifier, wherein a compensating circuit device having a characteristic with generally the same nonlinearity as the characteristic of the nonlinear circuit device is disposed in the feedback circuit or in the input signal circuit of the operational amplifier for compensating the distortion of the nonlinear circuit device.

TECHNICAL FIELD 
This invention relates to an electric signal processing circuit having an 
operational amplifier and a nonlinear circuit device having a 
characteristic with a distortion-producing nonlinearity and located in the 
feedback circuit of the operational amplifier. 
Such a signal processing circuit can be in particular a volume control for 
the audio signal processing section of a radio, for example a car radio. 
BACKGROUND OF THE INVENTION 
An example of a conventional volume control of this type is shown in FIG. 
5. It includes operational amplifier OA with noninverting signal input + 
connected with input signal connection E of the volume control, and 
inverting feedback input - and signal output A. Between input signal 
connection E and noninverting signal input + of operational amplifier OA 
there is switching point P connected via resistor R with signal ground 
connection SGND. Between output connection A and inverting feedback input 
- there is a feedback circuit with two cascade-connected resistor strings 
or voltage dividers ATT2 and ATT16. These two voltage dividers ATT2 and 
ATT16 have not only a different number of component voltage taps but 
different component voltage gradations. While ATT2 has eight component 
voltage taps and a component voltage gradation of 2 dB between adjacent 
component voltage taps in the shown embodiment, ATT16 has four component 
voltage taps and a component voltage gradation of 16 dB between adjacent 
component voltage taps. ATT2 thus forms a voltage divider with fine 
gradation while ATT16 is a voltage divider with coarse gradation. 
ATT2 is connected on its high-voltage side with output A of operational 
amplifier OA and on its low-voltage side with signal ground connection 
SGND. Each of the component voltage taps of ATT2 is connected via 
controllable electronic switch S21 to S28 with the high-voltage side of 
ATT16, whose low-voltage side is likewise connected with signal ground 
connection SGND. FIG. 5 shows only switch S21 associated with the 
uppermost component voltage tap of ATT2. The other seven switches S22 to 
S28 are not drawn but only indicated by their respective switch reference 
signs. All these eight switches S21 to S28 are connected on their side 
remote from ATT2 jointly with the high-voltage side of ATT16. 
Associated in turn with the four component voltage taps of ATT16 are 
controllable electronic switches S31, S32, S33, S34, respectively, whereby 
again only uppermost switch S31 is shown and the other three switches are 
only indicated by their reference signs S32 to S34. The sides of switches 
S31 to S34 remote from ATT16 are connected jointly with feedback input - 
of OA. 
The switching states of switches S21 to S28 are controlled by a digital 
switch control circuit, first decoder D1, while the switching states of 
switches S31 to S34 are controlled by another digital switch control 
circuit, second decoder D2. In the shown embodiment D1 is a 3/8 decoder 
which converts 3-bit control data words supplied to its input side into 
switch control signals which it can conduct via eight output lines to 
gates of electronic switches S21 to S28. In accordance with the data 
content of the particular 3-bit control data word, this renders a selected 
one of the eight switches S21 to S28 conductive while rendering the others 
nonconductive. In accordance with the control data value of the particular 
control data word, one of the component voltage taps of ATT2 is therefore 
connected with the high-voltage side of ATT16. 
In corresponding fashion, the control through second decoder D2 formed as a 
2/4 decoder renders one of the four switches S31 to S34 associated with 
second voltage divider ATT16 conductive while rendering the other three 
switches nonconductive, so that feedback input - of operational amplifier 
OA is connected with one of the four component voltage taps of ATT16 in 
accordance with the 2-bit control data word supplied to D2. 
In this way the digital switch control signals supplied to decoders D1 and 
D2 serve to adjust a very definite feedback for operational amplifier OA, 
which leads to a certain gain of the audio signal supplied to input signal 
connection E. 
The symbols used in FIG. 5 for switches S21 and S31 stand for electronic 
switches having a configuration shown in FIG. 2. These switches consist of 
a parallel connection of an NMOS transistor having an N-channel and a PMOS 
transistor having a P-channel. The gate electrodes of NMOS and PMOS are 
connected with control signal input SE, the gate electrode of NMOS 
directly and the gate electrode of PMOS via inverter INV. Therefore both 
transistors NMOS and PMOS of the electronic switch are always rendered 
conductive or nonconductive depending on the type of control signal 
supplied to control signal input SE. Control signal input SE is connected 
with a corresponding output signal line of first decoder D1 or second 
decoder D2. In FIG. 5 the eight and four output control lines of D1 and 
D2, respectively, are only shown in a simplified symbolical way by one 
dashed line in each case. 
The use of electronic switches with the configuration shown in FIG. 2 is 
known in the art. This type of switch is used in order to obtain better 
linearity than can be obtained using only one switching transistor. But 
this switch constructed from two parallel-connected transistors also has 
higher nonlinearity than is desirable for high-quality audio devices. 
FIG. 3 shows the characteristic of on resistance R.sub.ON of a switch of 
the type shown in FIG. 2 as a function of input voltage V across this 
switch. One can see that this switch also has considerable nonlinearity 
despite the use of the two parallel-connected transistors NMOS and PMOS. 
This resistance characteristic has the lowest nonlinearity in its middle 
area. Therefore one places the switch-on operating point of this 
electronic switch so that it is in the middle between the two "humps" in 
the resistance characteristic. One does so by setting the operating point 
at VS/2, whereby VS is the supply voltage of the integrated circuit. As 
the sinusoidal signal indicated in FIG. 3 shows, it is subject in the 
switch shown in FIG. 2 to a nonlinear transmission, which leads to 
distortion with a factor depending on the nonlinearity of the 
characteristic. 
The invention is intended to obtain a compensation of the distortion caused 
by this nonlinearity. 
SUMMARY OF THE INVENTION 
According to the invention one compensates the distortion of the nonlinear 
circuit device by disposing in the feedback circuit or in the input signal 
circuit of the operational amplifier a compensating circuit device having 
a characteristic with generally the same nonlinearity as the 
characteristic of the nonlinear circuit device. 
If the distortion-producing circuit device is located in the feedback 
circuit, the compensating circuit device is disposed in the input signal 
circuit of the operational amplifier. If the distortion-producing circuit 
device is located in the input signal circuit of the operational 
amplifier, however, the compensating circuit device is disposed in the 
feedback circuit of the operational amplifier. 
Operational amplifiers are usually high-gain difference amplifiers which 
only react to potential differences between their two inputs. Potentials 
supplied simultaneously to both inputs have no effect on the output signal 
of the operational amplifier. This applies not only to direct-voltage 
components but also to alternating-voltage components when they are 
supplied simultaneously to the two inputs of the operational amplifier. 
And this behavior of operational amplifiers is utilized by the invention. 
Without the inventive distortion compensation, the distortion-producing 
circuit device located in the feedback circuit or in the input signal 
circuit of the operational amplifier would cause a harmonic component due 
to nonlinearity to arise only at the feedback input or only at the signal 
input of the operational amplifier, and this harmonic component would 
produce at the difference input side of the operational amplifier a 
difference signal which would appear, amplified by the operational 
amplifier, in the output signal of the operational amplifier. 
Since a compensating circuit device having the same or at least generally 
the same nonlinearity as the circuit device whose distortion is to be 
compensated is disposed according to the invention in the input signal 
circuit or the feedback circuit depending on whether the 
distortion-producing circuit device is located in the feedback circuit or 
in the input signal circuit, a harmonic component of the signal to be 
processed is also supplied to the other input of the operational 
amplifier. If the circuit device to be compensated and the compensating 
circuit device have matching nonlinearities, the same harmonic pattern is 
supplied to both difference inputs of the operational amplifier, so that 
these harmonic components produce no difference voltage between the two 
difference inputs and therefore do not appear in the output signal. 
The inventive signal processing circuit is suitable in particular for 
volume controls in audio signal processing circuits, but can also be used 
for any other signal processing applications in which undesirably 
occurring distortion or harmonic components are to be eliminated by 
compensation. Examples of such other applications are measuring signal 
amplifiers or the like. 
The adjusting elements, for example switch-operated voltage dividers of the 
type shown in FIG. 5, are disposed in the feedback circuit or in the input 
signal circuit of the operational amplifier depending on whether the 
signal is to be influenced in an amplifying or attenuating way. That is, 
in the case of a signal-amplifying circuit layout the distortion-producing 
circuit device is located in the feedback branch, and in the case of a 
signal-attenuating circuit layout in the input signal circuit. Therefore 
the distortion-compensating circuit device is disposed in the former case 
in the input signal circuit of the operational amplifier and in the latter 
case in the feedback circuit. 
If an inventive signal processing circuit is used as a volume control, the 
adjusting device used is preferably a cascade connection of two resistor 
strings serving as voltage dividers with digitally controlled switch 
devices, as explained above in connection with FIG. 5. In the case of 
signal-amplifying volume control this cascade connection is located in the 
feedback circuit of the operational amplifier, in the case of 
signal-attenuating volume control it is disposed in the input circuit. 
In the case of an amplifying signal processing circuit with the 
distortion-producing circuit device located in the feedback circuit of the 
operational amplifier, the distortion-compensating circuit device need not 
necessarily be disposed in the input signal circuit of the same 
operational amplifier. It can also be disposed somewhere in the signal 
processing chain of the total circuit at a place before or after the 
operational amplifier in question, where signal processing is done by 
another operational amplifier formed as a difference amplifier OA2. If the 
distortion-compensating circuit device is disposed for example at a place 
in the signal processing chain located (in terms of signal flow) behind 
the operational amplifier in whose feedback circuit a distortion-producing 
circuit device is located, the distortion-causing harmonic component is 
initially contained in the output signal of this operational amplifier but 
is eliminated at the place where the distortion-compensating circuit 
device is located, with the aid of the operational amplifier located 
there. 
The height and the degree of nonlinearity of the characteristic shown in 
FIG. 3 depend on the junction of the MOS transistors of the switch device 
shown in FIG. 2. The greater the junction of these MOS transistors is, the 
lower and more compressed the characteristic shown in FIG. 3 is and the 
lower its nonlinearity is. If one makes the MOS junctions twice as great 
for example, the nonlinearity characteristic is only half as high and has 
only half as much nonlinearity, so that the distortion factor is also 
reduced to one half. To obtain a distortion factor below a certain 
threshold value in conventional circuits, for example with the 
configuration shown in FIG. 5, one must provide a certain MOS junction. If 
one wanted to halve the distortion factor in the conventional circuit for 
example, one would have to double the MOS junctions. Since a considerable 
number of switches of the type shown in FIG. 2 are necessary, this results 
altogether in a considerable increase in the space required by these MOS 
switches on the chip of the integrated circuit. Assuming as well that a 
signal processing circuit of the type shown in FIG. 5 occurs several times 
in an audio signal processing circuit, for example not only in the volume 
control but also for the loudness adjustment (aurally compensated volume 
adjustment), the bass adjustment and the treble adjustment, one ends up 
with an enormous number of switch devices of the type shown in FIG. 2 on 
such an integrated audio signal processing circuit. If the chip area of 
each of the many MOS transistors required must be doubled, the space 
required on the chip is greatly increased to achieve such a distortion 
factor reduction in the conventional way. 
This is all no problem using the inventive signal processing circuit, since 
one can afford to use MOS switch devices with a small MOS junction because 
the distortion produced thereby is compensated by the inventive measure. 
With the inventive measure one can thus not only obtain a considerable 
reduction of the distortion factor in a technically simple way, but also 
achieve a considerable saving of chip area at the same time.

DETAILED DESCRIPTION OF THE INVENTION 
The invention will now be explained more closely with reference to 
embodiments. 
FIG. 1 shows a signal-amplifying volume control with adjusting elements in 
the feedback circuit of an operational amplifier. Comparison of FIGS. 1 
and 5 show that some portions of the amplifying volume controls generally 
match each other. The inventive volume control of FIG. 1 differs from the 
volume control shown in FIG. 5 in that disposed between input signal 
connection B and switching point P is distortion-compensating switch 
device S1. This switch S1 having exactly the same circuit configuration as 
the switch devices used in the feedback circuit, i.e., having the circuit 
configuration shown in FIG. 2 with a parallel connection of an NMOS 
transistor and a PMOS transistor, the two MOS transistors having the same 
MOS junctions as are used for the switch devices located in the feedback 
branch. 
Since the circuits shown in FIGS. 1 and 5 otherwise match, reference is 
made to the circuit configuration and the functional description for FIG. 
5 for the parts of the circuit shown in FIG. 1 going beyond switch device 
S1, thus matching reference numbers are also used. 
One can also explain the distortion compensation as follows. A feedback 
operational amplifier works in such a way that it tries to equalize the 
voltages at its noninverting input and its inverting input, i.e., at its 
signal input and its feedback input in the case of FIGS. 1 and 5. The 
current flowing through S1 and through load resistor R in FIG. 1 is as 
great as the current flowing for example through S21 and ATT16. If one 
equalizes the resistance values of load resistor R and ATT16 and uses 
switch devices of the type shown in FIG. 2 with the same design, in 
particular MOS junction, for the switch devices in the feedback branch, 
for example S21, and distortion-compensating switch device S1, the 
currents flowing into the signal input and into the feedback input of 
operational amplifier OA are not only equal, but distortion-producing 
switch device S21 and distortion-compensating switch device S1 are also 
operated at the same operating point of the nonlinear characteristic shown 
in FIG. 3, so that both switch devices cause the same harmonic component 
and thus the same distortion. Due to the operational amplifier's attempt 
to equalize the voltages at its two inputs + and -, the distortion 
produced in the feedback circuit is therefore compensated by the 
distortion produced by compensating switch device S1. 
FIG. 4 shows an embodiment of an inventive signal processing circuit in the 
form of an attenuating volume control. It has the cascade connection of 
finely graded voltage divider ATT2 and coarsely graded voltage divider 
ATT16 in the input circuit of operational amplifier OA, so that the 
distortion-producing switch device is also located in the input circuit. 
Since the distortion-compensating circuit device must always be disposed 
in the other input circuit of the operational amplifier so that distortion 
compensation can occur, the distortion-compensating switch device is 
disposed in the feedback branch of the operational amplifier in this 
embodiment. 
Since the same circuit components are used in the embodiment shown in FIG. 
4 as in the embodiment shown in FIG. 1, the same reference signs are also 
used. Since voltage divider cascade connection ATT2 and ATT16 has the same 
configuration and same mode of operation as in the volume controls shown 
in connection with FIGS. 1 and 5, reference is made to FIGS. 1 and 5 for 
the configuration and operation of the voltage divider cascade in FIG. 4. 
Both for the embodiment shown in FIG. 1 and for the embodiment shown in 
FIG. 4, load resistor R belonging to compensating switch device S1 should 
have the same resistance value as the resistor string of voltage divider 
ATT16 for optimal distortion compensation. And this applies to the 
resistance value which ATT16 has between signal ground connection SGND and 
that component voltage tap whose switch device is rendered conductive. In 
the most relevant attenuation range of the voltage divider cascade, which 
is between 0 to 14 dB, voltage divider ATT16 is adjusted to 0 dB so that 
the total resistance of ATT16 is between the particular effective switch 
device of switch devices S21 to S28 associated with ATT2 and signal ground 
connection SGND. Therefore one preferably gives load resistor R the same 
resistance value as the total resistor string of voltage divider ATT16 in 
practical embodiments. 
If one of the other component voltage taps of ATT16 is activated by the 
corresponding one of switch devices S31 to S34 being rendered conductive, 
no complete distortion compensation occurs but rather a partial 
compensation, which in any case leads to a better result than in the 
conventional embodiment of FIG. 5, in which there is no distortion 
compensation whatsoever. 
Switch devices S31 to S34 belonging to voltage divider ATT16 likewise have 
a nonlinearity of the type shown in FIG. 3, but produce no distortion 
because they are connected at one end with very high-ohmic feedback input 
- of operational amplifier OA so that they conduct virtually no current. 
In the case of an amplifying signal processing circuit with the 
distortion-producing circuit device located in the feedback circuit of the 
operational amplifier, the distortion-compensating circuit device S1 need 
not necessarily be disposed in the input signal circuit of the same 
operational amplifier. It can be disposed somewhere in the signal 
processing chain of the total circuit at a place before (FIG. 6) or after 
(FIG. 7) the operational amplifier (OA) in question, where signal 
processing is done by another operational amplifier (OA2) formed as a 
difference amplifier. If the distortion-compensating circuit device S1 is 
disposed for example at a place in the signal processing chain located (in 
terms of signal flow) behind the operational amplifier (OA) in whose 
feedback circuit a distortion-producing circuit device is located (FIG. 
7), the distortion-causing harmonic component is initially contained in 
the output signal of this operational amplifier but is eliminated at the 
place where the distortion-compensating circuit device S1 is located, with 
the aid of the operational amplifier (OA2) located there. The signal 
processing circuit wherein the compensating circuit device is disposed, 
not in the input signal circuit of that operational amplifier (OA) in 
whose feedback circuit the distortion-producing circuit device is located, 
but in the input circuit of another operational amplifier (OA2) located 
before or after the first mentioned operational amplifier (OA), is shown 
in FIGS. 6 and 7.