Circuit arrangement with electronically controllable transfer characteristic

In a circuit arrangement with a electronically controllable transfer characteristic, the input signal is applied to an amplifier with a controllable negative feedback. The amplifier has its output connected to a voltage-divider circuit having a frequency-dependent characteristic. This circuit has a plurality of taps (9 to 13, 31 to 35) which are connected to an inverting input of the amplifier via a first electronically controllable switch (8) and to the output of the arrangement via a second electronically controllable switch (30). The two switches (8, 30) are controlled in a manner so as to influence the Q-factors of different filter curves. Therefore, in order to boost the input signal in a frequency band determined by the frequency-dependent voltage divider, the first switch (8) is set a position such that boosting is effected with the desired frequency bandwidth. Furthermore, the position of the second switch (30) is selected so as to obtain boosting to the desired degree. For attenuating the input signal in the frequency band determined by the frequency-dependent voltage divider, the second switch (30) is set a position such that attenuation is effected with the desired frequency bandwidth, and the first switch (8) is set to such a position as to obtain the desired degree of attenuation.

BACKGROUND OF THE INVENTION 
This invention relates to a circuit arrangement with an electronically 
controllable transfer characteristic, in particular a control circuit, in 
which an amplifier with controllable negative feedback is capable of 
receiving an input signal, which amplifier has an output connected to a 
voltage-divider circuit with a frequency-dependent characteristic, which 
circuit has a plurality of taps connected to an inverting input of the 
amplifier via a first electronically controllable switch and to an output 
of the arrangement via a second electronically controllable switch. 
Such a circuit arrangement is known from German Patent Specification 31 32 
402, which corresponds to U.S. Pat. No. 4,439,739 (Mar. 27, 1984). In this 
prior-art arrangement the frequency response to the input signal is 
influenced by suitably controlling the two electronically controllable 
switches. The two switches are controlled in a manner such that for a 
boost of the input signal in a frequency band determined by the 
frequency-dependent voltage divider, the second switch is set to that 
position in which the signal of the highest level is coupled out. The 
degree of boosting is changed by setting the first switch to its various 
positions. Conversely, the frequency range is attenuated in that the first 
switch is set to the position for maximum negative feedback and the second 
switch is actuated between its various positions. Particularly when used 
in a frequency-dependent configuration for, for example, equalizers or 
tone controls, this circuit arrangement has the drawback that depending on 
the selected boost or attenuation always a specific frequency band of the 
same width is influenced. Only the degree of boosting or attenuation is 
changed. Put in different terms, this means that for comparatively small 
boosts or attenuations the filtering provided by the circuit arrangement 
exhibits a smaller Q-factor than for stronger boosts or attenuations in 
the relevant frequency band. Here, the Q-factor is to be understood to 
mean the ratio of the frequency at the maximum boost or attenuation to the 
frequency difference between the frequencies for which the boost or 
attenuation is 3 dB below the maximum value. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a circuit arrangement which is 
capable of producing comparatively small boosts or attenuations with the 
same filter quality as larger boosts or attenuations. 
According to the invention this object is achieved in that the two switches 
are controlled in a manner such that 
for boosting the input signal in a frequency band determined by the 
frequency-dependent voltage divider 
the first switch is set a position such that boosting is effected with the 
desired frequency bandwidth, and that 
the position of the second switch is selected so as to obtain boosting to 
the desired degree, and that 
for attenuating the input signal in the frequency band determined by the 
frequency-dependent voltage divider 
the second switch is set to a position such that attenuation is effected 
with the desired frequency bandwidth, and that 
the first switch is set a position so as to obtain attenuation to the 
desired degree. 
The invention is based on the recognition of the fact that for boosting the 
input signal in the frequency range determined by the frequency-dependent 
voltage divider, the Q-factor of the filter curve then produced by the 
circuit arrangement depends on the position of the first switch, i.e. on 
the degree of negative feedback from the output to the inverting input of 
the amplifier. Thus, the quality of the resulting filter curve, i.e. the 
frequency bandwidth in which the input signal is boosted, can be 
influenced by an appropriate setting of the first switch. Conversely, the 
degree of boosting is influenced by setting the second switch to an 
appropriate position. When the input signal in the relevant frequency band 
is boosted, the various positions of the second switch do not influence 
the Q-factor of the resulting filter curves. This means that the position 
of the first switch dictates the quality of the resulting filter curves, 
the degree of boosting being determined by an appropriate control of the 
second switch. 
To attenuate the input signal in the frequency band the situation is 
exactly the other way around because now the Q-factor of the resulting 
filter curves is determined by the position of the second switch. Thus, 
the second switch must be set to such a position that the attenuation is 
obtained with the desired frequency bandwidth and Q-factor. The degree of 
attenuation is now defined by setting the first switch to an appropriate 
position. 
In many cases it will be desirable to have a boost or attenuation with a 
constant Q-factor in the frequency band even when the degree of boosting 
or attenuation varies. In the case of boosting this means that the first 
switch is set to a fixed position which results in a boost in the desired 
frequency band or with the desired Q-factor. The degree of boosting is 
then influenced by an appropriate setting of the second switch, the first 
switch remaining in the same position. The same applies to the attenuation 
for which the quality factor is selected by setting the second switch to a 
position which is subsequently maintained and for which the attenuation 
degree is selected by an appropriate setting of the first switch. 
Thus, by an appropriate setting of the switches the circuit arrangement in 
accordance with the invention not only allows the degree of boosting but 
also the width of the influenced frequency band to be influenced, i.e. the 
Q-factor of the resulting filter curves. In particular, in the present 
circuit arrangement, for comparatively small boosts or attenuations of the 
input signal within the frequency band, the Q-factor of the filter curves 
no longer exhibit comparatively small values. 
In an embodiment of the invention for boosting the input signal in the 
frequency band, the first switch is set to the position for minimum 
negative feedback, and for attenuating the input signal in the frequency 
band the second switch is set to that position in which the maximal 
attenuation of the output signal is obtained. 
As in many fields of use the frequency band to be influenced should be as 
narrow as possible, particularly also in the case of comparatively small 
boosts or attenuations, it is advantageous for these cases to set the 
switches in such a way that the maximum attainable Q-factor of the 
resulting filter curves is obtained when the frequency range is influenced 
differently. For boosts of the input signals in the frequency range this 
is effected in that the first switch is set to the position for minimum 
negative feedback because in this position filter curves with maximal 
Q-factor are produced for a boost of the input signal. Conversely, for the 
attenuation of the input signal the second switch is set to that position 
for which this switch provides the maximum attenuation of the output 
signal. In the case of attenuation of the input signal this position of 
the second switch provides the maximum Q-factor of the filter curves. 
This embodiment is particularly advantageous in those cases in which a 
frequency band of minimal width should also be influenced when only 
minimal boosts or attenuations are required, as in the case of, for 
example, equalizers. 
In a further embodiment of the invention the switches are semiconductor 
circuits whose switching states can be controlled by digital data words. 
This simply enables both the Q-factor of the resulting filter curves, i.e. 
the width of the frequency band being influenced, and the desired degree 
of boosting or attenuation to be adjusted in a simple manner. 
In a further embodiment of the invention a plurality of circuit 
arrangements with electronically controllable transfer characteristic are 
arranged after one another to influence the input signal in different 
frequency bands. 
In this way it is comparatively easy to construct an equalizer which has 
the special advantage that even comparatively small changes of the 
frequency response can be obtained with the same Q-factor as larger 
changes of the frequency response.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a circuit arrangement in accordance with the invention which 
enables its input signal to be influenced within a specific frequency 
band. Such a circuit arrangement is particularly suitable for use in audio 
equipment, if desired as a plurality of such arrangements arranged one 
after another to influence different frequency bands. 
The circuit arrangement shown in FIG. 1 comprises an operational amplifier 
1 whose non-inverting input 2 receives the input signal to be influenced. 
An output 3 of the operational amplifier 1 is connected to a resistor 
chain comprising a series arrangement of resistors 4, 5, 6 and 7. 
There has also been provided an electronically controllable switch 8 having 
five input terminals 9, 10, 11, 12, 13 and one output terminal 14. In the 
Figure the switch is shown diagrammatically as a pointer 15 by means of 
which one of the inputs 9 to 13 is connected to the output 14. In 
practice, the switch 8 is an electronic switch, which can be controlled, 
for example, externally by appropriate data words in a manner not shown. 
The first input 9 of the electronic switch 8 is connected to the output 3 
of the operational amplifier 1. The second input 10 of the switch 8 is 
connected to the node between the resistors 4 and 5, the third input 11 is 
connected to the node between the resistors 5 and 6, the fourth input 12 
is connected to the node between the resistors 6 and 7, and the fifth 
input 13 of the switch 8 is connected to the end of the resistor chain. 
The output 14 of the first switch 8 is connected to an inverting input 16 
of the operational amplifier 1. The negative feedback of the operational 
amplifier 1 is adjustable in accordance with the selected input 9 to 13 of 
the first switch 8. 
A frequency-dependent circuit element 20 is connected in parallel with the 
resistor chain comprising the resistors 4 to 7. This circuit element 
comprises two series-connected capacitors 21 and 22 whose node is 
connected to ground via a resistor 23. The resistor chain, which comprises 
the resistors 4 to 7, and the circuit element 20 constitute the 
frequency-dependent voltage-divider circuit. In the example shown in FIG. 
1 this voltage divider circuit has been dimensioned to influence the input 
signal in a frequency band around 1 kHz. 
There has further been provided a second electronically controllable switch 
30 which in the same way as the first switch 8 is shown only 
diagrammatically in FIG. 1 and which comprises five inputs 31, 32, 33, 34 
and 35. The input 31 of the second switch 30 is arranged in the same way 
as the input 9 of the first switch 8. Likewise, the further inputs 32 to 
35 are arranged similarly to the inputs 10 to 13 of the first switch 8. 
The inputs 31 to 35 of the second electronically controllable switch 30 
can be connected to its output 36. The output level of the output signal 
of the arrangement appearing on the output 36 of the second switch 30 is 
adjustable by means of the second switch 30. 
The circuit arrangement shown in FIG. 1 enables the input signal applied to 
the non-inverting input 2 of the operational amplifier 1 to be boosted or 
attenuated depending on the control of the two electronic switches 8 and 
30. The corresponding output signal is made available on the output 36 of 
the second switch 30. 
If the input signal is to be boosted in the frequency band determined by 
the frequency-dependent voltage divider, the width of the frequency band 
in which boosting is effected is influenced by the position of the first 
electronic switch 8. However, the degree of boosting is influenced by the 
position of the second electronic switch 30. Conversely, if the input 
signal is to be attenuated in the frequency band determined by the voltage 
divider, the width of the frequency band to be influenced is determined by 
the position of the second switch 30 and the degree of attenuation is 
influenced by the position of the first electronic switch 8. 
The operation of the circuit arrangement shown in FIG. 1 will now be 
explained with reference to the frequency diagram shown in FIG. 2. 
It is assumed hereinafter that the frequency-dependent voltage divider 
circuit comprising the chain of resistors 4 to 7 and the 
frequency-dependent circuit element 20 influences the frequency response 
of the input signal in the band around 1 kHz, the maximum influence being 
obtained at 1 kHz. 
FIG. 2 shows a family of curves representing the output signal of the 
circuit arrangement in the case of a linear input signal for different 
positions of the switches 8 and 30. In the example shown in FIG. 2 the 
circuit arrangement shown in FIG. 1, i.e. its electronically controllable 
switches 8 and 30, is controlled in such a way that also for small boosts 
or attenuations of the input signal in the frequency band around 1 kHz 
filter curves with substantially the same quality factor are obtained as 
for stronger boosts or attenuations. 
For the curve 41 shown in FIG. 2, when the input signal appears 
substantially unchanged on the output of the circuit arrangement, the 
first electronic switch 8 has been switched to its input 13 and the second 
electronic switch 30 to its input 35. These positions of the switches 8 
and 30 yield a linear frequency response, which is obtained for all those 
switch positions of the two switches for which the switches are connected 
to the same node in the resistor chain. Thus, the curve 41 shown in FIG. 2 
is also obtained, for example, if the first electronic switch 8 is 
switched to its input 9 and the second electronic switch 30 to its input 
31. In these switch positions the signal does not pass through the 
frequency-dependent voltage divider circuit and the entire circuit 
arrangement exhibits a gain of zero dB. 
FIG. 2 also shows curves 42 to 45 for which the input signal is boosted in 
the frequency band around 1 kHz. In the examples shown in FIG. 2 these 
curves have been selected in such a way that the Q-factor is substantially 
the same for the filter curves 42 to 45. For this purpose the first 
electronic switch 8 in the circuit arrangement shown in FIG. 1 is switched 
to its input 13. The curve 42 shown in FIG. 2 is obtained in that the 
second electronic switch 30 is switched to its input 34. By switching the 
second electronic switch 30 from its input 35 to its input 34 the output 
attenuation is reduced, so that now the output signal is influenced by the 
frequency-dependence of the voltage divider circuit. If a stronger boost 
in accordance with the curve 43 in FIG. 2 is required the electronic 
switch 30 is switched to its input 33. The curve 44 of FIG. 2 is obtained 
by switching the electronic switch 30 to its input 32 and the curve 45 is 
obtained by switching this switch to its input 31. The curve 45 represents 
the maximum boost of the input signal in the frequency band around 1 kHz, 
which is approximately 12 dB in the example shown in FIG. 2. 
Since for an attenuation of the input signal with filter curves of constant 
Q-factor the position of the second electronic switch 30 should not be 
changed, the second electronic switch is switched to its input 35 for the 
family of curves 46 to 49 shown in FIG. 2. The different degrees of 
attenuation represented by the family of curves 46 to 49 are obtained by 
appropriately switching the first electronic switch 8. The curve 46 is 
obtained when the first electronic switch 8 is switched to its input 12. 
The maximum attenuation of the input signal in the frequency band around 1 
kHz in accordance with the curve 49 is obtained when the first electronic 
switch 8 is switched to its first input 9. 
The example shown in FIG. 2 has been selected in such a way that both a 
boost and an attenuation of the input signal in the frequency band are 
always effected with the maximal Q-factor. This will be desirable in, for 
example, equalizers which should generally influence a frequency band 
which is as narrow as possible. Since the circuit arrangement in 
accordance with the invention enables the Q-factor of the filter curve 
produced by the arrangement to be adjusted selectively, it is possible to 
obtain filter curves with different Q-factors for a specific degree of 
boosting or attenuation. FIG. 3 shows some filter curves which can be 
obtained in such a case by means of the circuit arrangement shown in FIG. 
1. 
FIG. 3 shows four filter curves 51 to 54 for which the input signal is each 
time boosted to the same extent, i.e. approximately 3 dB, in the frequency 
band around 1 kHz. However, this boosting is effected in frequency bands 
of different widths, i.e. with filter curves having different Q-factors. 
The curve 51 shown in FIG. 3 is the curve with the maximum Q-factor, i.e. 
the curve for which boosting is effected in a comparatively narrow 
frequency band. In this case the first switch 8 in the circuit arrangement 
shown in FIG. 1 is switched to its input 13 and the second electronic 
switch 30 is switched to its input 34. In order to obtain the curve 52 in 
FIG. 3, the first electronic switch 8 in the circuit arrangement shown in 
FIG. 1 is switched to its input 12 and the second electronic switch 30 is 
switched to its input 33. For the curve 53 in FIG. 3 the first electronic 
switch 8 is switched to its input 11 and the second electronic switch 30 
is switched to its input 32. In order to obtain the curve 54 with the 
smallest Q-factor in FIG. 3, the first electronic switch 8 in the circuit 
arrangement shown in FIG. 1 is switch to its input 10 and the second 
electronic switch 30 is switched to its input 31. The family of curves in 
FIG. 3 shows that the Q-factor of the resulting filter curves, i.e. the 
width of the influenced frequency band, is influenced by the position of 
the first switch 8 in the case of a boost of the input signal. This first 
switch 8 determines the degree of negative feedback in the circuit 
arrangement and therefore not only the Q-factor of the filter curves but 
also the gain. For the filter curves in FIG. 3 with the same degree of 
boosting this different negative feedback and hence the different gain in 
the circuit arrangement can be compensated for by an appropriate setting 
of the second switch 30. However, the different positions of the second 
switch 30 do not influence the Q-factors of the resulting filter curves 
when the input signal is boosted. 
In the circuit of FIG. 4, the first and second switches 8 and 30 of FIG. 1 
are shown in block form. A digital data control circuit 60 generates a 
first digital data word 61 and a second digital data word 62 in order to 
control the switches 8 and 30. The circuit otherwise operates in a manner 
similar to that described above for the circuit of FIG. 1. 
FIG. 5 shows a plurality of circuit arrangements each with an 
electronically controllable transfer characteristic. In FIG. 5, a first 
circuit arrangement 99 with an electronically controllable transfer 
characteristic and a last circuit arrangement 199 with an electronically 
controllable transfer characteristic are shown. These circuit arrangements 
may be implemented in accordance with the circuit arrangements of FIG. 1 
or FIG. 4 and may be part of a plurality of circuit arrangements with 
electronically controllable transfer characteristics. The first circuit 
arrangement 99 is provided with an input 2 and an output 36, and the last 
circuit arrangement 199 is provided with an input 102 and an output 136. 
When an input signal is supplied to the input 2 and the output 36 is 
coupled to the input 102, either directly or by means of one or more 
further such circuit arrangements, an output signal can be obtained from 
the output 136. The circuit arrangements (99, . . . and 199) are thus able 
to influence the input signal in a plurality of different frequency bands.