Note: Descriptions are shown in the official language in which they were submitted.
1175505 ~
PHD 80141C 1 10.9.1981
Circuit arrangement with electronically controllable
transfer characteristic.
The invention relates to a circuit arrangement
with an electronically controllable transfer characteris-
tic, specifically a tone or volume control circuit, the
input signal being applied to an amplifier having con-
trollable negative feedback.
Such a circuit arrangement is known from DE-PS
24 04 331 and DE-PS 22 62 089. The negative feedback of
the amplifier, and consequently the transfer characteris-
tic of the circuit arrangement, is varied in that the cur-
rent distribution between two differential amplifiers is
varied in an opposite sense by a direct voltage. The in-
puts of the two differential amplifiers are coupled to
each other either via a frequency-independent network so
that a volume control device is obtained (DE PS 24 04 331),
15 or via a frequency-dependent network (DE-PS 22 62 o89), so
that a tone control device (treble or bass control) is ob-
tained O
A disadvantage of circuit arrangements which are
based on the variation of the current distribution between
two differential amplifiers is that they exhibit a compa-
ratively large amount of noise. Moreover, if such a cir-
cuit arrangement is constructed as an integrated circuit
there need be provided at least three external connection
terminals for connection of the negative feedback network
and the external connections to such an integrated circuit
are comparatively intricate, at least when said circuit
is used as a tone control device.
It is an object of the present invention to de-
sign a circuit arrangement of the type mentioned in the
opening paragraph in such a way that the transfer charac-
teristic can be controlled electronically without the useof current distribution circuits of the said type.
1175505
PHD 80141C 2 10.9.1981
According to the invention this object is
~ achieved in that a voltage divider circuit having a plu-
rality of taps is connected to the output of the amplifier
and in that the taps are connected to an inverting input
of the amplifier via a first electronicall~ controllable
switch and to the output of the circuit arrangement via a
second electronically controllable switch.
The signal voltage on each tap of the voltage
divider circuit has a different value. If the first switch
is connected to a tap carrying a comparatively high signal
voltage, the negative feedback will be comparatively
strong nd the gain will consequently be comparatively
low. However, if the second switch is connected to such a
tap, the gain will be comparatively high. The situation
is exactly reversed when the switch is connected to a
tap carrying a comparatively low voltage. In a further
embodiment of the invention, intended for use as a treble
control device, the frequency-dependent voltage divider
circuit is designed so that the signal voltage on at least
2Q some of the taps decreases at increasing frequency. In one
embodiment of this circuit arrangement the frequercy-de-
pendent voltage divider circuit comprises the series ar-
rangement of a capacitor and a resistor chain with a plu-
rality of taps.
When such a circuit arrangement is constructed
as an integrated circuit only a single connection is re-
quired, which via the integrated resistor chain is con-
nected to the amplifier output and via the capacitor to
a reference point (earth).
In a further embodiment of the invention, which
is suitable as a base control device, the signal voltage
on at least some of the taps decreases as the frequency
decreases. In an embodiment of this circuit arrangement
a resistor chain comprising a plurality of taps is con-
nected to the amplifier output and a capacitor is arrang-
ed between the amplifier output and one of the taps. If
the amplifier, the switches and the resistor chain are
1175505
PHD 80141C 3 10.9.1981
again constructed in an integrated form only one external
component, the capacitor, needs to be connected, For the
connection of this capacitor the integrated circuit should
comprise two connections.
In a further embodiment of such a bass control
device the terminal of the resistor chain which is remote
from the amplifier output is connected to a reference
point (earth) via a capacitor and the impedance of the
capacitor is low relative to the-impedance of the resis-
tor chain. The capacitor in sëries with the resistor
chain, which capacitor is suitably an electrolytic capaci-
tor, serves to make the d.c. impedance between the tap`and
the reference point (for example, earth) so high that the
d.c. negative feedback is strong and the d.c. gain is unity.
This minimizes offset voltages on the amplifier output,
which voltages as is known increase as the d.c. gain in-
creases.
In a further embodiment of the invention the
two switches are controlled in such a way that one of the
two switches always conveys the full output voltage of
the amplifier. If, for example, the resistor chain com-
prises n divider resistors and each of the two switches
has n inputs, with one input at any time being connected
to its output, this yields n combinations of switch po-
sitions, some of which are redundant, because, for exam-
ple, a treble reduction is fully or partly compensated for
by a treble boost. Owing to the steps in accordance with
this embodiment the entire treble (bass) control range re-
quires only 2(n-1) switch positions.
In this respect it is to be noted that from the
magazine "Funkschau" 1980, Vol. 5, a circuit arrangement
is known for influencing audio signals, in which circuit
arrangement the semiconductor switches are actuated by
digital signals. However, the known circuit arrangement,
which serves as a volume control device, is comparatively
expensive, because it requires the use of an analog-to-
digital converter with a resolution of 17 bits.
117S50s
PHD 80141C 4 10.9.1981
The invention will now be described in more
~ detail with reference to the drawings. In the drawings
Fig. 1 is a circuit diagram of the circuit ar-
rangement in accordance with the invention, which func-
tions as a treble control device,
Fig. 2 represents the frequency response forvarious positions of the two switches in the arrangement
of Fig. 1,
Fig. 3a to 3c represent equivalent diagrams of
the circuit arrangement of Fig. 1 for various switch po-
sitions,
Fig. 4 is a circuit diagram of a bass controldevice in accordance with the invention,
Fig. 5 represents the frequency response of the
circuit arrangement of Fig. 4 for various positions of the
two switches,
Figs. 6a to 6c-represent equivalent diagrams of
the circuit arrangement of Fig. 4 for various switch posi-
tions.
Fig. 1 shows a circuit arrangement, which may
be used as a treble control device in an audio amplifier
to which a loudspeaker is connected, which loudspeaker
serves for the reproduction of audio signals supplied by
a radio tuner, a record player or the like. The circuit
arrangement comprises an operational amplifier 1 with an
open-loop gain of 80 dB, whose non-inverting input 12 re-
ceives the input signal ui. The output terminal 13 of the
amplifier is connected to a capacitor 5 via a resistor
chain 4 comprising five series-connected resistors 41
... 45, the other end of said capacitor being connected
to earth. The output 13 of the amplifier and the four
junction points between the resistors 41 ... 45 are pro-
vided with five taps 55 ... 51, which are connected to
the five input terminals 25 ... 21 and 35 .~.. 31 respec-
tively of two switches 2 and 3. Depending on the positionsof said switches, which are symbolically represented by a
switch arm 28 and 3B respectively, one of the control in-
1175SOS
PHD 80141C 5 10.9.1981
puts 21 ... 25 or 31 ... 35 is connected to the respective
output terminal 26 or 36 of the relevant switch 2 or 3.
Which input is connected to the output 26 or 36 is deter-
mined by a digital data word applied to the five control
inputs 27 or 37 of the respective switches 2 and 3.
The output 26 of the switch 2 is connected to
the inverting input 11 of the amplifier 1, whilst the out-
put 36 of the switch 3 constitutes the output of the/-cir-
cuit arrangement, on which output the output signal uO is
10 available. The higher frequencies are then influenced in
different ways depending on the switch positions of the
switches 2 and 3.
In the switch position shown, in which the out-
put of the circuit arrangement is connected directly to
15 the output of the amplifier via the swi~ch 3 and in which
the output 26 is connected to the tap 31 at the lower end
of the resistor chain~ the equivalent diagram of Fig. 3
is valid. The resistance R between the output of the ampli-
fier and its inverting input corresponds to the sum of
20 the values of the divider resistors 42 45 of the re-
sistor chain 4~ whilst the resistance Ro~ which is low in
comparison with R and which serves to limit the treble
boost or reduction~ corresponds to the value of the resis-
tor 41 and, in series with the capacitor 5, is included
25 between the inverting input 11 and earth. At comparatively
low frequencies the impedance of the capacitor 5 is high
in comparison with the impedance of the resistance R~ so
that the signal on the inverting input 11 substantialIy
corresponds to the signal on the output of the amplifier
30 1. In this case a voltage gain of 1 or 0 dB is obtained.
At higher frequencies the impedance of the capacitor 5
may no longer be ignored in comparison with the resistance
R, so that the negative feedback decreases, which corres-
ponds to an increase of the gain at higher frequencies
35 (treble boost). Thus, the gain as a function of the fre-
quency will vary as represented by the uninterrupted line
121 in Fig. 2.
- 1175505
PHD 80141C 6 10.9.1981
If~ with the same position of the switch 3, the
position of the switch 2 is changed so that the inputs 22~
23, 24 and 25 are consecutively connected to the output 26
of the switch 2, the resistance Ro will increase stepwise,
- 5 whilst the resistance R will decrease by the same amount.
Thus~ at the higher frequencies the negative feedback will
increase continually, that is, the boost at the higher
frequencies will be reduced continually, as is represented
by the broken lines, 122.... 124. If finally the input 25
of the switch 2, which input is connected to the output
13 of the amplifier 1, is connected to the switch output
26, the equivalent diagram shown in Fig. 3b is valid. The
resistance Ro + R then corresponds to the sum of the values
of the resistors 4t ...45. The full output voltage is then
always fed back to the amplifier input, so that the re-
sulting characteristic (continuous line 125) is perfectly
linear if the internal resistance of the amplifier is sub-
stantially smaller than the impedance existing at the out-
put.
If, in contradistinction to the switch position
shown in Fig~ 1, the input 25 of the switch 2 is connected
to its output 26 and the input 31 of the switch 3 to the
output 36, the equivalent diagram of Fig. 3c is valid, the
resistance R again corresponding to the sum of the values
of the resistors 42....45 and the resistance Ro corres-
ponding to the value of the resistor 41. Since, as already
stated, the impedance of the capacitor 5 at low frequenci~
is high in comparison with the resistance R, the output
signal of the amplifier 13 appears substantially unchanged
on the output terminal 36 of the circuit arrangement. At
higher frequencies, however, the resistance R is no longer
negligible in comparison with the impedance of the capaci-
tor 5, so that the gain decreases towards the higher fre-
quencies, as represented by the uninterrupted line 131 in
Fig. 2. The resistance Ro~ which corresponds to the divider
resistor 41 of the resistor chain 4, then prevents an ex-
cessive reduction of the gain at high frequencies. If the
1175505
PHD 80141C 7 10.9,1981
switch 3 is now changed over, so that consecutively the
inputs 32~ 33, 34 are connected to the output 36 of the
switch 3 (the input 25 of the switch 2 remaining connected
to its output 26), the resistance Ro in the equivalent
diagram of Fig. 3~ increases continuously, whilst the
resistance R is reduced, so that the gain reduction at
higher frequencies decreases continuously, as is represent-
ed by the broken lines 132...134 in Fig. 2. In the upper
position of the switch 3 the equivalent diagram of Fig. 3b
and the frequency-independence of the gain as represented
by the line 125 are obtained again.
In addition to the switch positions described,
other positions are possible. For example, the inputs 23
and 33~ to which the tap 53 is connected~ may be connected
15 to the outputs 26 and 36 of the respective switches 2 and
3, but the treble reduction and treble boost then exactly
compensate for each other so that again a linear frequency
response is obtained. In other switch positions only a
partial compensation is obtained, so that either a treble
boost or treble reduction is obtained, but the variation
of the frequency response still varies between the limits
121 and 131, so that these switch positions are super-
fluous.
When the divider resistors 141...45 and the ca-
pacitor 5 are suitably dimensioned, it can be achieved
that the gain for an upper limit frequency fg, for exam-
ple at 20 kHz, between adjacent characteristics shown in
Fig. 2 varies by, for example 3 dB. For the specified
gain variation steps of 3 dB, overall treble boosts or
reductions of + 12 dB can be obtained with the circuit
arrangement of Fig. 1. If even greater boosts or reduc-
tions are required without the gain variations being
increased, or if the gain variation between adjacent
characteristics should be smaller than 3 dB~ a resistor
chain with even more divider resistors should be employed.
As already stated~ the switches 2 and 3 are
changed over by digital data words on their respective
1175SOS
PHD 80141C 8 10.9.1981
inputs 27 and 37. Such switches are known, for example
in the form of the integrated circuit Philips TDA 1029,
which, however~ is designed for switching over from four
inputs to two outputs. A greater number o~ switching pos-
sibilities is obtained by cascading, as is for exampledescribed in the Magazine "Technische Informationen f~r
die Industrie~ no. 780530 (Fig. 23) published by Valvo.
The two 5-bit data words required for controlling the
switches 2 and 3 are supplied by a code converter circuit
6~ which converts a four_bit data word appearing on its
input 62 into a ten-bit data word~ of which five bits are
each applied to the control inputs 27 and 37 of the
switches 2 and 3. The code converter circuit may comprise
a memory having a number of ten-bit storage locations
corresponding to the number of possible switch positions
(nine), which locations are addressed by the signal on
the input line 62; it may alternatively comprise a logic
circuit comprising gates~ which circuit produces a ten-
bit data word on the output for each four-bit data word
on the input~
The inputs 62 of the code converter circuit 6
are connected to the output of a memory 7, whose content
determines the position of the switches 2 and 3. The me-
mory 7 may for example be an up-down counter~ which can
count to nine and whose counting direction can be con-
trolled by the user via the unit 8, consecutive countsbeing assigned to the switch positions which correspond
to adjacent characteristics, for example the characteris-
tics 133 and 134 in Fig. 2, so that during a count-up
(down) cycle the characteristics 121 to 131 (Fig. 2) are
covered quasi-continuously in the one or the other direc-
tion.
However, the memory 7 may also be controlled by
a microprocessor. If said microprocessor serially supplies
the four bits of a data word~ a shift register is required
into which the data supplied by the microprocessor is load-
ed, which data is then transferred in parallel to the me-
1175505
PHD 80141C 9 10.9.1981
!mory 7.
The circuit arrangement described produces a
smaller amount of noise than the known circuit arrange-
ments, because the operation of both the amplifier and the
switches 2 and 3 is not based on the current distribution
principle. The treble boost and reduction curves are
exactly mirror-inverted relative to each other, because
for both functions the same resistor chain and the same
capacitor are used. If the circuit arrangement shown in
Fig. 1 is manufactured as an integrated circuit, all the
elements shown, except for the capacitor 5~ can be in-
tegrated on one chip~ Thus, only one external circuit ele-
ment (the capacitor 5) must be connected to such an in-
tegrated circuit and for this purpose only one external
connection (pin) is needed.
For frequencies above the limit frequency fg in
Fig. 2 which are so high that the impedance of the capa-
citor 5 is low in comparison with the resistance Ro and
for frequencies higher than the first-mentioned frequen-
cies the transfer characteristics is again in a linearfunction of the frequency for a specific switch position~
but it is situated at different levels for different
switch positions. If the capacitance of the capacitor 5
is now made so high that the "higher" frequencies, for
which the gain is frequency-dependent for a specific
switch position, are situated beyond the range of audi-
bility, the circuit arrangement shown in Fig. 1 will
function as a volume-control device. Suitably, if the
switch 2 is in such a switch position, the inverting in-
put 11 of the amplifier then remains connected to the tap55 or the output 13 of the amplifier 1, because in that
case the negative feedback is a maximum and the distor-
tion is minimal. Then, only the second switch 3 needs to
be controlled for adjusting the volume. - If the gain
variation steps for different switch positions should be
further reduced, a further resistor having a value higher
than the resistor 41 may be arranged in series with said
~175SO~i
PHD 80141C 10 10.9.1981
last-mentioned resistor.
~ Fig. 4 represents a so-called bass-control de-
vice~ that is a circuit arrangement by means of which the
lower frequencies can be boosted or attenuated at option.
The circuit design of the circuit arrangement of Fig. 4~
which arrangement can be controlled by the control units
6, 7, 8 in the same way as the circuit arrangement of Fig.
1, dlffers from t~e last-mentioned circuit arrangement
only in that the capacitor 5 between the output 13 of the
amplifier 1, which output corresponds to the tap 55, and
the tap 51 is arranged between the two lower resistors 42
and 41 of the resistor chain, and in that the terminal of
the lower resistor 41 of the resistor chain 4 which is
remote from the output 13 of the amplifier 1 is connected
to earth via a capacitor 56 with a very higher capacitance~
suitably an electrolytic capacitor. The function of the
last-mentioned capacitor is to reduce the d.c. gain of
the amplifier 1 to unity, so that the offset voltages on
the output become comparatively small. Said capacito 56
may be dispensed with if the terminal of the resistor 41
which is connected to said capacitor is d.c. coupled to
the point to which the input 12 of the amplifier 1 is con-
nected.
In the switch position of the switches 2 and 3
as shown in Fig. 4 the equivalent diagram of Fig. 6a is
valid for the circuit arrangement, R1 representing the
series arrangement of the resistors 42 to 45, the capaci-
- tor C1 corresponding to the capacitor 5, and the resistor
R2 corresponding to the resistor 41 (for all the frequen-
cies to be transmitted the impedance of the capacitor 56is negligible relative to the other resistances). For the
lower frequencies of the transmission range the impedance
of the capacitor C1 is not yet negligible relative to that
of the resistor R2, so that the negative feedback in-
creases and the gain decreases at increasing frequency,until at the mid-range frequencies the capacitor C1, in
comparison with the resistor R2, substantially presents
1175505
PHD 80141C 11 10.9.1981
a short-circuit to the signal, as a result of which the
gain assumes the value 1, as is represented by the gain
cur~e 221 in Fig. 1. If the switch arm 28 is set to the
upper position and one of the taps 52 . . . 54 is connected
to the inverting input 11 of the amplifier 1, the extent
to which the low frequencies (bass tones) are boosted de-
creases continually, as is represented by the gain curves
224 in Fig. 5. If finally the tap 55 of the resistor chain
4, or the output 13 of the amplifier 1, is connected to
the inverting input 11 of the amplifier via the switch 2,
the equivalent diagram of Fig. 6b will be valid.' In this
switch position the resistor chain and the capacitor
function as a frequency-dependent load of the amplifier
output, but its negative feedback is frequency-independent,
so that the gain also becomes frequency-inde~endent if the
output resistance of said amplifier is low in comparison
with the impedance acting on its output. The gain then
varies as a linear function of the frequency, as is re-
presented by 225.
If the switch position of the switches 2 and 3
is changed in such a way that the inverting input 11 is
connected to the tap 55 and the output to the tap 51, the
equivalent diagram shown in Fig. 6c is obtained. The out-
put voltage of the amplifier is then divided by the vol-
tage divider comprising the elements R1, C1 and R2, thetransfer coefficient or the ga~n then being reduced at
decreasing frequency. This results in the gain curve 231.
If the switch 3 is connected to the taps 52, 53 or 54
instead of to the tap 51, the gain characteristics 232
30 . . .234 are obtained.
For the bass-control device shown in Fig. 4 and
the treble-control device shown in ~ig. 1 the distortion
and noise are reduced substantially in comparison with
the known electronically controllable treble and bass-
control devices. The bass-boost and reduction curves 221
.. 231 are then again strictly mirror-inverted relative
to each other~ because the same components are utilized
for both functions.
