Amplifier with compressor and expander function for ground symmetrical electrical signals

An amplifier for ground symmetrical signals includes a compressor at the input and an expander at the output, with the slope of at least one of its expansion or compression characteristics being adjustable for at least one half-wave polarity with the aid of an adjustment voltage. In order to reduce second order distortions at the amplifier output, a circuit is provided for determining the arithmetic mean value of the voltage across the amplifier output. A control value is produced from the determined mean value and the slope of the expansion or compression characteristic is regulated with this control value in the opposite direction so that the distortions are reduced.

BACKGROUND OF THE INVENTION 
The present invention relates to an amplifier with a compressor and an 
expander function for positive as well as negative half-waves of ground 
symmetrical signals with the slope of the compression or expander 
characteristic being controllable for at least one half-wave polarity. 
The best known representative of such an amplifier with a compressor and 
expander function is the so-called VCA (voltage controlled amplifier). If 
it were required only for half-waves of one polarity, it could be composed 
of a circuit which produces an output equal to the logarithm of the input 
signal with a subsequently connected circuit which produces an output 
equal to the exponential power of its input signal, i.e., an antilog 
circuit, (see Tietze, Schenk, "Halbleiter-Schaltungstechnik" 
[Semiconductor Circuit Technology], 2nd Edition, published by 
Springer-Verlag, 1971, pages 282 et seq.). However, with such a circuit 
arrangement care would have to be taken that the slope of the compression 
and/or expansion characteristic is controllable. These characteristics 
represent the curve of the amplifier output level as a function of the 
input level. 
FIG. 1 is a basic circuit diagram of such an amplifier for an input signal, 
which may have both half-wave polarities, at its input E, with the gain of 
the amplifier not being controllable. Between its input terminal E and a 
center terminal M, the amplifier has an instantaneous compressor with an 
approximately logarithmic gain characteristic. The compressor is composed 
of an operational amplifier 1 having a series input circuit composed of a 
coupling capacitor C1 and an input resistor R1, and a pair of feedback 
diodes D1, D2 connected in antiparallel. With the output voltage of 
operational amplifier 1 rising linearly in magnitude, a superproportional 
current, i.e. an exponentially increasing current, is returned via the 
feedback diodes D1 and D2 to the input of operational amplifier 1 so that 
greater amplitude values are amplified less than smaller values in the 
compressor. 
In the subsequent instantaneous expander, which is disposed between center 
terminal M and amplifier output terminal A and which includes a pair of 
antiparallel connected diodes D3 and D4 connected in series with the input 
of an operational amplifier 2 having a feedback resistor R2, conditions 
are reversed. That is, in the expander, the currents through the input 
diodes D3 and D4, which increase exponentially with linearly increasing 
input voltage of the expander, are amplified by an operational amplifier 2 
and its feedback resistance R2 and produce a correspondingly amplified 
output voltage across output terminals A. 
To realize a controllable gain for the amplifier, the diodes D1-D4 of FIG. 
1 are each replaced in FIG. 2 by the emitter-collector path of a 
respective transistor T1 to T4. More specifically, the diodes D1 and D2 of 
the compressor of FIG. 1 are replaced by a pair of opposite polarity type 
transistors T1 and T2 which have their respective emitters connected to 
the output of the operational amplifier 1 via respective resistors R3 and 
R4 and have their collectors connected together and to the inverting input 
of the operational amplifier 1. A source of operating potential is 
connected across the series connected emitter-collector paths of the 
transistors T1 and T2. The diodes D3 and D4 of the expander of FIG. 1, in 
turn, are replaced by a pair of opposite polarity type transistors T3 and 
T4 which are of the same polarity type as the transistors T1 and T2 
respectively and which have their emitters connected to the respective 
emitters of the transistors T1 and T2 and their collectors connected 
together and to the inverting input of the operational amplifier 2. In 
order to control the slope of the compression and expansion 
characteristics of both the compressor and the expander for both 
half-waves polarities, the bases of transistors T1 and T4 are connected to 
ground via a resistor R5 and to a terminal 3, while the bases of 
transistors T2 and T3 are connected to ground via a resistor R6 and to a 
terminal 4; an adjustable d.c. voltage is applied to the pair of terminals 
3-4. 
In an emitter-collector path as shown in FIG. 2, the current through the 
transistor also rises exponentially with the linearly increasing 
emitter-base voltage as long as the associated base-collector voltage 
remains constant. If the potentials at the pair of adjustment terminals 
3-4 are initially kept constant, the potential differences between the 
constant base potentials, on the one hand, and the half-wave shaped, 
pulsating emitter potentials, on the other hand, produce collector 
currents as an exponential function of the base-emitter voltages. 
Controlling the gain is now possible in that an adjustment value 
(adjustment voltage) Us is applied to the pair of adjustment terminals 
3-4, thus permitting the slope of the collector current/base-emitter 
voltage characteristic of each transistor to be varied because a change in 
potential at the bases causes a change in the collector-base voltage. It 
must here be considered that the potential at the collectors is almost 
zero because the collectors are each connected with one of the inverted 
inputs (so-called virtual zero points) of the respective operational 
amplifiers 1 and 2. In detail, the gain control operates such that, for 
example, an increase in potential at adjustment terminal 3 and a reduction 
at adjustment terminal 4 adjusts transistors T3 and T4 to conduct better 
and transistors T1 and T2 to conduct less well, with the result that the 
degree of expansion in the expander (2, R2, T3, T4) increases and the 
degree of compression occurring in the compressor (1, R1, T1, T2) 
decreases, so that the overall gain between input terminals E and output 
terminals A increases. 
When an amplifier according to FIG. 2, or another amplifier for symmetrical 
signals with a compressor and/or expander function, is employed, second 
order distortions or harmonics may occur at the output although such 
distortions actually should not occur. The cause of these distortions is 
due to differences in behavior, sometimes as a function of temperature, of 
the two branches with respect to positive and negative half-waves. 
To reduce such distortions, it is known to use selected pairs of 
transistors T1-T2 and T3-T4 with the individual transistors of each pair 
being thermally coupled together. This technique, however, requires 
undesirable expenditures. 
Another way to reduce distortions is disclosed in Federal Republic of 
Germany DE-OS No. 3,021,788. According to this reference, behind input 
terminal E of FIG. 2 of the present application, a parallel path branches 
off for the symmetrical signals. In this parallel path a phase inverter 
amplifier is followed by a further circuit as shown in FIG. 2 which is 
then again followed by a further phase inverter amplifier before the 
output signals of the parallel path are added to the signals at output 
terminal A of FIG. 2. If now in FIG. 2, for example, the positive 
half-wave were given less preferential treatment (due to a weaker gain for 
transistor T3 than transistor T4), thus producing second order 
distortions, the inversion in the parallel path would put the other 
half-wave (through the transistor corresponding to T3) at a disadvantage. 
The addition at the output terminal A balances out the disadvantaged 
different half-waves (and thus the preferential treatment of the 
respectively other half-waves), presuming again that sufficient thermal 
couplings are provided. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an amplifier of the 
above type wherein distortions in the form of undesirable second harmonics 
in the output signal are reduced in such a manner that the accuracy of the 
thermal coupling is no longer so important. 
The above object is accomplished according to the present invention by an 
amplifier with a compressor and expander function for positive and 
negative half-waves of ground symmetrical signals which comprises: a 
compressor circuit means, having its input connected to a signal input 
terminal, for compressing negative and positive half-waves of an input 
signal; expander circuit means, having its input connected to the output 
of the compressor circuit means and its output connected to an amplifier 
signal output terminal, for expanding positive and negative half-waves of 
an input signal; circuit means for applying a variable desired voltage to 
at least the expander or compressor circuit means to control the slope of 
the expansion or compression characteristic respectively for at least one 
half-wave polarity to control the gain of the amplifier; and control 
circuit means for reducing distortions in the output signal from the 
amplifier caused by even number harmonics including means, connected to 
the amplifier output terminal, for determining the arithmetic mean value 
of the signal at the amplifier output terminal and feeding a corresponding 
control value to the expander or compressor circuit means to change the 
slope of the expansion characteristic for the at least one half-wave 
polarity relative to the other half-wave polarity so as to reduce the 
distortions. 
According to a feature of the invention the control circuit means includes 
adjustable means for superposing a desired constant value on the 
corresponding control value. 
Preferably, the circuit means for applying a variable voltage is connected 
to the expander circuit means to control the slope of the expansion 
characteristic for both half-wave polarities, the control circuit means 
feeds the control value to the expander circuit means to simultaneously 
change the slope of the expansion characteristic for both half-wave 
polarities, and the control circuit means comprises an integrator having 
its input connected to the amplifier output terminal and its output 
connected to the expander circuit means to superpose the control value on 
the variable desired voltage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 3, there is shown a preferred embodiment of an 
amplifier according to the present invention which amplifier generally 
coincides with that of FIG. 2 with respect to the compressor at the input 
and its components 1, T1, T2, etc. and with respect to the expander near 
the output and its components 2, T3, T4, etc. The compressor and expander 
circuits of FIG. 3 differ, as shown, from the corresponding circuits of 
FIG. 2 in that the bases of the respective transistor T1-T4 are each 
connected to ground via respective separate resistors R7-R10 and to the 
associated terminals 3 or 4 via respective decoupling resistors R11-R14. 
In the circuit of FIG. 3, the compressor near the input has an 
approximately logarithmic compression characteristic and the expander near 
the output has an approximately exponential expansion characteristic for 
half-waves of the same polarity. Semiconductor components or transistors 
T1 to T4 are provided to generate these characteristics. The input 
coupling capacitor C1 takes care that the input signal to be processed 
reaches operational amplifier 1 with an arithmetic mean value of zero. 
According to the invention at output terminal A there now is disposed a 
circuit for determining the arithmetic mean of the amplifier output 
signal. This circuit, which is an integrator, includes an operational 
amplifier 6 having its inverting input connected via an input resistor R15 
to the output terminal A and to its output via a feedback capacitor C. The 
non-inverting input of operational amplifier 6 is connected to ground via 
resistor R16. The output of operational amplifier 6 is connected via 
respective decoupling resistors R17 and R18 to the bases of transistors T3 
and T4 so that the output voltage of the integrator is superposed in the 
same sense on the voltages across the bases of transistors T3, T4. If, for 
example, a positive half-wave is amplified less than required through T3 
and operational amplifier 2, an asymmetrical output voltage appears across 
output terminals A and a negative direct voltage (control value S) appears 
at the output of the integrator. This negative control value S will be 
applied to the bases of transistors T3 and T4, causing transistor T3 to be 
more conductive and transistor T4 to be less conductive until the 
asymmetry of the output voltage and the second order distortion have been 
eliminated. 
Preferably, an adjustable desired potential can be applied across the 
non-inverting input of operational amplifier 6. As shown in FIG. 3, this 
adjustable desired potential is produced by a potentiometer P having its 
ends connected across a source of d.c. voltage and having its moveable tap 
connected via a resistor R19 to the non-inverting input of operational 
amplifier 6 so that the resistors R16-R19 form a voltage divider. 
Potentiometer P permits the control circuit 2-6-T3 or T4 to be set to 
provide minimum second order distortion at output terminal A. At the same 
time, offset errors of operational amplifiers 2 and 6 can be compensated 
as well. The reduction to a minimum of the second order distortion output 
voltages is based on the fact that the control circuit takes care that 
compansion is effected in the same manner for positive half-waves as for 
negative half-waves. 
Additionally, it should be noted that the adjustment voltage Us need not 
necessarily control the gain of the compressor and of the expander. For 
example, the compressor could be constructed according to FIG. 1 and thus 
not be controllable. 
##EQU1## 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes and adaptations, and the 
same are intended to be comprehended within the meaning and range of 
equivalents of the appended claims.