Circuit for DC control of a compressor

Abstract: A compressor with a DC bias control circuit (17) is provided. The DC bias control circuit provides a predetermined DC current to a variable gain stage of the compressor circuit such that when the input signal to the compressor is substantially equal to zero, the variable gain stage is biased by the DC current and provides a DC feedback path for the compressor.

Description: Background of the Invention This invention relates to compandors, in particular, a circuit for providing DC bias control of a compressor. Companding is a widely used technique for gaining a signal to noise ratio improvement in systems where a signal is passed through a noisy transmission medium. Companding is utilized when one wishes to transmit a signal with a large dynamic range through a channel that has limited dynamic range. The process of companding is a method whereby data with a large dynamic range is first "compressed" thereby attenuating the high voltage signals and amplifying the low voltage signals. This compressed limited dynamic range signal is then usually transmitted over a channel. Upon receipt, the data is "expanded" thereby amplifying the high voltage signals and attenuating the low voltage signals. The basic building blocks of a compandor are an operational amplifier (op amp), a rectification and average circuit, and a variable gain stage. The op amp is typically connected in a negative feedback mode whereby the rectification and average circuit and the variable gain stage are configured in the feedback loop for a compressor or connected to the inverting input of the op amp for an expander. The rectification and average circuit performs rectification of the input signal, for the expander, or of the output signal, for the compressor, and then averages the rectified signal to obtain a DC signal that is proportional to the average level of the input or output signal for an expander or compressor, respectively. This DC signal then feeds the variable gain stage which determines the overall gain of the compandor. Additionally, the gains of a compressor and an expander are typically complementary wherein the output of the former is a function of the square root of the input signal, while the output of the latter is a function of the square of the input signal. With respect to a compressor, a variable resistor is utilized in the feedback path and the variable resistor is typically infinite when no input signal is present. As a result, a separate DC feedback path is required to properly bias the compressor in the absence of an input signal. One approach to bias the compressor when there is no input signal present is described in an article by Craig C. Todd entitled "A Monolithic Analog Compandor", IEEE JSSC vol. sc-11. In particular, the subject compandor includes a separate DC feedback path comprised of external resistors and an external capacitor. However, in addition to requiring these three additional components, two additional pins are also required which may not be suitable on an 8-pin integrated circuit or any pin limited application. European Patent Application No. EP-A-0305301 and US Patent No. 3,919,654 give examples of compandor and compressor circuits. Hence, a need exists for a circuit that provides DC bias control of a compressor in the absence of an input signal which minimizes the number of external components and pins required. Summary of the Invention The present invention provides a compressor circuit responsive to an input signal applied at an input and providing an output signal at an output, the compressor circuit comprising: an operational amplifier circuit having first and second inputs and an output, said first input being coupled to receive a bias voltage, said second input being coupled to the input of the compressor circuit, and said output of said operational amplifier circuit being coupled to the output of the compressor circuit; and a rectification and average circuit for providing a DC signal at an output of said rectification and average circuit in response to the output signal, said rectification and average circuit having an input coupled to the output of the compressor circuit, the compressor circuit being characterised by: a variable gain circuit having an adjusting gain in response to said DC signal of said rectification and average circuit, said variable gain circuit having an input coupled to the output of the compressor circuit and an output coupled to said second input of said operational amplifier circuit; and a bias circuit coupled to said rectification and average circuit and to said variable gain circuit for providing a DC bias current to said variable gain circuit when the input signal is substantially equal to zero thereby maintaining a DC feedback path from said output of said operational amplifier circuit to said second input of said operational amplifier circuit, wherein said bias circuit includes a current mirror circuit having an input coupled to a current source, a first output coupled to a bias terminal of the variable gain circuit and a second output coupled to the rectification and average circuit, and wherein said DC bias current does not affect the biasing of the variable gain circuit when the input signal is a non-zero input signal. It is an advantage of the present invention to provide a DC bias current to a compressor circuit when an input signal to the compressor circuit is substantially equal to zero. It is also an advantage of the present invention to provide a DC feedback path from the output of a compressor circuit to the input of the compressor circuit with minimum external components. The above and other features and advantages of the present invention will be better understood from the following detailed description taken in conjunction with the accompanying drawing. Brief Description of the Drawing The sole figure is a detailed schematic diagram illustrating a compressor circuit in accordance with the present invention. Detailed Description of the Invention Referring to the sole figure, a detailed schematic diagram illustrating a compressor circuit 10 comprising rectification and average circuit 12, variable gain stage 13, operational amplifier (op amp) 14, resistors 11 (RIN), 15 (R1) and 16 (R2) and bias circuit 17. Rectification and average circuit 12 includes op amp 28 configured in a negative feedback mode having a non-inverting input coupled to a bias voltage, V B , an inverting input coupled to circuit node 19 via resistor 15 (R1) wherein circuit node 19 is the input of rectification and average circuit 12. The inverting input of op amp 28 is also coupled to the emitter of transistor 20, to the base of transistor 21, and to the anode of diode 22. The output of op amp 28 is coupled to the base of transistor 20, to the emitter of transistor 21 and to the cathode of diode 22. Diode 22 may be formed by a transistor having its base and collector shorted together as is understood. The collector of transistor 20 is coupled to the collector of transistor 21 and to the base and collector of diode-connected transistor 23. The emitters of transistors 23 and 24 are coupled to a first supply voltage terminal at which the potential V CC is applied. Transistor 24 also has a base coupled to the base of transistor 23 and an emitter returned to ground reference through capacitor 25. It is understood that transistors 23 and 24 form a current mirror having an input at the collector of transistor 23 and an output at the collector of transistor 24. The collector of transistor 24 is further coupled through resistor 26 to circuit node 31. Op amp 27 has a non-inverting terminal coupled to bias voltage V B , and an output coupled to the base of transistor 33. The inverting terminal of op amp 27 is coupled to circuit node 31 and to the emitter of transistor 33. The collector of transistor 33 is coupled to the base and collector of transistor 34. The emitters of transistors 34 and 35 are returned to ground reference. The base of transistor 35 is coupled to the base of transistor 34, and the collector of transistor 35 is coupled to circuit node 18 wherein circuit node 18 is the output of rectification and average circuit 12. It is understood that transistors 34 and 35 form a current mirror having an input at the collector of transistor 34 for receiving current I G /4 and an output at the collector of transistor 35 for providing rectification current I REC .. Further, capacitor 25 and resistor 26 comprise an average circuit for averaging an AC signal appearing at the output of the collector of transistor 24 and providing a DC average current signal of I G /4 which is proportional to the input voltage V IN . Variable gain stage 13 includes op amp 40 having a non-inverting input coupled to bias voltage V B and an output coupled to the bases of transistors 42 and 44. The inverting input of op amp 40 is coupled through current source 46 to operating potential V CC . The inverting input of op amp 40 is also coupled to the base and collector of transistor 48 and through resistor 16 (R2) to circuit node 19. The emitters of transistors 42 and 48 are returned to ground reference through current source 50. Transistor 42 has a collector coupled to operating potential V CC . Transistor 44 has a collector coupled to the collector of transistor 52 and to the inverting input of op amp 14. The base of transistor 52 is coupled to the base and collector of transistor 54 and to the collector of transistor 56. The emitters of transistors 52 and 54 are coupled to operating potential V CC , and the base of transistor 56 is coupled to bias voltage VB. The emitters of transistors 44 and 56 are coupled to circuit node 18. Bias circuit 17 includes current source 60 which supplies current I DC and is coupled between operating potential V CC and the base and collector of transistor 62. The base of transistor 62 is also coupled to the bases of transistors 64 and 66. The emitters of transistors 62, 64 and 66 are all returned to ground reference. The collector of transistor 64 is coupled to circuit node 18 while the collector of transistor 66 is coupled to the bases of transistors 34 and 35 of rectification and average circuit 12. In addition, compressor circuit 10 includes resistor 11 (R IN ) coupled between the inverting input of op amp 14 and input terminal 72 at which input voltage signal V IN is applied. Further, the output of op amp 14 is fed back to circuit node 19 and to output terminal 74 at which voltage signal V OUT is provided. In operation, compressor circuit 10 is a variable gain amplifier which provides increasing gain as voltage input signal V IN decreases in amplitude, as is understood. For example, an input signal, having a 100 dB dynamic range, say from 0 dBV to -100 dBV, that undergoes a 2:1 compression, whereby every 2 dB change in input amplitude is compressed to a 1 dB change in output amplitude, results in the output signal having only a 50 dB dynamic range, say from -10 dBV to -60 dBV. In summary, as the level of the input signal increases, the overall gain of compressor circuit 10 decreases thereby compressing an input dynamic range signal to a smaller dynamic range output signal. The operation of rectification and average circuit 12 is fully described in US patent number 5,012,139 entitled "Full Wave Rectifier/Averaging Circuit" and issued on date of April 30,1991. Briefly, rectification and average circuit 12 receives a voltage signal at its input (circuit node 19) and provides a DC current, I REC , which is used to set the gain of variable gain stage 13. It should be noted that the current mirror comprised of transistors 34 and 35 is designed to provide a current gain of, for example, four such that the output current (I REC ) is four times the input current (I G /4). Variable gain stage 13 receives signal V OUT at the base of transistor 48 via resistor R2. In a steady state condition, transistors 42 and 48 conduct currents substantially equal to I REF /2 since current I REF of current source 50 is equally divided through transistors 42 and 48. Further, current source 46 provides current I 1 which is substantially equal to current I REF /2. Further, transistors 44 and 56 conduct currents substantially equal to I G /2 wherein current I G is provided by rectification and average circuit 12. As voltage V OUT increases, transistor 48 conducts more current than transistor 42 and op amp 40 responds to decrease the voltage at the bases of transistors 42 and 44 to compensate for the increase in voltage at the base of transistor 48. As a result, the voltage at the base of transistor 56 increases thereby increasing the current through transistor 56. This increased collector current of transistor 56 is subsequently mirrored through transistors 54 and 52 and supplied to the inverting input of op amp 14. When voltage V IN is substantially equal to zero, that is, no input signal present, there is no DC feedback path from the output of op amp 14 to the inverting input of op amp 14. As a result, current I G /4 is substantially equal to zero and transistor 34 is rendered non-operative. Further, there is no input current to be mirrored into transistor 35 and, thus, transistor 35 is also rendered non-operative. Thus, in a typical compressor, currents I G and I REC would be substantially equal to zero and, thus, variable gain stage 13 would no longer be biased. However, the present invention comprises bias circuit 17 which includes a current mirror circuit which has an input at the collector of transistor 62 for receiving current I DC from current current source 60 and a first output at the collector of transistor 64 for providing current I DC to variable gain stage 13. In addition, the current mirror circuit has a second output at the collector of transistor 66 for providing current I DC /4 where the current gain between transistors 66 and 62 is, for example, four. Bias circuit 17 provides a constant bias current, I DC , to variable gain stage 13 such that when there is no input signal, current I DC biases variable gain stage 13. Thus, when no input signal is present, current IG is substantially equal to current I DC and, thus, provides proper biasing for variable gain stage 13. In addition, current I DC /4 is pulled from the base of transistor 35 thereby assuring that transistor 35 is rendered non-operative and allowing all of current I DC to bias variable gain stage 13. On the other hand, when voltage V IN is not equal to zero, that is, there is an input signal present, it must be realized that current I DC does not introduce an error current into current I G . This can be shown by first summing the currents at circuit node 18: I G = I DC + I REC Further, current I REC which is, for example, four times the current flowing into transistor 34, can be expressed as: I REC = 4 x (I G /4 - I DC /4) = I G - I DC Upon substitution of the expression for I REC of Eqn. 2 into Eqn. 1, one obtains that IG = I G . Thus, when there is an input signal, bias current I DC has no affect on the value of current I G . Intuitively, one should realize that since current I DC /4 is pulled from the collector of transistor 34, and current I DC is pulled from the collector of transistor 35, current I DC is cancelled from current I G when the current mirrors (transistors 34 and 35, and transistors 62 and 66) have the proper current gain. A current mirror gain of four was used in the above description, but is not intended to be restricted to four. However, it should be realized that the current gain between transistors 34 and 35 should be substantially equal to the current gain between transistors 66 and 62 (or 64) so that an error current is not added to current I G . In summary, bias circuit 17 provides current I DC such that when there is no input signal present, current I DC biases variable gain stage 13 thereby providing a DC feedback path from the output of op amp 14 to the inverting input of op amp 14. However, when there is an input signal present, current I DC does not affect the biasing of variable gain stage 13. By now it should be appreciated that there has been provided a novel circuit that provides DC bias control of a compressor in the absence of an input signal while minimizing the number of external components and pins required. Further, the novel circuit does not affect the biasing of variable gain stage 13 when there is an input signal present.

Claims: A compressor circuit (10) responsive to an input signal applied at an input (72) and providing an output signal at an output (74), the compressor circuit comprising: an operational amplifier circuit (14) having first and second inputs and an output, said first input being coupled to receive a bias voltage (V B ), said second input being coupled to the input of the compressor circuit (10), and said output of said operational amplifier circuit (14) being coupled to the output (74) of the compressor circuit (10); and a rectification and average circuit (12) for providing a DC signal at an output of said rectification and average circuit (12) in response to the output signal, said rectification and average circuit (12) having an input coupled to the output (74) of the compressor circuit (10), the compressor circuit being characterised by: a variable gain circuit (13) having an adjusting gain in response to said DC signal of said rectification and average circuit (12), said variable gain circuit (13) having an input coupled to the output of the compressor circuit (10) and an output coupled to said second input of said operational amplifier circuit (14); and a bias circuit (17) coupled to said rectification and average circuit (12) and to said variable gain circuit (13) for providing a DC bias current to said variable gain circuit (13) when the input signal is substantially equal to zero thereby maintaining a DC feedback path from said output of said operational amplifier circuit (14) to said second input of said operational amplifier circuit (14), wherein said bias circuit (17) includes a current mirror circuit (62, 64, 66) having an input coupled to a current source (60), a first output coupled to a bias terminal of the variable gain circuit (13) and a second output coupled to the rectification and average circuit, and wherein said DC bias current does not affect the biasing of the variable gain circuit (13) when the input signal is a non-zero input signal. The compressor circuit according to claim 1 wherein said bias circuit includes: a current source (60) having first and second terminals, said first terminal being coupled to a first supply voltage terminal; a first transistor (62) having a collector, a base and an emitter, said collector and said base being coupled to said second terminal of said current source, and said emitter being coupled to a second supply voltage terminal; a second transistor (64) having a collector, a base and an emitter, said collector being coupled to said variable gain circuit, said base being coupled to said base of said first transistor, and said emitter being coupled to said second supply voltage terminal; and a third transistor (66) having a collector, a base and an emitter, said collector being coupled to said rectification and average circuit, said base being coupled to said base of said first transistor, and said emitter being coupled to said second supply voltage terminal. The compressor circuit according to claim 1 or 2 further including: an input resistor (11) coupled between said second input of said operational amplifier circuit (14) and the input of the compressor circuit (10); a first resistor (15) coupled between the output of the compressor circuit (10) and said input of said rectification and average circuit (12); and a second resistor (16) coupled between the output of the compressor circuit (10) and said input of said variable gain circuit (13). A compandor comprising a compressor circuit according to claim 3.

Inventors: BADER SCOTT K, BADER, SCOTT K.
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1992
Main classification: H03G7
IPC codes: H03G7, H03F3, H03G11