Abstract:
A remotely controlled microphone amplifier has a first amplifier stage with a variable gain circuit operatively connected in the overall feedback loop of the first stage for controlling the gain in first increments. A unity gain second amplifier stage has variable attenuator operatively coupled between the output of the first amplifier stage and the input of the second amplifier stage. The variable attenuator controls the input to the second stage in second increments less than the first increments. The overall gain is linearly adjustable and varies in equal increments.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to and claims the benefit of U.S. Provisional Application Ser. No. 60/128,305 filed Apr. 8, 1999, the teachings of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention pertains to a microphone amplifier having a digital control and more particularly to a two stage amplifier in which the first stage employs a feedback circuit providing relatively large gain control increments and a second stage employs an attenuator providing relatively small gain control increments so that the resulting overall gain control is substantially linear with equal increments. 
     Variable-gain microphone preamplifiers are employed in various applications including mixing consoles for sound reinforcement or recording, architectural electro-acoustic systems, public address (PA) systems and the like. 
     In large systems, the audio equipment is installed in a different area or location remote, from the control room. This makes remote control of the microphone amplifier gain highly desirable and necessary. However, satisfactory remote control is difficult to achieve. 
     It is desirable to digitally control the gain of a microphone amplifier (MA) in equal logarithmic increments, assuring at the same time good resolution and continuity. This is likewise difficult to achieve economically. 
     Typically the processing or analog to digital (A/D) conversion, if any, of an audio signal occurs at signal levels of about 1V or higher. A typical dynamic microphone has an output of about 1 mV. An amplifier gain of about 60 dB (×1000) is necessary to elevate the low microphone output signal to a level that can be either converted into digital form or further processed in the analog domain. 
     On the other hand, different microphone designs and wide dynamic range of the audio-source signal can cause the microphone output level to vary drastically, requiring the microphone amplifier gain to be made variable within the same wide range. 
     In a typical application, the control element is a potentiometer with a special taper located on the front panel of the piece of audio equipment. This makes remote gain control difficult if not impossible, leaving manual control as the only practical control option. 
     A microphone amplifier (MA) is typically designed as a linear (no distortion), high-gain (60 dB), variable gain (0˜60 dB), high headroom (20 dB) amplifier with a relatively low equivalent input noise (EIN) level of less than −125 dBu at maximum gain. A well designed MA should also meet the following requirements: 
     1. Balanced input for high Common Mode Rejection Ratio (CMRR). 
     2. Operative with low input levels 0.5˜1 mV. (−60 dBu) and high output (about 0 dBu) suitable for further processing or A/D conversion. 
     3. Low noise: SNR˜70 dBu @ maximum gain of 60 dB and EIN&lt;−125 dBu. 
     4. Wide gain range (0˜60 dB) in virtually all cases accomplished by a potentiometer. To achieve continuity and uniform gain change in dB over the entire control range the potentiometer has a special, close to, but steeper than reverse-logarithmic (rev-audio) taper. 
     The gain change should not affect the CMRR. 
     5. Sufficient headroom of 20+dB above the nominal level of usually 0 dBu. 
     In addition, an MA desirably includes the option of: 
     6. Balanced output for better interface with a consequent piece of equipment or A/D) converters. 
     A microphone amplifier with digital gain control should have the same electrical and noise characteristics as a conventional high quality microphone amplifier plus the added flexibility of digitally controlled gain in: 
     7. Equal (uniform) logarithmic increments (dB). The incremental control comes from the nature of any digitally controlling or processing system, where all parameter changes can be implemented only in discrete steps. 
     Finally, an MA should: 
     8. provide smooth, uniform gain control in a sufficient number of steps without an unreasonable increase in hardware. This is directly related to the practicality of the design. 
     One conventional approach uses a digital volume control integrated circuit. These are so called digital potentiometers basically consisting of a resistive ladder and analog switches providing attenuation, or sometimes a gain, that works like a mechanical potentiometer with large number of tabs or steps. An associated digital circuit controls the transfer (attenuation) ratio and provides the interface with a CPU. 
     Some exemplary digital potentiometers are passive attenuators. Some are combined with operational amplifiers. Most can be connected in different feedback circuits to replace variable resistors and to perform a variety of gain control functions. Some provide linear or logarithmic change of the transfer ratio in 8 to 256 increments of 0.5 to 1 dB in wide range (80˜126 dB). Used by themselves, however, digital potentiometers all are unsuitable for a microphone amplifiers, because they do not meet requirements 1, 3, 5 and 6. 
     In another approach, an analog microphone amplifier is used. This approach may meet some of the desired requirements, but does not meet requirements 4, 7, and 8. 
     A significant problem is that even if a suitable analog circuit for a microphone amplifier is selected in order to meet the balance, noise, and headroom requirements, implementing a uniform gain change of 60 dB in equal, logarithmic increments of 0.5 to 1 dB would require between 60 and 120 separate steps. Although having such large number of steps, existing digital potentiometers can not provide a uniform incremental gain change, because none meet the requirement 4. 
     The trade-off appears to be between good microphone amplifier performance with non-uniform gain change in equal increments in dB, or using an analog circuit combined with an integral digital potentiometer, providing uniform incremental gain change in dB, but mediocre or poor microphone amplifier performance. 
     Yet another approach uses an appropriate MA with a discrete resistive ladder and switches in the feedback loop. The way to achieve a uniform step in dB is to utilize an appropriate MA and to optimize the resistor values in the feedback resistive ladder. This can only be accomplished with a discrete design, which may solve the noise problem by using an appropriate MA. However such an implementation would require a prohibitively high number of switches and precise resistors, one each per step: 60˜120. As a result, the discrete design is not practical and does not meet requirement 8. 
     Nevertheless, it is desirable to use discrete switches because the discrete approach offers a complete solution. Therefore an alternative solution using discrete switches is desirable if it can be made in a practical manner. 
     SUMMARY OF THE INVENTION 
     The invention reduces the number of switches and discrete components to a reasonable, i.e. practical number by adding a second amplifier to a high quality, analog microphone amplifier and splitting the gain control finction between the two stages. The circuit allows the gain of the first stage to change in relatively large (10 dB) increments and the gain of the second stage to change in more precise relatively small (1 dB) increments within each one of the larger sub-ranges set by the first stage. The circuit reduces by five times the number of switches necessary, and reduces digital controlling ports to one-fifth (20%) of the number of the steps. Also, each additional switch produces a three-fold (10 dB) increase in the gain range maintaining the same resolution (1 dB). 
     In accordance with the invention there is provided a remotely controlled microphone amplifier having a first amplifier stage with a variable gain circuit operatively connected in the overall feedback loop of the first stage for controlling the gain in first increments. A unity gain second amplifier stage has a variable attenuator operatively coupled between the output of the first amplifier stage and the input of the second amplifier stage. The variable attenuator controls the input to the second stage in second increments less than the first increments. The overall gain is linearly adjustable and varies in equal logarithmic increments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a microphone amplifier with digitally controlled gain according to the invention; 
     FIG. 2 is a schematic illustration of an exemplary variable resistance circuit for use in the circuit of FIG. 1; and 
     FIGS. 3A &amp; 3B illustrate a more detailed exemplary schematic diagram of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with the invention, there is provided a microphone amplifier  10  shown schematically in FIG.  1 . The circuit  10  comprises two major sections or stages  12  and  14 . The first stage  12  is a high quality, high-gain analog microphone amplifier incorporating low noise transistors Q 1  and Q 2  and operational amplifiers U 1  (non-inverting) and U 2  (inverting). The first stage  12  has a balanced differential input  16  which is the balanced input of U 1 . The two low noise transistors Q 1  and Q 2  on each of the inverting and non-inverting input of U 1  provide preamplification to keep the noise at the lowest possible level and to increase the overall loop gain. Transistors Q 1  and Q 2  work in a common emitter mode with separate current sources CS 1  and CS 2  to keep the respective collector currents constant The inverting amplifier U 2  in conjunction with non-inverting amplifier U 1  furnishes a balanced output signal to the second stage  14 , and at the same time provides equivalent driving conditions for both sides of a feedback circuit. In the exemplary embodiment, the feedback circuit comprises resistors R 5  and R 6  which extend from the output to the input and a parallel combination of a fixed resistor R 11  and a remotely controllable variable-resistance cell VRC 1  in the input circuit. In accordance with the invention, the variable-resistance cell VRC 1  determines the gain Af 1  of the first stage as follows: 
     
       
           Af   1 =(1+2 Rf   1 /( VRC   1 ∥ R   11 ))=(+2 Rf   1 /( VRC   1 * R   11 /( VRC   1 + R   11 ))), 
       
     
     where: 
     
       
           R   5 = R   6 = Rf   1 . 
       
     
     The gain can be made variable by changing the resistance of VRC 1  in the overall feedback circuit of the first stage  12  in an appropriate manner as hereinafter described. 
     The second stage  14  operates as an active attenuator with a unity gain buffer. The attenauator comprises input resistors R 9  and R 10  which are bridged by a parallel circuit comprising fixed resistor R 12  and remotely controllable variable resistance cell VRC 2 . The attenuator feeds the non-inverting operational amplifiers U 3  and U 4  which act as a buffer to the output. The attenuation ratio is: 
     
       
           Af   2 =(1/(2 Rf   2 /( VRC   2 ∥ R   12 )+1))=(1/(2 Rf   2 /(( VRC   2 * R   12 )/( VRC   2 + R   12 ))+1)) 
       
     
     where: 
     
       
           R   9 = R   10 = Rf   2 . 
       
     
     The attentuation ratio of the second stage  14  may be made variable by changing the resistance of the variable resistance cell VRC 2 . 
     The overall transfer ratio (gain and attenuation) of the microphone amplifier  10  with digitally controlled gain in accordance with the present invention is: 
     
       
           Af=Af   1 * Af   2   
       
     
     
       
           Af =(1+2 Rf   1 /( VRC   1 * R   11 /( VRC   1 + R   11 )))*(2 Rf   2 /( VRC   2 * R   12 /( VRC   2 + R   12 ))−1). 
       
     
     The VRC circuit shown in FIG. 2 employs parallel connected resistors Ra-Rx in series; with corresponding remotely controllable switches Sa-Sx. Control input lines Ca-Cx provide remote control codes or signals for the respective switches Sa-Sx. Both stages of the MA ( 10 ) from FIG. 1 use a variable resistance cells VRC 1  and VRC 2  with the same overall exemplary configuration. However, the resistor values (Ra-Rx) and control codes (Ca-Cx) are different. By selecting the appropriate resistor values and control codes, VRC 1  is capable of providing gains from 0 to 60 dB, controlled in equal increments of 10 dB; and VRC 2  produces an attenuation change of 0 dB to 9 dB in 1 db increments. As a result, the invention simultaneously maintains high MA performance and meets all of the previously noted requirements 1˜8. Other parameters include: 
     Balanced input (Input resistance Rin=2.2 k, CMRR=60 dB @ 1 kHz) 
     Nominal Input level −60 dBu, Nominal output level 0 dBu 
     Equivalent Input Noise (EIN)=−128.5 dBu 
     Gain range 0˜60 dB (doesn&#39;t affect the CMRR) 
     Number of steps N=60 
     Maximum output level (headroom)=+22 dBu 
     Balanced output 
     Total Harmonic Distortion (THD+N)=0.05% 
     In FIG. 2, the switches Sa . . . Sx are selectively operable by respective remote control inputs Ca . . . Cx to connect the corresponding parallel connected resistors Ra . . . Rx between the terminals  30 - 32 . Such selective operation provides a circuit having large number of combinations of resistance values which then provide a large number of increments of the gain or attenuation, as the case may be. The switches Sa-Sx may be solid state devices selectively operable by means of a corresponding input signal for each switch. 
     A schematic of an exemplary circuit is shown in FIGS. 3A-3B. The transistors Q 101  and Q 106  are the two low noise common emitter inputs. Q 102  and Q 105  are the corresponding constant current sources. Resistor R 107  is parallel connected with resistors R 108 -R 113  and corresponding series switches S 108 -S 113  to form variable resistance cell VRC 1 . Resistors R 104  and R 124  provide feedback for operational amplifiers U 101 A and U 101 B. The resistor R 104  is 51 KΩ and resistor R 107  is 100 KΩ. Resistors R 104  and R 107  together with resistors R 108 -R 113 , and corresponding switches S 108 -S 113 , responsive to respective control input lines C 108 -C 113 , produce a combined resistance of about 22 KΩ to about 22Ω resulting in a gain range of 0-60 dB in 10 db increments for the first stage  12 . 
     Resistors R 128  and R 143  are about 2.7 kΩ and parallel resistors R 130 -R 135  together with corresponding switches S 130 -S 135 , likewise responsive to control input lines C 130 -C 135 , result in an attenuation of 0-9 dB in 1 dB increments at the input of unity gain operational amplifiers U 103 A and U 103 B. 
     While there have been provided what are considered to be exemplary embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications therein may be made without departing from the invention, and it is intended in the appended claims to cover such changes and modifications as fall within the true spirit and scope of the invention.