Abstract:
An electronic amplifier delivers to a load an output signal related to an input, typically with increased power. As the power output, volume, or gain of the amplifier is changed, so may the spectral characteristics of the signal. In order to maintain the desired spectral or tonal character of the output signal over the dynamic range of output power, biasing of the amplifier must be adjusted. Particular ratios of drive and bias currents and/or voltages for different implementations of amplifier technologies should be relatively constant to produce substantially invariant input-output spectral relationships from low power output through high power output settings. Several techniques are presented which provide these relationship in amplifiers.

Description:
FIELD OF THE INVENTION 
     Embodiments of the present invention relate to amplifiers that retain the power amplifier spectral characteristics over a wide range of output power. 
     BACKGROUND OF THE INVENTION 
     An electronic amplifier (amplifier, amp) is an apparatus that enables an input electrical signal to control power from a source independent of the signal and thus is capable of delivering an output that bears some relationship to, and is generally greater than, the input signal. An amplifier may be designed for a specific purpose. For example, radio frequency (RF) amplifiers may convert low-power signals with frequencies generally in the portion of the electromagnetic spectrum between audio and infrared into a larger signal with more power, typically for driving the antenna of a transmitter. In another example, an audio amplifier may amplify audio signals (e.g., signals in the range of human hearing) to a suitable level (magnitude) for driving loudspeakers or other devices. A guitar amplifier is another example of an amplifier designed for a specific purpose. A guitar amplifier is designed to amplify the electrical signal of guitar or an acoustic pickup. 
     An amplifier may strive to reproduce the electromagnetic spectrum (e.g., spectral characteristics, frequencies, tone) of the input signal. Alternatively, the amplifier may alter the spectrum of the input signal. The output spectrum may depend on the output power level (magnitude). For example, a guitar amplifier may add effects such as distortion at high output power levels. A musician may find these effects desirable. However, guitar amplifiers may fail to reproduce the same effects at lower output power. For example, a musician using a guitar amplifier in a concert hall or arena setting with a high power output may desire to have the same effects at a lower output power while playing in a smaller room or location. Maintaining the relationship of input to output spectrum over the dynamic range (the ratio between the largest and smallest possible values) of output power may not be achievable with typical amplifiers. 
     In other examples, it may be desirable for audio amplifiers to faithfully reproduce the electromagnetic spectrum of the input signal at the output regardless of output power levels. Audio amplifiers that introduce distortion or other effects alter the original spectrum (e.g., sounds) which listeners may find objectionable. In this example, it is desirable to maintain the relationship of input spectrum to output spectrum over the dynamic range of output power without distortion. 
     Electronic amplifiers that maintain a desired relationship of input spectrum to output spectrum over the dynamic range of output powers alleviate the problem of power output dependent spectral variations. Thus, maintaining a spectral relationship between an input and output signal in an amplifier is a need felt by many users across multiple fields. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Embodiments of the present invention will be described with reference to the drawings, wherein like designations denote like elements, and: 
         FIG. 1  is a functional block diagram of an apparatus to amplify electrical signals in accordance with various aspects of the present invention; 
         FIG. 2  is a functional block diagram of the amplifier of  FIG. 1  for audio signals in accordance with various aspects of the present invention; 
         FIG. 3  is a schematic diagram of a circuit of a fixed bias power amplifier of the audio amplifier of  FIG. 1 ; 
         FIG. 4  is a schematic diagram of a circuit showing a voltage regulator biasing of the amplifier of  FIG. 2 ; 
         FIG. 5  is a schematic diagram of a circuit of an implementation of the voltage regular bias of  FIG. 4  according to various aspects of the present invention; 
         FIG. 6  is a schematic diagram of the circuit of  FIG. 4  showing a tracking control grid bias; 
         FIG. 7  is a schematic diagram of a circuit showing an implementation of a tracking control grid bias of  FIG. 6  according to various aspects of the present invention; 
         FIG. 8  is a schematic diagram of a circuit of  FIG. 6  showing a phase splitter bias; 
         FIG. 9  is a schematic diagram of a circuit of  FIG. 8  showing a control grid phase splitter bias according to various aspects of the present invention; and 
         FIG. 10  is a schematic diagram of  FIG. 8  showing a phase splitter with a voltage controlled differential amplifier according to various aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A signal is used to convey information. An electrical signal may be characterized by voltage (e.g., electromotive force), current (e.g., flow of electric charge), electromagnetic waves (e.g., spectrum, frequencies, wavelengths, tones), power (e.g., rate of transferring or transforming energy), and/or other quantities. As used herein, the term “signal” means an electrical signal that conveys information. 
     An amplifier boosts (e.g., enlarges, magnifies, increases, raises, gains) one or more characteristics (e.g., voltage, current, power) of one or more signals. Amplifiers may have unity gain (e.g., no amplification). Amplifiers may also attenuate a signal. Amplifiers may be designed for particular applications (e.g., guitar amplifier), frequencies ranges (e.g., audio amplifier, radio frequency amplifier), and/or to boost particular characteristics (e.g., current amplifier, voltage amplifier, differential amplifier, inverting amplifier, integrating amplifier). As used herein, the term “amplifier” or “amp” means any electrical or electronic equipment that amplifies one or more characteristics of a signal. 
     An amplifier may include a preamplifier (pre-amp or preamp), a phase splitter, a power amplifier, and a power supply ( FIG. 1 ). An amplifier may also include a tone stack and a transformer ( FIG. 2 ). A pre-amp may electrically couple (e.g., establish an electrical connection, establish a path for current to flow) to a low-level input signal (source). The components of an amplifier may be contained in a single enclosure (e.g., housing, box, assembly, case). The components may be contained in multiple enclosures with any combination of components in each enclosure. 
     A pre-amp may present a suitable impedance (e.g., matched impedance) to a signal source. A pre-amp may provide gain (e.g., amplification) to the input signal to produce a signal suitable for further processing. A user interface may be provided to a user to adjust the gain of the pre-amp. A pre-amp may provide suitable output impedance to a component for further processing. A pre-amp may provide equalization and/or mixing of the input signal. A pre-amp may produce distortion in a signal. A pre-amp may contain any combination of conventional circuit elements (e.g., electron tubes, semiconductors, integrated circuits, transistors, resistors, capacitors, inductors, transformers) to perform these functions. A pre-amp may be omitted (e.g., left out, not included) from an amplifier if its function is performed by another component or if its function is not required. 
     A phase splitter may electrically couple to a pre-amp to provide further processing. A phase splitter may produce one or more signals from an input signal which differ in phase (e.g., different polarities, quadrature signals) from one another. A phase splitter may provide a suitable impedance for an input and/or an output circuit coupled to the phase splitter. For example, a phase splitter may separate an input signal into two signals with opposite polarities for further processing by a push-pull amplifier circuit. In another example, a phase splitter may produce a single signal for further processing by single phase amplifier. A phase splitter may be omitted from an amplifier if only a single phase of the signal is required for further processing. A phase splitter may contain any combination of conventional circuit elements to perform these functions. 
     A power amplifier amplifies an input signal to a sufficient power level (magnitude) to drive a load (e.g., utilization device, antenna, loudspeaker, circuit that consumes electric power). A load may be one or more devices. A device that provides additional processing may be a load. A power amplifier may be the last stage in an amplifier before a load. A user interface may be provided to a user to adjust an amount of power output by the power amplifier. A power amplifier may employ any class of operation or service (e.g., Class A, Class AB, Class C, Class D). The signal output by the power amplifier may include distortion (e.g., harmonic distortion, crossover distortion). The output of the power amplifier may be proportional to the input signal (e.g., linear). There may be a non-linear relationship between the output of the power amplifier and the signal input to the power amplifier. A power amplifier may contain any combination of conventional circuit elements to perform these functions. 
     A power supply includes a supply of energy. Energy may be used for enabling the operation of electronic circuits (e.g., devices) such as an amplifier, processing circuit, and/or a user interface. A power supply may include any conventional component for providing energy such as a battery, a transformer that transforms line power, and/or a capacitor. A power supply may store energy for providing energy. Energy from a power supply may be used as a force (e.g., voltage, current) for an amplifier as discussed herein. 
     Tone refers to the pitch, quality, and strength of musical or vocal sounds. A tone stack may output a processed input signal that has been modified in accordance with a user interface. A tone stack may provide a user interface to adjust a frequency response (e.g., the quantitative measure of the output spectrum) of an input signal. A tone stack may adjust timbre (e.g., tone color, tone quality) of an audio signal. The user interface may provide for user adjustment of treble (e.g., tones at the higher range of human hearing), bass (e.g., tones at the lower frequency or range of human hearing), and/or middle (e.g., tones at the midrange of human hearing). A tone stack may allow a user to control equalization, reverberation, and/or mixing of the input signal. A tone stack may provide an effects output port (e.g., connection, socket, plug) and an effects input port for an external device to connect. The external device may provide modification (e.g., additional effects) to the signal from the effects output port and return the modified signal to the effects input port of the tone stack. A tone stack may contain any combination of conventional circuit elements to perform these functions. 
     A transformer may provide impedance matching of an amplifier output to an impedance of a load. A transformer may provide galvanic isolation (e.g., blocking of direct current). A transformer may provide alternating current restoration (e.g., converting direct current in a transformer primary winding to alternating current in the transformer secondary winding). Impedance matching may maximize the power transfer from an amplifier to a load. Impedance matching may minimize a signal reflection from a load. A transformer may provide a center tap for connecting to a bias voltage. A transformer may provide connections to winding ends to accept output signals from a power amplifier. A transformer may contain any combination of conventional circuit elements to perform these function. 
     A user interface may include electronic devices (e.g., switches, push buttons, touch screen, potentiometers, rheostats, wireless transceiver, remote controls) for receiving information (e.g., data) from a user. A user may manually manipulate one or more electronic devices of a user interface to provide information. Electronic devices for receiving information from a user may include a wireless receiver that receives information from an electronic device (e.g., smartphone, tablet, watch). A user may manually provide information to a user interface via an electronic device. A user interface may include electronic devices for providing information to a user. A user may receive visual and/or auditory information from a user interface. A user may receive visual information via devices (e.g., LCDs, LEDs, light sources, graphical and/or textual display) that display information. A user interface may include a wireless transmitter for transmitting information to an electronic device for presentation to a user. 
     For example, amplifier  100 , shown in  FIG. 1 , includes pre-amp  120 , phase splitter  140 , power amplifier  150 , and power supply  160 . Pre-amp  120  electrically couples to an input signal and provides the functions of a pre-amp as described above. Pre-amp  120  processes the input signal for further processing by phase splitter  140 . The input of phase splitter  140  electrically couples to the output of pre-amp  120 . Phase splitter  140  provides the functions of a phase splitter as described above. The output of phase splitter  140  electrically couples to the input of power amplifier  150 . Power amplifier  150  provides the functions of a power amplifier as described above. Power amplifier  150  electrically couples to a load. Power supply  160  provides the energy required by pre-amp  120 , phase splitter  140 , and power amplifier  150 . A user interface (not shown) may provide the user a means for controlling the amount of power output from amplifier  100 . A user interface may provide the user with a means of controlling other characteristics and/or functions of amplifier  100 . 
     In another example, amplifier  200 , shown in  FIG. 2  includes input port  210 , pre-amp  220 , tone stack  230 , phase splitter  240 , power amplifier  250 , transformer  260 , loudspeaker  270 , and power supply  280 . Input port  210  may provide an electrical connection for an input signal and couples that signal to an input of pre-amp  220 . A gain (e.g., amplification, boost, volume, increase in power) of pre-amp  220  may be set via a user interface. Pre-amp  220  performs the functions of a pre-amp on the input signal as described above. Tone stack  230  couples to pre-amp  220  and takes as an input the signal output by pre-amp  220 . A user may adjust (e.g., modify, alter) the tonal qualities (e.g., timbre, bass, treble, midrange, reverberation) of the signal processed by tone stack  230  via a user interface. Tone stack  230  performs the function of a tone stack as described above and outputs a signal for phase splitter  240 . Phase splitter  240  couples to tone stack  230  and performs the function of a phase splitter as described above. Phase splitter  240  may separate a signal into one or more phases to be processed by power amplifier  250 . 
     Power amplifier  250  couples to phase splitter  240  and receives the signal output by phase splitter  240 . The output power of the signal from power amplifier  250  may be controlled through a user interface. Power amplifier  250  may provide distortion (e.g., harmonics, crossover) to the signal. Power amplifier  250  performs the function of a power amplifier as described above. 
     Power amplifier  250  may be a push pull amplifier which has an output stage that can drive a current in either direction through a load. The output stage of a typical push pull amplifier may include at least one electron tube (e.g., vacuum tube, receiving tube, gas tube). Electron tubes for amplifiers may be conventional amplifier tubes (e.g., triode, tetrodes, pentodes). The output stage may include at least one semiconductor device (e.g., transistor, BJT, FET). Bipolar junction transistors (BJTs or bipolar transistors) are devices that rely on the contact of types of semiconductor (e.g., PNP, NPN) for its operation. Field-effect transistors (FETs) use an electric field to control the shape and therefore the conductivity of a channel of one type of charge carrier in a semiconductor. FETs may be junction field-effect transistors (JFETs), metal oxide semiconductors (MOSFETs) or any other conventional FET transistor. 
     A push pull amplifier may operate in a particular class of service (e.g., Class A, Class B, Class AB) with any of the devices described above (e.g., electron tube, BJT, FET). The class of service may be changed by altering the bias parameters of a device. 
     Transformer  260  couples to, and receives a signal from, power amplifier  250 . Transformer  260  provides a matching impedance to loudspeaker  270 . Transformer  260  provides the function of a transformer as described above. Transformer  260  may provide galvanic isolation. Transformer  260  may provide alternating current restoration. 
     Power supply  280  provides a source of energy for pre-amp  220 , tone stack  230 , phase splitter  240 , power amplifier  250 , and transformer  260 . Power supply  280  performs the function of a power supply as described above. 
     The components of amplifier  200  may be contained within a single housing (e.g., enclosure, cabinet). A plurality of housings may contain any combination of components, each housing electrically coupled to another housing to provide the electrical connections between components described above. 
     Power amplifier  302  in  FIG. 3  provides an example of a push pull amplifier performing the functions of power amplifier  250 . The output stage in this example uses two pentode electron tubes, tubes  310  and  314 . The suppressor grids of tubes  310  and  314  are connected to their respective cathodes which are in turn connected to the circuit ground. The suppressor grids may be connected to a biasing circuit with any combination of resistors, capacitors, diodes, or other conventional circuit elements. The heater connections to a power supply are not shown. VB+ provides a fixed voltage through a center tap of transformer  360  to the plates of tubes  310  and  314 . VB+ also provides a fixed voltage, filtered through RFLTR and CFLTR, with current limited by RSG 1  and RSG 2 , to the screen grids of tubes  310  and  314 , respectively. Phase splitter  340  provides two signals to the input of power amplifier  302 . The input signals are filtered by CDRV 1  and RDRV 1 , which also provides AC (alternating current) coupling (e.g., capacitive coupling, blocking of direct current signals), to tube  310  and biased by fixed voltage Vbias through RCG 1  and RDRV 1 . Similarly, CDRV 2  and RDRV 2  provides filtering and AC coupling, and RCG 2  and RDRV 2  with Vbias provides biasing for tube  314 . The biasing in this example is set (e.g., predetermined, established) by a circuit designer. 
     For push pull Class AB 1  operation, the plate voltage, screen voltage and total zero signal plate current must be maintained in accordance with tube  310  and  314  specifications. As an example, with a 6BQ5 (EL84) power amplifier pentode for tubes  310  and  314 , plate and screen grid voltages are 300 volts and total zero signal plate (quiescent) current is 36 mA (milliamperes) per tube according to the manufacturer&#39;s specifications (e.g. data sheet, application note). The voltages and current may be different for other tube selections. As the power output or amplifier gains changes, tubes  310  and  314  may not remain within the design specifications for push pull Class AB 1  operation. 
     In an embodiment of the present invention, power amplifier  402  in  FIG. 4  shows the use of a voltage regulator to supply a regulated voltage to the screen grids of tubes  310  and  314 . Regulator  420  may supply a predetermined voltage. Rcontrol may adjust the output voltage of regulator  420  which, in turn, controls the output power. Regulator  420  may automatically maintain a voltage level (typically within a ±5% output voltage tolerance) and thus may reduce unwanted voltage variations (e.g., ripple, spiking). Voltage regulator  420  may be implemented with a non-linear regulator. A linear regulator may be used to implement the functions of regulator  420 . Any combination of conventional circuit components may be used to perform the functions of voltage regulator  420 . 
     In other embodiments of the present invention, the voltage regulator may supply a regulated voltage to the plates of tubes  310  and  314 . The voltage regulator may supply a regulated voltage to the screen grids and plates of tubes  310  and  314 . 
     Regulator  500  in  FIG. 5  provides an example circuit of voltage regulator  420 . A convention linear voltage regulator (e.g., Microchip LR8 high input voltage, adjustable 3-terminal linear regulator) may be used for regulator U 51 . Unregulated input power is supplied to the IN connection. The regulated output voltage is provided at the OUT connection. The ratio of R 52  and R 53  determines the output voltage level. The output voltage may be controlled via a variable resistance between the CONTROL connection and circuit ground. Thus, the CONTROL input determines the output power of the power amplifier. Bypass transistor Q 41  boosts the current available through regulator  500 . 
     In another embodiment of the present invention, power amplifier  602  in  FIG. 6  includes control grid (CG) bias  630 . CG bias  630  may follow a control law in which the output voltage is determined by the voltage at the input at any given instant. The control law relationship may be linear or may be non-linear. CG bias  630  maintains a relationship between a screen grid voltage and a control grid voltage on tubes  310  and  314  while a constant plate voltage may be maintained. The tonal characteristics of an output audio signal are thus substantially invariant (e.g., relatively constant, little or no change) when the screen grid voltage changes because the bias voltage changes in a control law relationship to the screen grid voltage. By controlling the CG bias voltage, the zero-signal (quiescent) plate current can be maintained according to the tube specifications. 
     As used herein, “substantially invariant” and “relatively invariant” when used with tonal characteristics or input-output spectral relationships means that any change in the frequency composition of the output signal is imperceptible (e.g., unnoticeable, undetectable, indistinguishable, indiscernible) to an ordinary person listening to the sounds produced through a loudspeaker or that any change in frequency composition does not alter a result of any further processing of the output signal. 
     CG bias  700  in  FIG. 7  is an example of a circuit to perform the function of CG bias  630 . Operational amplifier (OPAMP) U 701  provides an inverted (e.g., opposite polarity) signal proportional to fixed reference voltage V 715 . The ratio of impedances R 702  to R 707  determines the gain of OPAMP U 701 . OPAMP U 702  produces a signal proportional to the difference of an IN signal and the output from OPAMP U 701 . In general, the output of OPAMP U 702  may be the product of the impedance value of R 704  with the sum of IN divided by the value of impedance R 709  and the output voltage of OPAMP U 701  divided by the impedance of R 710 . The resulting output of OPAMP U 702  follows a control law relationship to the IN input signal (the screen grid voltage). The functions of an operation amplifier may be implemented with any combination of conventional electronic components. 
     If U 310  and U 314  are both 6BQ5 (EL84) electron tubes, for example, and the screen grid voltage changes from 300 volts to 100 volts while the plate voltage is maintained at 300 volts, the control grid voltage must decrease by 773 millivolts to maintain a zero signal (quiescent) plate current of 36 mA, as specified by the manufacturers for the particular class of service. Thus, if a linear relationship is assumed, CG bias  700  produces the following relationship:
 
 V   CGB =−0.053135 V   SGB +4.54 Volts  Equation 1:
 
     where V CGB  is the control grid bias voltage (output) and V SGB  is the screen grid bias voltage (input). 
     The values of fixed reference voltage V 715  and impedances R 702 , R 704 , R 707 , R 709  and R 710  may be appropriately selected to achieve the relationship in Equation 1 in this example. Capacitors C 702  and C 704  may be included to provide low-pass filtering or may be omitted from CG bias  700 . 
     In another embodiment of the present invention, circuit  800  in  FIG. 8  includes a voltage controlled differential amplifier (VCDA) in phase splitter  840 . The VCDA is controlled by control law amplifier  850 . The combination of amplifier  850  and VCDA may prevent the loss of dynamic range of output power due to the control grid voltage of tube  310  and/or tube  314  becoming positive with respect to the cathode. A change in the power amplifier class of service from Class AB 1  to Class AB 2  would result in an electron tube grid voltage becoming positive with respect to the cathode. Amplifier  850  produces a signal in relation to the control grid bias voltage of tubes  310  and  314  that, in turn, changes the drive level within phase splitter  840 . 
     An example of a circuit implementing a control law amplifier is shown in amplifier  900  in  FIG. 9 . OPAMP U 901  produces a voltage proportional to fixed reference voltage V 915 . The voltage is proportional to the ratio of the values of impedances R 902  to R 907 . OPAMP U 902  produces a voltage proportional to the difference in voltage between an IN input signal and an output of OPAMP U 901 . The values of impedances R 902 , R 904 , R 907 , R 909 , and R 910  determine the relationship of the voltage at output OUT to reference voltage V 915  and the IN input voltage. The functions of OPAMPs U 901  and U 902  may be implemented with any combination of conventional electronic circuit components. OPAMPS U 901  and U 902  may be implemented with integrated circuit operational amplifiers (e.g., Texas Instruments (TI) TL082, TI TL072, TI LF353). 
     An example of a circuit implementing a phase splitter with a VCDA is shown in phase splitter  1000  in  FIG. 10 . Transistors Q 1001  and Q 1002  form a differential amplifier controlled by input VGB. Input voltage VGB determines the current through Q 1002  and thus the cathode current in tubes U 1001  and U 1002 . In this example, tubes U 1001  and U 1002  are triode electron tubes arranged in a common cathode configuration (e.g., long-tailed pair, differential pair). Phase splitter  1000  produces two output signals, DRIVE-A and DRIVE-B, with opposite polarities (e.g., 180 degree phase difference) which may serve as inputs to power amplifiers  302 ,  402 ,  602 , and  802 . 
     The differential amplifier in phase splitter  1000  may be implemented with transistors as shown. The differential amplifier may be implemented with operational amplifiers. Any combination of conventional electronic components that performs the function of a VCDA may be used. 
     The functions of tubes U 1001  and U 1002  may be implemented with electron tubes. Transistors may be used to implement the functions of U 1001  and U 1002 . Operational amplifiers may be used to implement the functions of U 1001  and U 1002 . Any combination of conventional electronic components that perform the functions of U 1001  and U 1002  may be used. 
     Implementations of the present invention in an amplifier may include fixed or variable voltage regulator  420  of  FIG. 4 . Other implementations of the present invention may include control law amplifier  850  and a VCDA of phase splitter  840 . In still other implementations, an amplifier may include fixed or variable voltage regulator  420 , control law amplifier  630 , control law amplifier  850 , and a VCDA of phase splitter  840  of  FIGS. 4-10 . 
     The foregoing description discusses preferred embodiments of the present invention, which may be changed or modified without departing from the scope of the present invention as defined in the claims. Examples listed in parentheses may be used in the alternative or in any practical combination. As used in the specification and claims, the words ‘comprising’, ‘including’, and ‘having’ introduce an open ended statement of component structures and/or functions. In the specification and claims, the words ‘a’ and ‘an’ are used as indefinite articles meaning ‘one or more’. When a descriptive phrase includes a series of nouns and/or adjectives, each successive word is intended to modify the entire combination of words preceding it. For example, a black dog house is intended to mean a house for a black dog. While for the sake of clarity of description, several specific embodiments of the invention have been described, the scope of the invention is intended to be measured by the claims as set forth below. In the claims, the term “provided” is used to definitively identify an object that not a claimed element of the invention but an object that performs the function of a workpiece that cooperates with the claimed invention. For example, in the claim “an apparatus for aiming a provided barrel, the apparatus comprising: a housing, the barrel positioned in the housing”, the barrel is not a claimed element of the apparatus, but an object that cooperates with the “housing” of the “apparatus” by being positioned in the “housing”.