Single supply direct drive amplifier
A driver amplifier operative from a single DC voltage supply, coupled directly to the output load without the need for DC coupling capacitors used for preventing DC reaching the output load. An onboard power supply generates a negative voltage rail that powers the output amplifiers, allowing driver amplifier operation from both positive and negative rails. Since the amplifiers can be biased at ground potential (0 volts), no significant DC voltage exists across the load and the need for DC coupling capacitors is eliminated.
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The present application is a continuation of U.S. patent application Ser. No. 11/039,386, filed Jan. 18, 2005, which is a continuation-in-part of U.S. patent application Ser. No. 10/056,994, filed Jan. 24, 2002, entitled SINGLE SUPPLY HEADPHONE DRIVER/CHARGE PUMP COMBINATION, both of which are incorporated herein by reference. The present application also claims priority to U.S. Provisional Patent Application No. 60/592,868, filed Jul. 29, 2004, entitled SINGLE SUPPLY DIRECT DRIVE AMPLIFIER, which is incorporated herein by reference.
FIELD OF THE INVENTIONThis invention relates generally to amplifier circuits and more particularly to single supply direct drive amplifier circuits.
BACKGROUND OF THE INVENTIONDirect drive amplifiers are used in a variety of applications. These include a host of applications where miniaturization is important, such as video and audio applications. The following background discussion focuses on prior art related to headphones, but the limitations described below are common to all prior art direct drive amplifiers. In particular, prior art direct drive amplifiers that operate from a single power supply require series output capacitors or other costly and space inefficient schemes.
PRIOR ART
PRIOR ART
Each headphone may be represented by a resistive headphone load to be driven by the incoming signals. Typical value for the resistive load of a headphone speaker is in 16 to 32 Ω (ohm) range.
PRIOR ART
As shown in PRIOR ART
In order to prevent high currents from flowing through the headphones and having the headphones in a continuously on state, direct current (DC) coupling capacitors such as 40 and 42 are inserted in series with the output of the amplifiers 32 and 34, in order to prevent a DC current from reaching the headphones. The DC coupling capacitors 40 and 42 act as a high pass filter preventing DC and very low frequency signals from reaching the headphones. In order to reproduce low frequency input signals into the 16-32 Ω (ohm) load of a typical headphone, the value of these DC coupling capacitors needs to be in the 100-470 μF (micro Farad) range. However, the physical size of a 100-470 μF capacitor is prohibitively large and prevents miniaturization of the headphone circuitry. The physical size and cost of these DC blocking capacitors 40 and 42 is of a greater importance in the design of portable equipment and therefore implementing an amplifier topology that either completely eliminates the DC blocking capacitors or reduces their value and size is desirable.
Returning to PRIOR ART
PRIOR ART
Therefore, it is desirable to provide a direct drive amplifier system that operates from a single voltage supply, and which does not require the usual large DC coupling capacitors or need the physical isolation of the common return of the amplifier.
BRIEF DESCRIPTION OF THE DRAWINGS PRIOR ART
PRIOR ART
PRIOR ART
PRIOR ART
Prior art amplifier driver systems for video and audio devices that operate from a single power supply require biasing the output at mid-range of the power supply in order to fully represent the incoming signal without the danger of any clipping. As a result, these prior art systems require that DC blocking capacitors be used in series with the amplifiers driving the output. The value and physical size of these DC coupling capacitors are prohibitively large and limit miniaturization, which is highly desired in most systems.
One aspect of the present invention allows for driver amplifier circuits that operate from a single voltage supply, without requiring the usual series coupling capacitors necessary for preventing DC current from reaching the output. An on-board power supply generates a negative voltage rail, which powers the output amplifiers, allowing driver amplifier operation from both positive and negative rails. In this way, the amplifier can be biased at ground (0 volts) potential, generating no significant DC voltage across the output load (speakers, video device, etc.).
The term “charge pump” refers to a type of DC voltage-to-voltage converter that uses capacitors, and in an alternative embodiment inductors, to store and transfer energy. One type of charge pump (also referred to as switched-capacitor converters) includes a switch/diode network that charges and discharges one or more capacitors. Alternatively, in implementing the present invention, a DC voltage-to-voltage converter may be used that includes an inductor.
The charge pump circuitry of the present invention generates a negative voltage rail −VDD with respect to ground, powering the output amplifiers and allowing driver amplifier operation from both positive and negative rails. Providing a negative voltage rail with respect to ground allows for the headphone amplifiers to be biased at ground voltage, allowing for the incoming signals to be amplified without clipping. As shown in
Returning to
The headphone amplifier circuit 45 allows for the headphone 10 to be biased at zero volts, operating between VDD and −VDD which in turn allows for the leads 50 and 52 of the respective headphone amplifiers 46 and 48 to directly couple the headphone speakers 10 to the headphone amplifiers 46 and 48 without the need for any DC coupling capacitors in series.
In the simple capacitor based IC charge pump circuitry 66, the switch network 74 and 76 toggles between charge and discharge states. An oscillator (OSC) 72 controls the two switches (74 and 76) that alternately charge a flying capacitor (C1) from an input voltage supplied by the amplifier 68 and 70, and discharge the flying capacitor (C1) into an output capacitor (C2). The voltage thus produced across the output capacitor C2 may be output as the output voltage (VOUT). Typically, the oscillator 72, the switches 74 and 76, and still other controls are all commonly contained in a single integrated circuit (IC).
The simple capacitor based IC charge pump circuitry 66 is of the inverting type, and it operates by lowering the potential of the charge in the flying capacitor C1 below ground, and then discharging the output capacitor C2 with this. The optimal result of this is an output voltage VOUT that is the negative of the input voltage.
One very common type of inverting charge pump operates in this way, but further includes an appreciable resistance in the charge path to the flying capacitor. The resistance intentionally introduces a delay in the charging of the flying capacitor, and appropriate control of the oscillator is then used to switch the charge before it is able to reach the full input voltage potential. This type of charge pump may accordingly transfer charge quanta having only one-half, two-thirds, etc. of the input voltage, and thereby produce an output voltage which is correspondingly lower than the input voltage. This type of step-down charge pump is probably overwhelmingly the most common today, but it is not the only type possible. Alternative circuit arrangements allow for the generation of an output voltage VOUT that is equal to some negative quanta of the input voltage.
The foregoing examples illustrate certain exemplary embodiments of the invention from which other embodiments, variations, and modifications will be apparent to those skilled in the art. As will be further appreciated, the circuit of the present invention is well suited for use in portable applications such as cellular telephones, digital cameras, portable computers, etc. The invention should therefore not be limited to the particular embodiments discussed above, but rather is defined by the following claims.
Claims
1. A circuit enabling a driver amplifier to operate from a single voltage supply comprising:
- an amplifier having an output arranged for coupling to an output load, said amplifier having first and second power supply leads, said first power supply lead connected to a power supply voltage, wherein the output of the amplifier is coupled to the output load without an intervening DC coupling capacitor;
- a charge pump DC voltage-to-voltage converter having an output, said DC voltage-to-voltage converter having a power source lead connected to the supply voltage, the output of said DC voltage-to-voltage converter connected to the second power supply lead, and said DC voltage-to-voltage converter generating an output voltage at the output that is substantially equal in magnitude to some negative quanta of the power supply voltage; and a shutdown circuit operable to control operation of said circuit.
2. The circuit of claim 1 connected to a common ground by two external capacitors in the range of 0.47 to 3.3 micro farads.
3. An amplifier circuitry for directly driving stereo headphones, said amplifier circuitry being driven by a single supply voltage VDD, said amplifier circuitry comprising:
- a first and a second amplifier, the first amplifier having an output coupled to a first headphone without any intervening DC coupling capacitor and the second amplifier having an output coupled to a second headphone without any intervening DC coupling capacitor, each of the first and second amplifier having a VDD power supply lead connected to a positive voltage supply VDD; and
- a charge pump circuitry output connected to a −VDD supply voltage of the first and second amplifier, wherein said charge pump circuitry output provides a voltage substantially equal in magnitude to a negative quanta of the VDD supply, said charge pump further having a power supply lead connected to the VDD supply voltage.
4. A circuit enabling a driver amplifier to operate from a single voltage supply comprising:
- an amplifier having an output arranged for coupling to an output load, said amplifier having first and second power supply leads, said first power supply lead connected to a power supply voltage, wherein the output of the amplifier is coupled to the output load without an intervening DC coupling capacitor;
- a charge pump having an output, said charge pump having a power source lead connected to the supply voltage, and said charge pump generating an output voltage at the output that is substantially equal in magnitude to some negative quanta of the power supply voltage; and
- a linear regulator having an input and an output, the output of the charge pump connected to the input of the linear regulator, and the output of the linear regulator coupled to the second power supply lead of the amplifier.
5. A driver amplifier circuit comprising:
- an amplifier having an output coupled to a load without an intervening DC coupling capacitor, the amplifier having a positive power lead and a negative power lead,
- a voltage supply coupled to the positive power lead, the voltage supply providing a positive voltage, and
- a charge pump coupled to the voltage supply, the charge pump having an output coupled to the negative power lead of the amplifier, the output of the charge pump being a negative voltage;
- wherein the negative voltage is substantially equal in magnitude to a quanta of the positive voltage.
6. A direct drive charge pump enabled stereo headphone system comprising the following formed on a single integrated circuit:
- a single power input for providing, internal to said integrated circuit;
- a VDD supply voltage originating external to said integrated circuit;
- a charge pump coupled to said single power input and operable to provide, internal to said integrated circuit,
- a voltage substantially equal in magnitude to a negative quanta of said external VDD supply;
- a first headphone amplifier power by both said external VDD supply and said voltage substantially equal in magnitude to the negative quanta of said external VDD supply, said first headphone amplifier having a first audio input driven by a first audio signal provided external to said integrated circuit, and a first audio output suitable for driving a stereo headphone without any intervening DC coupling capacitor;
- a second headphone amplifier powered by both said external VDD supply and said voltage substantially equal in magnitude to the negative quanta of said external VDD supply, said second headphone amplifier having a second audio input driven by a second audio signal provided external to said integrated circuit; and a second audio output suitable for driving said stereo headphone without any intervening DC coupling capacitor.
7. A direct drive charge pump enabled stereo headphone system as recited in claim 6, wherein said first audio input is an audio in for a right stereo headphone and said second audio input is an audio in for a left stereo headphone.
8. A direct drive charge pump enabled stereo headphone system as recited in claim 6, wherein said first audio input is coupled to an inverting terminal of said first headphone amplifier and said second audio input is coupled to an inverting terminal of said second headphone amplifier.
9. A direct drive charge pump enabled stereo headphone system as recited in claim 6, wherein a non-inverting terminal of said first headphone amplifier is coupled to a non-inverting terminal of said second headphone amplifier.
10. A direct drive charge pump enabled stereo headphone system as recited in claim 9, wherein said non-inverting terminal of said first headphone amplifier and said non-inverting terminal of said second headphone amplifier are coupled to ground.
11. A direct drive charge pump enabled stereo headphone system as recited in claim 6, further comprising:
- a first resistor coupled to said first audio input and an inverting terminal of said first headphone amplifier,
- a second resistor coupled to said first audio output and said inverting terminal of said first headphone amplifier,
- a third resistor coupled to said second audio input and an inverting terminal of said second headphone amplifier, and
- a fourth resistor coupled to said second audio output and said inverting terminal of said second headphone amplifier.
12. A direct drive charge pump enabled stereo headphone system as recited in claim 6, further comprising:
- a first capacitor having a first lead and a second lead, said first lead coupled to said charge pump and said second lead coupled to said charge pump, said first capacitor located off said integrated chip, and
- a second capacitor having a first lead and a second lead, said first lead coupled to said charge pump and said second lead couple to ground, said second capacitor located off said integrated chip.
13. A direct drive charge pump enabled stereo headphone system as recited in claim 6, further comprising a short circuit protection device.
14. A direct drive charge pump enabled stereo headphone system as recited in claim 6, further comprising a bias circuitry device.
15. A direct drive charge pump enabled stereo headphone system as recited in claim 6, further comprising a click/pop suppression device coupled to said first headphone amplifier and said second headphone amplifier.
16. A direct drive charge pump enabled stereo headphone system as recited in claim 6, further comprising a shutdown control device coupled to said charge pump.
17. A direct drive charge pump enabled stereo headphone system as recited in claim 15, further comprising a shutdown control device coupled to said charge pump and said click/pop suppression device.
18. A direct drive charge pump enabled stereo headphone system as recited in claim 6, further comprising:
- a first capacitor having a first lead and a second lead, said first lead coupled to a non-inverting terminal of said first headphone amplifier and said second lead coupled to said first audio input, said first capacitor located off said integrated chip, and
- a second capacitor having a first lead and a second lead, said first lead coupled to a non-inverting terminal of said second headphone amplifier and said second lead coupled to said second audio input, said second capacitor located off said integrated chip.
Type: Application
Filed: Feb 26, 2007
Publication Date: Oct 4, 2007
Applicant:
Inventors: Tony Doy (Sunnyvale, CA), Ronald Koo (Mountain View, CA)
Application Number: 11/711,480
International Classification: H03F 3/04 (20060101);