Circuit, system and method for isolating a transducer from an amplifier in an electronic device

1. An electronic circuit for an output signal of an amplifier in an electronic device, comprising:

a speaker;

a digital to analog converter (DAC);

a microprocessor;

a memory device storing instructions operable on the microprocessor to

sum a first output signal from a source input signal with a volume signal for a volume control signal for the source input signal to produce a resultant signal;

boost the resultant signal to a boosted signal to use available headroom in the DAC, the boosted signal being within a threshold of a maximum digital input level for the DAC; and

provide the boosted signal to the DAC;

an amplifier connected to an output of the DAC; and

an interface circuit connected to an output of the amplifier and connected to the speaker, the interface circuit attenuating the output of the DAC to a second output signal representing a resultant volume level for the volume control signal, the interface circuit comprising

a first resistor coupled to the output of the amplifier in series, the first resistor having a resistance value Rs that limits current provided to the speaker from the electronic circuit;

a second resistor coupled to the first resistor in series, the second resistor having a resistance value Rp to reduce a load impedance sensitivity for the speaker, where the Rs and Rp values are related by an inversely proportional relationship; and

an output terminal for the speaker connected to the electronic device, the output terminal connected in parallel to the second resistor and providing the second output signal,

wherein

the second output signal has a noise floor that varies with an output level of the amplifier.

2. The electronic circuit of claim 1 , wherein:

Rp=Rs *α/(1−α), where α is a non-zero constant.

3. The electronic circuit of claim 2 , wherein a value for a relates to a preset attenuation value for the electronic circuit.

4. The electronic circuit of claim 1 , wherein:

Rp=Rs *α/(1−α), where α is a non-zero variable.

5. The electronic circuit of claim 1 , wherein

the attenuation=−20 log(α), where α is a non-zero value.

6. The electronic circuit of claim 1 , wherein:

Rs is set between approximately 10Ω and 32Ω; and

Rp is set between approximately 39Ω and 10Ω.

7. The electronic circuit of claim 1 , wherein:

Rs=Rx /α; and

Rp=Rx /(1−α),

where Rx is a reference series resistance used for the electronic circuit with no attenuation and α is an attenuation value.

8. The electronic circuit of claim 1 , wherein:

the speaker is in a headset; and

the electronic circuit is provided in a communication device.

9. An electronic device, comprising:

a microprocessor;

a speaker;

a digital to analog converter (DAC);

an amplifier connected to an output of the DAC;

a memory device storing instructions operable on the microprocessor to

sum a first output signal from a source input signal with a volume signal for a volume control signal for the source input signal to produce a resultant signal;

boost the resultant signal to a boosted signal to use available headroom in the DAC, the boosted signal being within a threshold of a maximum digital input level for the DAC;

provide the boosted signal to the DAC; and

program resistance values for a first programmable resistor and a second programmable resistor;

an interface circuit connected to an output of the amplifier and connected to the speaker, the interface circuit attenuating an output of the DAC to a second output signal representing a resultant volume level for the volume control signal, the interface circuit comprising

the first programmable resistor coupled to the output of the amplifier in series, the first programmable resistor having a resistance value Rs that limits current provided to the speaker; and

the second programmable resistor coupled to the first resistor in series, the second programmable resistor having a resistance value Rp to reduce a load impedance sensitivity for the speaker;

and

a terminal for the speaker, the terminal connected in parallel with a circuit containing the second programmable resistor,

wherein

the values Rs and Rp are related by an inversely proportional relationship; and

the second output signal at the terminal has a noise floor that varies with an output level of the amplifier.

10. The electronic device of claim 9 , further comprising:

a programmable voltage divider circuit connected in series to an input terminal of the amplifier.

11. The electronic device as claimed in claim 9 , wherein:

the output signal of the electronic circuit has a noise floor with at least approximately a 30 dB gain range.

12. The electronic device as claimed in claim 9 , further comprising:

a digital to analog converter (DAC) connected to the electronic circuit to boosting a digital value for the output signal to a boosted value to utilize available signal headroom in the signal.

13. A method for controlling an output signal of an electronic circuit in an electronic device, comprising:

summing a first output signal from a source input signal with a volume signal for a volume control signal for the source input signal to produce a resultant signal;

boosting the resultant signal to a boosted signal to use available headroom in a digital to analog converter (DAC), the boosted signal being within a threshold of a maximum digital input level for the DAC;

providing the boosted signal to the DAC;

providing an output of the DAC to an amplifier; and

attenuating the boosted output signal through an interface circuit connected to the output of the amplifier and connected to a speaker, the interface circuit attenuating the output of the DAC to a second output signal representing a resultant volume level for the volume control signal, the interface circuit comprising

a first resistor coupled to the output of the amplifier in series, the first resistor having a resistance value Rs that limits current provided to the speaker from the electronic circuit;

a second resistor coupled to the first resistor in series, the second resistor having a resistance value Rp to reduce a load impedance sensitivity for the speaker, where the Rs and Rp values are related by an inversely proportional relationship; and

an output terminal for the speaker connected to the electronic device, the output terminal connected in parallel to the second resistor and providing the second output signal,

wherein

the second output signal of the electronic circuit has a noise floor that varies with an output level of the amplifier.

14. The method for controlling an output signal of an electronic circuit as claimed in claim 13 , further comprising:

utilizing a voltage divider circuit connected in series to an input terminal of the amplifier to provide an input attenuation stage for the electronic circuit;

providing a first value to the voltage divider when the electronic circuit is producing an output above a first threshold; and

providing a value for the second resistor when the electronic circuit is producing an output below the first threshold.

15. The method for controlling an output signal of an electronic circuit as claimed in claim 13 , wherein:

an output signal at the output terminal has a dynamic range of at least approximately 90 dB.

16. A portable audio system for an electronic device, the portable audio system comprising:

a speaker;

a digital to analog converter (DAC);

a microprocessor;

a memory device storing instructions operable on the microprocessor to

sum a first output signal from a first source input signal with a first volume signal for a first volume control signal for the first source input signal to produce a first summed signal;

sum a second output signal from a second source input signal with a second volume signal for a second volume control signal for the second source input signal to produce a second summed signal;

sum the first summed signal with the second summed signal to produce a resultant signal;

boost the resultant signal to a boosted signal to use available headroom in the DAC, the boosted signal being within a threshold of a maximum digital input level for the DAC; and

provide the boosted signal to the DAC;

an amplifier connected to an output of the DAC; and

an interface circuit connected to an output of the amplifier and connected to the speaker, the interface circuit attenuating the output of the DAC to a third output signal, the interface circuit comprising

a first resistor coupled to an output of the amplifier in series, the first resistor having a resistance value Rs that limits current provided to the speaker; and

a second resistor coupled to the first resistor in series, the second resistor having a resistance value Rp to reduce a load impedance sensitivity for the speaker, where the Rs and Rp values are related by an inversely proportional relationship; and

an output terminal for the speaker connected to the electronic device, the output terminal connected in parallel to the second resistor and providing the third output signal,

wherein

the third output signal has a noise floor that varies with an output level of the amplifier.

17. The portable audio system as claimed in claim 16 , wherein:

the first source input signal is a music file;

the first volume signal is for a volume control for a headphone amplifier for the electronic device; and

the second source input signal relates to a telephone call being processed by the electronic device.