Circuit for reducing pop noise
The invention concerns an amplifier circuit comprising: an amplifier having a first input coupled to an input node of the amplifier circuit via a first resistor and an output coupled to a load via a coupling capacitor, the output being coupled to the first input via a second resistor; and a current ramp generator adapted to supply a current ramp to the first input of the amplifier during a power up phase or power down phase of the amplifier circuit to control the rate of charge or discharge of the coupling capacitor.
1. An amplifier circuit comprising:
an amplifier having a first input coupled to an input node of the amplifier circuit via a first resistor and an output coupled to a load via a coupling capacitor, the output being coupled to the first input via a second resistor; and
a current ramp generator adapted to supply a linear or non-linear current ramp to the first input of the amplifier during a power up phase of the amplifier circuit to control the rate of charge or discharge of the coupling capacitor, the linear or non-linear current ramp starting at a first level and falling to a second level.
2. The amplifier circuit of claim 1 , wherein during a power down phase of the amplifier circuit the current ramp generator is adapted to supply a further linear or non-linear current ramp to the first input of the amplifier, the further linear or non-linear current ramp starting at the second level and rising to the first level.
3. The amplifier circuit of claim 1 , wherein the first level is high enough to cause saturation of the amplifier.
4. The amplifier circuit of claim 1 , wherein the second level is equal to or less than 1 μA.
5. The amplifier circuit of claim 1 , wherein the linear or non-linear current ramp has a gradient that results in a voltage gradient at the output of the amplifier circuit of at least 1 V/s.
6. The amplifier circuit of claim 1 , wherein the current ramp generator is adapted to convert a first voltage ramp into the linear or non-linear current ramp.
7. The amplifier circuit of claim 6 , wherein the current ramp generator comprises a differential pair, a first of the transistors of the differential pair being controlled by the first voltage ramp.
8. The amplifier circuit of claim 7 , wherein a second of the transistors of the differential pair is controlled by a second voltage ramp having a gradient of opposite sign to the first voltage ramp.
9. The amplifier circuit of claim 1 , wherein the amplifier is coupled in an inverting amplifier configuration.
10. The amplifier circuit of claim 1 , wherein the input node of the amplifier circuit couples to a digital-to-analog converter (DAC) and receives an input signal from the DAC.
11. The amplifier circuit of claim 10 , wherein the current ramp generator is coupled between the first and second resistors.
12. An amplifier circuit comprising:
an amplifier having a first input coupled to an input node of the amplifier circuit via a first resistor and an output coupled to a load via a coupling capacitor, the output being coupled to the first input via a second resistor; and
a current ramp generator adapted to supply a linear or non-linear current ramp to the first input of the amplifier during a power down phase of the amplifier circuit to control the rate of charge or discharge of the coupling capacitor, the linear or non-linear current ramp starting at a second level and rising to a first level.
13. The amplifier circuit of claim 12 , wherein the first level is high enough to cause saturation of the amplifier.
14. The amplifier circuit of claim 12 , wherein the second level is equal to or less than 1 μA.
15. The amplifier circuit of claim 12 , wherein the input node of the amplifier circuit couples to a digital-to-analog converter (DAC) and receives an input signal from the DAC.
16. The amplifier circuit of claim 15 , wherein the current ramp generator is coupled between the first and second resistors.
17. A method of powering up an amplifier circuit, comprising:
supplying a linear or non-linear current ramp to a first input of an amplifier of the amplifier circuit, the first input being coupled to an input node of the amplifier circuit via a first resistor an output of the amplifier being coupled to a load via a coupling capacitor and to the first input via a second resistor, wherein the linear or non-linear current ramp starts at a first level and falls to a second level and controls the rate of charge or discharge of the coupling capacitor; and
applying an input signal to the input node of the amplifier circuit.
18. The method of claim 17 , further comprising generating the linear or non-linear current ramp by a current ramp generator and generating the input signal by a digital-to-analog converter (DAC).
19. A method of powering down an amplifier circuit, comprising:
supplying a linear or non-linear current ramp to a first input of an amplifier of the amplifier circuit, the first input being coupled to an input node of the amplifier circuit via a first resistor an output of the amplifier being coupled to a load via a coupling capacitor and to the first input via a second resistor, wherein the linear or non-linear current ramp starts at a second level and rises to a first level and controls the rate of charge or discharge of the coupling capacitor; and
applying an input signal to the input node of the amplifier circuit.
20. The method of claim 19 , further comprising generating the linear or non-linear current ramp by a current ramp generator and generating the input signal by a digital-to-analog converter (DAC).