Variable gain amplifier with gain-based compensation

1. A method for amplifying an analog signal read from a magnetic media disk, the method comprising:

receiving the analog signal from a read head configured to generate the analog signal based on digital data stored on the magnetic media disk;

determining a gain setting for a variable gain amplifier having a gain stage, an output stage, a compensation stage coupled to an output of the output stage, and a capacitor coupled between an output of the gain stage and an output of the compensation stage;

configuring, based on the gain setting, a gain of the variable gain amplifier by which to amplify a signal;

configuring, based on the gain setting, a gain of the compensation stage to alter an effective capacitance of the capacitor;

compensating, at the output of the gain stage, the variable gain amplifier with the effective capacitance of the capacitor; and

amplifying the signal at the configured gain with the compensated gain stage of the variable gain amplifier.

2. The method for amplifying the analog signal read from the magnetic media disk as recited in claim 1 , wherein:

the compensation stage of the variable gain amplifier includes a resistor network or a resistor-divider stage; and

configuring the gain of the resistor network or resistor-divider stage comprises altering a resistance of at least one variable resistor of the resistor network or the resistor-divider stage.

3. The method for amplifying the analog signal read from the magnetic media disk as recited in claim 1 , wherein:

configuring the gain of the variable gain amplifier comprises altering a resistance of a first variable resistor coupled between the output of variable gain amplifier and an input of the gain stage; and

configuring the gain of the compensation stage comprises altering a resistance of a second variable resistor coupled to at least one of the output of the compensation stage or the capacitor.

4. The method for amplifying the analog signal read from the magnetic media disk as recited in claim 3 , further comprising:

determining, based on the gain setting, a digital gain code of bits indicative of the gain setting;

converting, via a digital-to-analog converter, the digital gain code to an analog gain control signal; and

altering, based on the analog gain control signal, the resistance of one or more of the first variable resistor and the resistance of the second variable resistor.

5. The method for amplifying the analog signal read from the magnetic media disk as recited in claim 4 , wherein the respective resistances of both the first variable resistor and second variable resistor are varied based on the same analog gain control signal.

6. The method for amplifying the analog signal read from the magnetic media disk as recited in claim 4 , wherein varying the resistance of the second resistor based on the analog gain control signal is effective to provide a range of compensation capacitance values at the output of the gain stage that correspond approximately with a bit resolution of the digital gain code.

7. The method for amplifying the analog signal read from the magnetic media disk as recited in claim 1 , further comprising:

receiving, from an analog-to-digital converter, an indication of an amplitude of the analog amplified signal that is amplified by the variable gain amplifier; and

determining, based on the indication of the amplitude, the gain setting for the variable gain amplifier.

8. A data storage apparatus comprising:

a media disk configured to store digital data;

a read head configured to generate an analog signal based on the digital data stored on the media disk;

an analog front-end comprising:

a variable gain amplifier that includes (i) a gain stage configured to amplify the analog signal to provide an amplified analog signal, (ii) an output stage configured to drive an output of the variable gain amplifier, (iii) a compensation stage coupled to an output of the output stage, and (iii) a capacitor disposed between an output of the gain stage and an output of the compensation stage;

an automatic gain controller configured to:

determine a gain setting for the variable gain amplifier;

set, based on the gain setting, a gain of the variable gain amplifier by which to amplify the analog signal;

selectively set, based on the gain setting, a gain of the compensation stage to alter an effective capacitance of the capacitor; and

apply the effective capacitance to the output of the gain stage to compensate the variable gain amplifier;

an analog-to-digital converter configured to convert the amplified analog signal to a digital signal; and

a digital processor configured to decode, based on the digital signal, the digital data stored on the media disk.

9. The data storage apparatus as recited in claim 8 , wherein:

the compensation stage of the variable gain amplifier comprises a resistor network coupled between the output of the output stage and the capacitor that is coupled to the output of the gain stage; and

the automatic gain controller is further configured to selectively set the gain of the resistor network by altering a resistance provided by a variable resistor of the resistor network.

10. The data storage apparatus as recited in claim 9 , wherein the variable resistor is a first variable resistor and automatic gain controller is further configured to:

generate an analog gain control signal to set the gain of variable gain amplifier via a second variable resistor that governs the gain of the variable gain amplifier; and

apply the analog gain control signal to a gate of a transistor by which the first variable resistor is implemented to selectively set the gain of the resistor network.

11. The data storage apparatus as recited in claim 10 , wherein:

the automatic gain controller is further configured to generate the analog gain control signal based on a digital gain code; and

applying the analog gain control to the gate of the transistor by which the first variable resistor is implemented is effective to provide a range of compensation capacitance values at the output of the gain stage that correspond approximately with a bit resolution of the digital gain code.

12. The data storage apparatus as recited in claim 11 , wherein the bit resolution of the digital gain code is at least eight bits and the range of compensation capacitance values applied to the output of the gain stage include approximately 256 different compensation capacitance values.

13. The data storage apparatus as recited in claim 8 , wherein:

the gain stage of the variable gain amplifier is implemented as a first differential amplifier;

the compensation stage of the variable gain amplifier is implemented as a second differential amplifier; and

a differential output of the first differential amplifier is connected directly to a differential input of the second amplifier.

14. The data storage apparatus as recited in claim 13 , wherein a differential output of the second amplifier is connected directly to a differential output of the variable gain amplifier.

15. The data storage apparatus as recited in claim 8 , wherein the apparatus is implemented in whole or part as a hard disk drive, a network-attached storage device, a set-top box, or a gaming console.

16. A System-on-Chip (SoC) for processing signals read from a magnetic media disk, the SoC comprising:

an input configured to receive an analog signal from a magnetic media read head;

a variable gain amplifier configured to amplify the analog signal, the variable amplifier including:

a gain stage configured to amplify the analog signal to provide an amplified analog signal;

an output stage configured to drive a variable resistor by which gain of the variable gain amplifier is selectably configurable; and

a compensation stage having an output coupled to a variable capacitor that is coupled to an output of the gain stage;

an analog-to-digital converter configured to convert the amplified analog signal to a digital signal;

an output configured to transmit the digital signal; and

a gain controller configured to:

adjust, based on the digital signal, a gain setting of the variable gain amplifier via the variable resistor; and

adjust, based on the gain setting of the gain stage, a gain setting of the compensation stage to alter an effective capacitance of the variable capacitor; and

apply, to the output of the gain stage, the effective capacitance of the variable capacitor to compensate the variable gain amplifier.

17. The SoC for processing signals read from a magnetic media disk as recited in claim 16 , wherein:

the variable resistor is a first variable resistor by which gain of the variable gain amplifier is governed;

compensation stage comprises a resistor-divider stage that includes a second variable resistor by which gain of the resistor-divide stage is governed; and

the gain controller is configured to adjust the respective gain settings of the variable gain amplifier and the resistor-divider stage by varying respective resistances of the first and second variable resistors using a same analog control signal.

18. The SoC for processing signals read from a magnetic media disk as recited in claim 17 , wherein:

the gain controller is further configured to generate the analog gain control signal based on a digital gain code; and

adjusting the second variable resistor based on the analog control signal is effective to provide a range of different capacitance values at the output of the gain stage that correspond to a bit resolution of the digital gain code.

19. The SoC for processing signals read from a magnetic media disk as recited in claim 18 , wherein the digital gain code has a bit resolution of eight bits and the range of different capacitance values includes approximately 256 different capacitance values.

20. The SoC for processing signals read from a magnetic media disk as recited in claim 16 , wherein the System-on-Chip is implemented in whole or part as an analog front-end module, media drive controller, media drive read element, radio-frequency (RF) front-end, or communication transceiver.