Class G
The Class G topology is a modification of another Class of amplifier (normally Class B or Class AB)to increase efficiency and reduce power dissipation. Class G takes advantage of the fact that musical and voice signals have a high crest factor with most of the signal content at lower amplitudes.
The Class G topology uses multiple power supplies, operating from the power rail that provides the optimum combination of headroom and power dissipation.The Class G topology improves amplifier efficiency by optimizing the power supply.
A Class G device uses a minimum of two different supply rails. The device operates from the lower supply until output headroom becomes an issue. At this point the device switches the output stage to the higher supply rail.
Once the output signal drops below a predetermined level, the device switches back to the lower rail. Power dissipation is greatly reduced for typical musical or voice sources. below figure illustrates
a simplified Class G implementation with a split supply Class AB output stage.
A MOSFET is used as a switch to change the supply rails from the lower voltage (LV) to the higher supply voltage (HV) for the output stage.There are several ways to control the switching of the supply rails. Feedback from the output stage may be used and/or the input and pre-amp stage may have the control. The control could have some time constant so that when a switch to the higher
rails occurs, there is some delay before the FETs turn back off and the output stage shifts back to the
lower supply rails. Th e speed of the change also will need to be controlled appropriately for minimal loss of power in the FETs.
Below figure shows an example of a music output signal.The dotted lines are the supply voltages for the output stage. When the output signal requires a higher supply voltage, the FETs are turned on and the supply voltage increases to the higher rail. Once the output signal falls below the internally-set threshold, and after some delay, the device switches back down to the lower rail, reducing power dissipation.
Designers must balance many trade-off s associated with Class G: selecting the proper number of supplies and the voltage difference between the supplies in order to optimize headroom at lower voltages, while minimizing power dissipation. Two different rails minimize the complexity of the power supplies, while providing sufficient voltage flexibility. Additional rails may reduce power dissipation further but at the cost of higher component count, complexity, and reliability. Another issue is the length of time the device operates from the higher rail. While operating from the higher supply rail, power dissipation increases. Switching back to the lower rail too early may result in distortion due to clipping, while remaining at the higher rail for an extended period of time will result in a degradation of efficiency.
An example of a Class G device is National Semiconductor’s LM48824 Class G headphone amplifier.Th e LM48824 amplifier operates from two voltage supplies (1.1V and 1.8V) generated by an integrated step-down (buck) regulator. When the audio output exceeds an internally-set threshold, buck converter output increases from 1.1V to 1.8V. When the audio signal falls below the required voltage rails for a set period of time, the buck converter output decreases back to 1.1V. Power dissipation is greatly reduced for typical musical or voice sources.above figure shows
how a musical output may look. Th e green line corresponding to HPVDD(HV) and HPVDD(LV) is the buck converter output. Th e green line corresponding to HPVSS(HV) and HPVSS(LV) is the inverting charge pump output.
Class G Efficiency
Class G efficiency depends largely on the source material (music or voice) and the characteristics
of the signal. With pure sine waves, depending on signal amplitude, there is no efficiency gain compared to a Class A, B, or AB amplifiers under the same conditions. If the amplitude remains at a level where the Class G device operates from its lower supply rail, then power dissipation does decrease compared to the other architectures that can only operate from a fixed, higher voltage supply.
For real-world Class G amplifiers, the maximum efficiency occurs when operating under the lowest
supply rails as opposed to operating at peak output power on the higher rails due to biasing conditions, current x voltage (I x V) loss, and IR losses in the FETs. Peak efficiency depends on the
supply voltage switch over threshold. A design that switches at a lower level improves efficiency when operating from the lower supply but causes the device to operate from the higher supply for a
longer period. Ideally, a Class G device operates for as long as possible from the lower supply, minimizing I x V losses incurred when operating at higher voltages. The power considerations must be balanced with maintaining proper headroom, so distortion due to clipping does not become an issue.
Extract From : national.com/powerwise |Texas Instruments
The Class G topology is a modification of another Class of amplifier (normally Class B or Class AB)to increase efficiency and reduce power dissipation. Class G takes advantage of the fact that musical and voice signals have a high crest factor with most of the signal content at lower amplitudes.
The Class G topology uses multiple power supplies, operating from the power rail that provides the optimum combination of headroom and power dissipation.The Class G topology improves amplifier efficiency by optimizing the power supply.
A Class G device uses a minimum of two different supply rails. The device operates from the lower supply until output headroom becomes an issue. At this point the device switches the output stage to the higher supply rail.
Once the output signal drops below a predetermined level, the device switches back to the lower rail. Power dissipation is greatly reduced for typical musical or voice sources. below figure illustrates
a simplified Class G implementation with a split supply Class AB output stage.
rails occurs, there is some delay before the FETs turn back off and the output stage shifts back to the
lower supply rails. Th e speed of the change also will need to be controlled appropriately for minimal loss of power in the FETs.
Below figure shows an example of a music output signal.The dotted lines are the supply voltages for the output stage. When the output signal requires a higher supply voltage, the FETs are turned on and the supply voltage increases to the higher rail. Once the output signal falls below the internally-set threshold, and after some delay, the device switches back down to the lower rail, reducing power dissipation.
An example of a Class G device is National Semiconductor’s LM48824 Class G headphone amplifier.Th e LM48824 amplifier operates from two voltage supplies (1.1V and 1.8V) generated by an integrated step-down (buck) regulator. When the audio output exceeds an internally-set threshold, buck converter output increases from 1.1V to 1.8V. When the audio signal falls below the required voltage rails for a set period of time, the buck converter output decreases back to 1.1V. Power dissipation is greatly reduced for typical musical or voice sources.above figure shows
how a musical output may look. Th e green line corresponding to HPVDD(HV) and HPVDD(LV) is the buck converter output. Th e green line corresponding to HPVSS(HV) and HPVSS(LV) is the inverting charge pump output.
Class G Efficiency
Class G efficiency depends largely on the source material (music or voice) and the characteristics
of the signal. With pure sine waves, depending on signal amplitude, there is no efficiency gain compared to a Class A, B, or AB amplifiers under the same conditions. If the amplitude remains at a level where the Class G device operates from its lower supply rail, then power dissipation does decrease compared to the other architectures that can only operate from a fixed, higher voltage supply.
For real-world Class G amplifiers, the maximum efficiency occurs when operating under the lowest
supply rails as opposed to operating at peak output power on the higher rails due to biasing conditions, current x voltage (I x V) loss, and IR losses in the FETs. Peak efficiency depends on the
supply voltage switch over threshold. A design that switches at a lower level improves efficiency when operating from the lower supply but causes the device to operate from the higher supply for a
longer period. Ideally, a Class G device operates for as long as possible from the lower supply, minimizing I x V losses incurred when operating at higher voltages. The power considerations must be balanced with maintaining proper headroom, so distortion due to clipping does not become an issue.
Extract From : national.com/powerwise |Texas Instruments
1 comment:
So the $64 question is... Do the 1970's Hitachi class G receivers touted as having 3dB's of headroom really supply short burst doubling of rated power output??? Does the SR-804 receiver rated at 50wpc really supply 100wpc short term??
Post a Comment