Operational amplifier with class-AB+B output stage

1. An operational amplifier, comprising:

a Class-AB stage operable to drive an output load; and

a Class-B stage coupled in parallel with the Class-AB stage, the Class-B stage operable to drive the output load and comprising a first level-shifting circuit and a second level-shifting circuit, the first level-shifting circuit comprising a first neutralizing transistor, and the second level-shifting circuit comprising a second neutralizing transistor.

2. The operational amplifier of claim 1 , the first level-shifting circuit further comprising a first current source and a first level-shifting transistor, and the second level-shifting circuit further comprising a second current source and a second level-shifting transistor.

3. The operational amplifier of claim 2 ,

the first current source comprising a first P-channel transistor having a source coupled to a power supply,

the first level-shifting transistor comprising a second P-channel transistor having a source coupled to a drain of the first P-channel transistor, a gate coupled to the Class-AB stage, and a drain coupled to ground, and

the first neutralizing transistor comprising a first N-channel transistor having a drain coupled to the source of the second P-channel transistor, a gate coupled to the gate of the first P-channel transistor and a source coupled to the gate of the second P-channel transistor.

4. The operational amplifier of claim 3 ,

the second current source comprising a second N-channel transistor having a source coupled to ground,

the second level-shifting transistor comprising a third N-channel transistor having a source coupled to a drain of the second N-channel transistor, a gate coupled to the Class-AB stage, and a drain coupled to the power supply, and

the second neutralizing transistor comprising a third P-channel transistor having a drain coupled to the source of the third N-channel transistor, a gate coupled to the gate of the second N-channel transistor and a source coupled to the gate of the third N-channel transistor.

5. The operational amplifier of claim 4 , the Class-AB stage comprising an input circuit and an AB output circuit, the AB output circuit comprising a fourth P-channel transistor having a source coupled to the power supply and a gate coupled to the gate of the second P-channel transistor and a fourth N-channel transistor having a source coupled to ground, a gate coupled to the gate of the third N-channel transistor, and a drain coupled to a drain of the fourth P-channel transistor.

6. The operational amplifier of claim 5 , the Class-AB stage further comprising a first biasing circuit and a second biasing circuit.

7. The operational amplifier of claim 4 , the Class-B stage further comprising a B output circuit, the B output circuit comprising a fourth P-channel transistor having a source coupled to the power supply, a gate coupled to the drain of the first P-channel transistor, and a drain operable to be coupled to the output load and a fourth N-channel transistor having a source coupled to ground, a gate coupled to the drain of the second N-channel transistor, and a drain operable to be coupled to the output load.

8. The operational amplifier of claim 7 , the fourth P-channel transistor operable to turn on as an output voltage of the operational amplifier increases towards a voltage of the power supply and the fourth N-channel transistor operable to turn on as an output voltage of the operational amplifier decreases towards ground.

9. A mobile telephone, comprising:

a radio-frequency (RF) transceiver for transmitting an outgoing RF signal and receiving an incoming RF signal;

baseband processing circuitry for processing a forward channel baseband signal received from the RF transceiver and processing a reverse channel baseband signal to send to the RF transceiver; and

an operational amplifier capable of amplifying an analog signal in the mobile telephone, the operational amplifier comprising:

a Class-AB stage operable to drive an output load, and

a Class-B stage coupled in parallel with the Class-AB stage, the Class-B stage operable to drive the output load and comprising a first level-shifting circuit and a second level-shifting circuit, the first level-shifting circuit comprising a first neutralizing transistor, and the second level-shifting circuit comprising a second neutralizing transistor.

10. The mobile telephone of claim 9 , the first level-shifting circuit further comprising a first current source and a first level-shifting transistor, and the second level-shifting circuit further comprising a second current source and a second level-shifting transistor.

11. The mobile telephone of claim 10 ,

the first current source comprising a first P-channel transistor having a source coupled to a power supply,

the first level-shifting transistor comprising a second P-channel transistor having a source coupled to a drain of the first P-channel transistor, a gate coupled to the Class-AB stage, and a drain coupled to ground, and

the first neutralizing transistor comprising a first N-channel transistor having a drain coupled to the source of the second P-channel transistor, a gate coupled to the gate of the first P-channel transistor and a source coupled to the gate of the second P-channel transistor.

12. The mobile telephone of claim 11 ,

the second current source comprising a second N-channel transistor having a source coupled to ground,

the second level-shifting transistor comprising a third N-channel transistor having a source coupled to a drain of the second N-channel transistor, a gate coupled to the Class-AB stage, and a drain coupled to the power supply, and

the second neutralizing transistor comprising a third P-channel transistor having a drain coupled to the source of the third N-channel transistor, a gate coupled to the gate of the second N-channel transistor and a source coupled to the gate of the third N-channel transistor.

13. The mobile telephone of claim 12 , the Class-AB stage comprising an input circuit and an AB output circuit, the AB output circuit comprising a fourth P-channel transistor having a source coupled to the power supply and a gate coupled to the gate of the second P-channel transistor and a fourth N-channel transistor having a source coupled to ground, a gate coupled to the gate of the third N-channel transistor, and a drain coupled to a drain of the fourth P-channel transistor.

14. The mobile telephone of claim 13 , the Class-AB stage further comprising a first biasing circuit and a second biasing circuit.

15. The mobile telephone of claim 12 , the Class-B stage further comprising a B output circuit, the B output circuit comprising a fourth P-channel transistor having a source coupled to the power supply, a gate coupled to the drain of the first P-channel transistor, and a drain operable to be coupled to the output load and a fourth N-channel transistor having a source coupled to ground, a gate coupled to the drain of the second N-channel transistor, and a drain operable to be coupled to the output load.

16. The mobile telephone of claim 15 , the fourth P-channel transistor operable to turn on as an output voltage of the operational amplifier increases towards a voltage of the power supply and the fourth N-channel transistor operable to turn on as an output voltage of the operational amplifier decreases towards ground.

17. An operational amplifier comprising a Class-AB+B output stage, the Class-B output stage comprising:

a B output circuit operable to drive an output load;

a first level-shifting circuit coupled to the B output circuit, the first level-shifting circuit comprising a first neutralizing transistor; and

a second level-shifting circuit coupled to the B output circuit, the second level-shifting circuit comprising a second neutralizing transistor.

18. The operational amplifier of claim 17 , the first level-shifting circuit further comprising a first current source and a first level-shifting transistor, and the second level-shifting circuit further comprising a second current source and a second level-shifting transistor.

19. The operational amplifier of claim 18 ,

the first current source comprising a first P-channel transistor having a source coupled to a power supply, the first level-shifting transistor comprising a second P-channel transistor having a source coupled to a drain of the first P-channel transistor, a gate coupled to the Class-AB stage, and a drain coupled to ground, and the first neutralizing transistor comprising a first N-channel transistor having a drain coupled to the source of the second P-channel transistor, a gate coupled to the gate of the first P-channel transistor and a source coupled to the gate of the second P-channel transistor, and

the second current source comprising a second N-channel transistor having a source coupled to ground, the second level-shifting transistor comprising a third N-channel transistor having a source coupled to a drain of the second N-channel transistor, a gate coupled to the Class-AB stage, and a drain coupled to the power supply, and the second neutralizing transistor comprising a third P-channel transistor having a drain coupled to the source of the third N-channel transistor, a gate coupled to the gate of the second N-channel transistor and a source coupled to the gate of the third N-channel transistor.

20. The operational amplifier of claim 19 , the B output circuit comprising a fourth P-channel transistor having a source coupled to the power supply, a gate coupled to the drain of the first P-channel transistor, and a drain operable to be coupled to the output load and a fourth N-channel transistor having a source coupled to ground, a gate coupled to the drain of the second N-channel transistor, and a drain operable to be coupled to the output load.