Many electronic circuits use amplifiers or buffers (e.g., unity gain amplifiers) to manipulate or maintain various signals within the circuit. The output of an amplifier or buffer stage is typically connected to provide an output voltage to a load circuit, or to provide circuit isolation via the stage's high input impedance and low output impedance. The design of an output stage may affect various operating aspects of the amplifier or buffer. For example, some amplifiers can deliver a high output current to the load, while others are designed to produce an output voltage swing approximately equal to the magnitude of the circuit's power supply. In other examples, amplifiers are designed to provide output signals with low crossover distortion. In still other examples, amplifiers must maintain gain and stability at relatively high frequencies.
The various operational requirements place constraints upon the design of the output stage. One of the most common output stage designs is that of the class AB amplifier. These output stages are used in a variety of applications including, for example, audio amplifiers and flat-panel display driver circuits. FIG. 1 illustrates a prior art class AB output stage. The output stage of class-AB amplifiers operates by using a current source and a current sink, here complimentary MOSFET transistors M1 and M5. This configuration is sometimes referred to as a “push-pull” configuration since a first branch of the output stage pushes or sources currents to a load while a second branch of the output stage pulls or sinks current from the load. Class-AB amplifiers avoid the high power consumption of a class-A amplifier by always having one output branch substantially turn off when the other output branch is turned on.
The input signal to this stage is applied at nodes g1 and g5, where the signal values rise and fall in the same direction, i.e., the nodes are the same phase. If the output node is accidentally shorted to ground, PMOS transistor M1 sources a great deal of current while NMOS transistor M5 turns off. If, instead, the output node is shorted to the power supply rail VDD, NMOS transistor M5 sinks a great deal of current and PMOS transistor M1 turns off. Such shorts can damage or destroy the output stage itself, and/or other circuitry associated with the output stage. Although shorts to VDD or ground are not common during normal operation of the circuit, they can occur during manufacture or testing of devices using the circuit, such as flat-panel displays including one or more integrated circuits with such output stages.
Accordingly, it is desirable to have protection circuitry for output stages such as the class AB output stage of FIG. 1, thereby reducing the likelihood that the circuit is damaged because an output node is shorted to ground or to the power rail.