MOSFET Amplifier Design

　　Figure 1 shows the basic block diagram of an amplifier using the LM4702. The amplifier consists of three　building blocks， the LM4702， the bias stage， and the output stage （there are no protection circuits）。 Each　stage will be covered in detail. The power supply design will not be covered as a basic unregulated supply　consisting of a transformer， a bridge rectifier， with noise and reservoir capacitors is well known and　common.

　　The LM4702 is a high voltage driver that includes the input stage and voltage amplifier stage （VAS） of a　power amplifier. The LM4702， with feedback from the output stage， sets the gain and is externally　compensated to set the slew rate. The outputs of the LM4702 drive the top and bottom sides of the bias　and output stages. The LM4702 performance details are limited to basic recommendations on ranges for　gain， slew rate， and the component types that achieve the best performance.

　　The bias stage performs two functions. First， it sets the DC bias voltage and resulting bias current in the　output stage for Class A， AB or B operation. Second， it allows thermal compensation that maintains steady　bias current as the output stage devices vary in temperature. As will be shown， certain devices do not　need temperature compensation and the bias stage becomes as simple as a resistor. Other devices will　need thermal tracking and temperature compensation controlled by the bias stage.

　　The output stage is a basic Source-Follower stage using a single pair of complementary N-channel and Pchannel　transistors for simplicity. The same output stage design will be used for all devices listed in　Table 1.