AN-261| Application Note
Low Distortion Wideband Power Op Amp
Low Distortion Wideband Power Op Amp
The LH0101 is a power operational amplifier capable of delivering a high-current low-distortion output The device is conservatively rated at 2 amps continuous current A novel design technique is used to eliminate the crossover distortion often plaguing power op amps Additional features include a frequency response from DC to greater than 4 MHz Excellent DC performance is attained by using FET input devices and the unity gain frequency compensation has been performed internally Finally the device is hermetically sealed in a standard 8-pin TO-3 power package The initial LH0101 design goal was to develop an easy to use wideband operational amplifier capable of driving a variety of loads This requirement focused a major portion of the design effort in the power output stage where considerable emphasis was placed on eliminating crossover distortion Another consideration was to remove the ground connection typically associated with power amplifiers in order to ease the usage with single or dual power supply config
urations Our discussion is sectioned into three subtopics where the first details the LH0101 internal circuitry the second presents a variety of user product development design precautions and the third presents typical applications for the LH0101 supported by circuit diagrams TABLE I LH0101 Typical Performance Characteristics at 25 C Ambient g 15V Supply Parameter Conditions Value 2A 5 mV 50 pA 25 pA 1012X 200V mV g 12 5V g 11 6V g 11V 10 Vms 300 kHz 10 ns 2 ms 4 MHz 0 005%
National Semiconductor Application Note 261 July 1981
CIRCUIT TOPOLOGY The LH0101 consists of 3 essential stages an operational amplifier a buffer and a power output stage Selection of a BI-FET operational amplifier was prompted by a balance between the desired AC and DC performance This decision was made in order to take advantage of the high performance BI-FET series' slew rate settling time and low bias current characteristics The added feature of internal frequency compensation aided in making it an ideal amplifier upon which to build The zero-crossing distortion associated with high current and high frequency conditions is an age-old problem of class B and class AB power amplifiers In order to minimize the distortion at crossover the amplifier must maintain a low output impedance throughout zero crossing This requires the push-pull output transistors to smoothly alternate current sourcing and sinking duty during the crossover To obtain a low output resistance the output stage must constantly remain in the active region The usual approach is to incorporate
a class AB output stage similar to that shown in Figure 1 During no load conditions both output transistors are biased ON thus providing a low output resistance and hence eliminating crossover distortion Under rated current load conditions however a potential source of distortion can develop Take the case of an output at a positive voltage delivering the rated current to a load The increased base-to-emitter voltage of the drive transistor tends to bias the bottom transistor OFF
Output Current Input Offset Voltage Input Bias Current Input Offset Current Input Resistance Large Signal Voltage Gain Output Voltage Swing RL e 100X RL e 10X RL e 5X Slew Rate AV e a 1 Full Power Bandwidth AV e a 1 RL e 10X Small Signal Rise Time AV e a 1 RL e 10X Small Signal Settling Time to 0 01% VIN e 10V AV e a 1 Gain Bandwidth Harmonic Distortion f e 1 kHz PO e 1W RL e 10X AV e a 1 f e 20 kHz PO e 1W RL e 10X AV e a 1
TL H 6865 1
0 05%
FIGURE 1 Class AB Output Stage as a Possible Solution to Minimize Distortion
AN-261
C1995 National Semiconductor Corporation
TL H 6865
RRD-B30M115 Printed in U S A
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