LINEAR Application Note 18: Power Gain Stages for Monolithic Amplifiers

Application Note 18 March 1986

Power Gain Stages for Monolithic Amplifiers Jim Williams

Most monolithic amplifiers cannot supply more than a few 150m A Output Stage hundred milliwatts of output power. Standard IC processing

Figure 1a shows the LT®1010 monolithic 150m A current

techniques set device supply levels at 36V, limiting avail-

booster placed within the feedback loop of a fast FET

able output swing. Additionally, supplying currents beyond

amplifier. At lower frequencies, the buffer is within the

tens of milliamperes requires large output transistors and

feedback loop so that its offset voltage and gain errors

causes undesirable IC power dissipation.

are negligible. At higher frequencies, feedback is through Many applications, however, require greater output power Cf, so that phase shift from the load capacitance acting than most monolithic amplifiers will deliver. When voltage against the buffer output resistance does not cause loop or current gain (or both) is needed, a separate output instability.

stage is necessary. The power gain stage, sometimes

Small-signal bandwidth is reduced by Cf, but considerable

called a “booster”, is usually placed within the monolithic

load isolation can be obtained without reducing it below the

amplifier’s feedback loop, preserving the IC’s low drift and

power bandwidth. Often a bandwidth reduction is desirable

stable gain characteristics.

to filter high frequency noise or unwanted signals.

Because the output stage resides in the amplifier’s feedback

The LT1010 is particularly adept at driving large capacitive

path, loop stability is a concern. The output stage’s gain

loads, such as cables.

and AC characteristics must be considered if good dynamic

performance is to be achieved. Overall circuit phase shift, The follower configuration (Figure 1b) is unique in that frequency response and dynamic load handling capabilities capacitive load isolation is obtained without a reduction in are issues that cannot be ignored when designing a power small-signal bandwidth, although the output impedance of gain stage for a monolithic amplifier. The output stage’s the buffer has a 10MHz bandwidth without capacitive load- added gain and phase shift can cause poor AC response ing, yet it is stable for all load capacitance to over 0.3μF.

or outright oscillation. Judicious application of frequency

Figure 1c shows LT1010s used in a bridge type differential

compensation methods is needed for good results (see

output stage. This permits increased voltage swing across

box section, “The Oscillation Problem”).

the load, although the load must float.

The type of circuitry used in an output stage varies with

All of these circuits will deliver 150m A of output current.

the application, which can be quite diverse. Current and

The LT1010 supplies short-circuit and thermal overload

voltage boosting are common requirements, although both

protection. Slew limit is set by the op amp used.

are often simultaneously required. Voltage gain stages are usually associated with the need for high voltage power High Current Booster supplies, but output stages which inherently generate such

Figure 2 uses a discrete stage to get 3A output capacity.

high voltages are an alternative.

The configuration shown provides a clean, quick way to

A simple, easily used current booster is a good place to increase LT1010 output power. It is useful for high current begin a study of power gain stages. loads, such as linear actuator coils in disk drives.

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