Paralleling Amplifiers Increases Output Drive

There are several circuit techniques that increase the output drive capability of an amplifier. One method uses external bipolar transistors as a push-pull circuit to provide additional drive. Another method uses two different amplifiers in a composite amplfier configuration as detailed in Application Note 21 to leverage the benefits of each individual amplifier.

A third method involves paralleling two or more similar amplifiers.

The following LT6020 circuit is an example of this. The LT6020 is a dual, 3V to ±15V, 400kHz GBW, 100µA quiescent current, ±30µV offset amplifier with an impressive 5V/­µs slew rate (with ±15V supplies) targeting low power precision applications. With two amplifiers in parallel, the output drive capability doubles. In general, the drive capability increases by N (where N is the number of amplifiers in parallel). The circuit can also be thought of as halving the output impedance. The LT6020 is specified with a 10kΩ load in the datasheet electrical tables and has a short circuit current of 3.5mA minimum with a 3V supply, and 5.5mA minimum with ±15V supplies; the load can be halved and the short circuit current doubled when using two amplifiers in parallel. Paralleling the two amplifiers also reduces the power dissipation of a single amplifier by sharing the load with multiple amplifiers. The 100Ω resistors at the output prevent the paralleled amplifier outputs from fighting with each other.

There are other benefits that arise from this circuit. For instance, the amplifier voltage noise is reduced, improving the signal to noise ratio. Another advantage is that the output offset error is reduced. Since offset error is not correlated, as more amplifiers are added, the distribution of offset errors will move toward a center value, such that the net value when all devices are considered will approach the typical mean value.

There are trade-offs that must be considered when using parallel amplifiers to increase amplifier output drive. These include cost and component count, and increased input current noise. However the benefits often outweigh the potential downsides, allowing the designer to choose the amplifier that best fits the application by paralleling the amplifiers. Contact your local sales office for additional assistance with your most challenging circuit output drive problems.