Methods for Improving Output Current Driving Capability of Operational Amplifiers

There are some applications that could be implemented with just an ideal op amp, but cannot be achieved in practice with just a real-life device due to certain physical limitations. Fortunately, it is often possible to enlist the help of a second amplifier so that the two, combined, aptly called a composite amplifier, can do what the primary amplifier alone cannot.

Composite Amplifier Stability Considerations

The secondary op amp is usually placed inside the feedback loop of the primary op amp, as shown in Figure 1(a). The phase lag introduced by the secondary device tends to erode the phase margin ɸ m of the composite amplifier, so appropriate frequency compensation measures may be required.

Figure 1. (a) Block diagram of a composite voltage amplifier. (b) Circuit for finding the open-loop gain ac and the noise gain 1/β of the composite amplifier.

Figure 2. Common phase margin cases for (a) frequency-independent and (b) frequency-dependent noise gain 1/β(jf).

Improving the output current drive capability of operational amplifiers

Most op amps are designed to provide no more than a few tens of milliamps of output current. For example, the venerable 741 op amp can handle a maximum of 25 mA of output current. Trying to exceed this will activate some internal watchdog circuitry, preventing the actual current from increasing any further.

In this case, the op amp will no longer function properly, but at least it will be protected from possible damage due to excessive power dissipation.

A popular method to increase the output current drive capability of an op amp is to use a voltage buffer, as shown in Figure 3(a).

Figure 3. (a) Using a buffer to boost the output current drive of an op amp. (b) Detailed buffer schematic.