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Boosting Op Amp Output Drive

Written by Andrew Levido

Selecting an op amp is always an exercise in compromise and trade-off. With so many parameters to consider you can’t always find an op amp that does everything you might want. One of the parameters that can be a limitation, especially in precision op amps is the output drive capability. There are however a few tricks you can use to boost the output current capability of almost any conventional op amp.

One way is to take advantage of one of the dedicated unity gain buffer chips that are available, such as the venerable LT1010. This can drive ±150mA and has a bandwidth of 20MHz. They come at a price of course, with the LT1010 costing around $10.00 (US) in low quantities. There are some other ways of course.

If you only need a single ended buffer (in other words, you only need to source or sink current) you can use the simple circuit shown in Figure 1. Here a single NPN or PNP transistor is used to provide current gain. You can easily achieve a gain of 50 to 100 by selecting the right transistor. Note that we lose about 0.7V of output swing due to the transistor’s VBE drop.

FIGURE 1. This figure shows a simple way of boosting op amp output current if you only need to source or sink current. With the right choice of transistor, is relatively easy to achieve a load current 50 times higher than the op amp’s capacity. The maximum output swing is reduced by the transistor’s VBE drop.

If we need to source and sink current, you can combine the circuits of Figure 1 to produce the circuit in Figure 2. Now we can source or sink current as required, but we have a “dead band” of about 1.4V right around zero, where neither of the transistors is biased on. A resistor between the op amp output and the output node may help if the current needed at zero output voltage is very low.

FIGURE 2. This circuit effectively combines the two circuits shown in figure one to allow both sourcing and sinking of current. Note that there is a dead band of ± 0.7V around zero output where neither transistor is conducting. Class AB biasing can overcome this at the expense of circuit complexity.

One solution to this dead band is to build a true class AB output stage with the appropriate biasing components to eliminate the crossover distortion. This quickly gets complicated, and class AB circuits are inefficient since there is always a small current flowing from rail to rail through both transistors to keep them both in conduction.

A better, but less intuitive solution is the circuit of Figure 3. At light loads, the output current is supplied directly to the load by the op amp. This load current also flows through R1 if the op amp is sourcing current, or R2 if the op amp is sinking current. R1 and R2 are sized such that they drop about 0.7V just below the op amp’s maximum drive current. At this point the appropriate transistor begins to turn on, contributing additional current to the load.

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FIGURE 3. In this circuit light loads are sourced or sunk directly by the op amp. As the load increases to the point that around 0.7V is dropped across R1 or R2, the corresponding transistor begins to conduct, adding additional current to the load. This circuit does not suffer the dead band issue of the circuit in Figure 2.

There is no dead band in this case. Like the other circuits, the output swing at full load is reduced by ±0.7V, however at light load when the transistors are turned off, pretty much the full op-amp swing is available at the load (a small amount will be lost due to the presence of R1 and R2).

I have used this simple circuit a number of times over the years and it has proven to be a reliable and low cost solution if you need high output drive at moderate bandwidths.

References

“LT1010 – Fast ±150mA Power Buffer,” n.d. https://www.analog.com/media/en/technical-documentation/data-sheets/LT1010.pdf.

Texas Instruments. “AN-272 Op Amp Booster Designs,” n.d. https://www.ti.com/lit/an/snoa600b/snoa600b.pdf?ts=1635202795398&ref_url=https%253A%252F%252Fwww.google.com%252F

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Andrew Levido (andrew.levido@gmail.com) earned a bachelor’s degree in Electrical Engineering in Sydney, Australia, in 1986. He worked for several years in R&D for power electronics and telecommunication companies before moving into management roles. Andrew has maintained a hands-on interest in electronics, particularly embedded systems, power electronics, and control theory in his free time. Over the years he has written a number of articles for various electronics publications and occasionally provides consulting services as time allows.

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Boosting Op Amp Output Drive

by Andrew Levido time to read: 3 min