Linear power driver works from single supply

Tom Gay

EDN

In low-power, single-supply analog applications, it is often desirable to maintain precise control of voltages much greater than the positive-supply rail. The circuit in Figure 1 allows you to amplify the input voltage, V_IN, by a factor, A, which resistors R₁ and R₂ set. The output voltage, V_OUT, equals AV_IN, where

The op amp receives its supply from a single 5 V source, and the discrete output stage operates from a rectified voltage, V_S, from a power source that meets the requirements of the application. When the circuit neither sinks nor sources current, the op amp’s output settles to a voltage higher than 1.9 V (Q₁ is completely off) but lower than the threshold voltage of the n-channel FET, Q₃, minus 1.2 V (two diode drops). When V_IN rises from a given state, the op amp’s output voltage drops and gradually turns on Q₁. This action results in a voltage drop across R₈, turning on Q₂. This process continues until the voltages at the op amp’s two inputs match. A decreasing V_IN causes the op amp’s output voltage to rise to the point at which Q₃ conducts enough to pull down V_OUT. Capacitors C₁, C₂, and C₃ are necessary to prevent oscillation.

Figure 1.	This simple circuit allows you to control voltages far in excess of the positive-supply rail.

The circuit, useful as a power driver for pulse generators, offers rise and fall times of less than 15 µsec, virtually independently of the supply voltage, V_S. You can test the design with op amps LMC7101, LT1013, and AD8551. All these op amps deliver load currents as high as 5 A at voltages as high as V_S = 40 V. One important feature of the design is its insensitivity to component tolerances. The values for R₁ and R₂ in Figure 1 yield A = 8. The value of R₉ depends on V_S. You could use a logic- level FET, such as the IRLZ34N for Q₃; you can then omit the components D₁, D₂, D₄, R₄, R₉, and C₄. In this case, the op amp’s output connects directly to the gate of Q₃, and you must reduce the value of R₆ to lower the base voltage of Q₁ to maintain the “idle window” in which both Q₁ and Q₃ are off. The op amp’s input-voltage range must include ground. D₃ is necessary only in cases in which sources connect to the output. D₃ prevents reverse current flow from the external source through Q₂. Such a situation can arise when the circuit serves in a battery-charger application.

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