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Increasing the Output Current

One of the very useful facts about operational amplifier circuits is that the amplifier itself will generate whatever output voltage is necessary to exactly match the feedback current to the total input current. In most cases, this means matching the output voltage to the applied combination of input voltages (remember the inversion!), as multiplied by their coefficients. We can use this fact to insert a number of different components between the amplifier output and the designated Vout, in order to modify the overall limits and behavior of the circuit. This page shows one such practical modification.

Adding output transistors for increased current capability.

In the circuit to the right, we have added a pair of transistors to the output circuit. Note that we have placed them inside the feedback loop, in such a way as to include their behavior in determining the output voltage and current of the overall circuit. As a result, any non-linearities of the transistors will automatically be compensated by the action of the op amp. The actual Vout will still be determined entirely by the ratio of Rf/Rin.

The effect of the added transistors, then, is utterly transparent to the operation of the circuit. What they accomplish here is to increase the available output current to any following circuits. Instead of a limit of 5 mA, we can get a limit of 50 to 100 mA or more, depending on the specific transistors used.

Often small resistors (less than 1 ohm) are placed in series with the transistor emitters to limit the current drain in the event the output gets short-circuited to ground or to one of the available voltages. If so, they still have no apparent effect on the normal operation of the circuit, because the op amp will necessarily compensate for any voltage dropped across them.

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