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Increasing OP-AMP output current.

Contents


Introduction.

This is a simple way to increase the output current of an OP-AMP. This can be necessary to drive low-impedance loads or to drive a low-impedance feed-back network.

Circuit.

The Howland current pump basic equation.
Fig 1 : Circuit. Components in dotted box are used to further increase output current.
Q1, Q3 and Q2, Q4 are current mirrors that drives a current similar to the OP-AMP's positive/negative supply current into the output node. With the components outside the dotted box, output current and idle current are doubled. With all 8 transistors, output current is 4 times the OP-AMP's output current. Current limit is provided by the OP-AMP. To prevent thermal runaway, the transistors must be thermally coupled to each other.
I have used the circuit with NE5534 with a gain of +3 and with an AD797 with a gain of -1 and ZTX653/ZTX753 for Q1..Q8.
The specs for this circuit is similar to those of the OP-AMP with a lower load current except that the output voltage is a couple of volts lower.


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