Might Diamonds

Posted on Feb 6, 2014

With only four transistors and four resistors, the diamond buffer does not require much. Some harsh critics would add that it does not deliver much either. I have covered the circuit and its modification here before, for example in Blog 232 and Blog 187. But for over a year, I have been meaning to write about a significant modification to the basic circuit that I

Might Diamonds
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came up with. In fact, I planned on including it in my last post. Here is the big problem with the naked diamond buffer circuit: it works great as long as you do not ask to work hard, which is a tad bit sad, as that`s exactly what we want from a buffer. The makers of OpAmps know about this problem and they have come up with some clever workarounds, such as this circuit found in the OPA633. As long as the input stage can handle driving the output transistors, the two transistors nested inside do nothing; in fact, they are not even turned on. But once the input stage falters, they begin conducting and take over the task of driving the output transistors. Clever, very clever. The LMH6321 tackles the same problem in a different way: it builds a very robust, dual, push-pull, input stage that can easily drive the output transistors to full output. (The actual circuit is more complex, but I erased the protection diodes to make the topology more readily apprehensible. ) Each output transistor gets its own three-transistor, push-pull, driver stage. These elaborate topologies are necessary in an OpAmp, as it is impossible to include large-valued capacitors within the OpAmp`s small plastic package. On the other hand, if we build up a diamond buffer out of discrete transistors and resistors, we can add as many big electrolytic capacitors as we please. Indeed, just adding one big electrolytic capacitor makes a big improvement in the...

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