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Transistor as Diode

Hi All,

So I was reviewing an old schematic for an opamp circuit that was built before the prevalent IC's (circa 1965). On the input, to protect from over-ranging there were two bipolar junction transistors, wired directly across the non-inverting and inverting input to the amplifier circuit. The base and collector were tied together to one side and the emitter to the other (one wired for each polarity).

What's the advantage of using a transistor like this as opposed to a simple diode? In the end, they don't conduct unless the inverting input moves away from the virtual ground potential (as when the amplifier output saturates). Seems like simple diodes would have been easier/cheaper, so I was wondering if anyone knew why this transistor method would be preferred.

Thanks,

daestrom

Reply to
daestrom
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The process has transistors and no diodes? ;-) The transistor is a vertical device so takes no more space than a diode would, so why not? They're all made at the same time, so it's no more "expensive" and is in fact cheaper (same device design). BTW, the base and collector are tied together to keep the transistor out of saturation. This makes the diode curves more "normal" and reduces capacitance.

Reply to
krw

Actually quite common at one time and I would suspect is still common in IC structures. I have seen discrete designs where the transistors used were significantly cheaper that the equivalent diodes. I suspect it is a volume of production thing.

Don Young

Reply to
Don Young

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