Transistor as Diode

[daestrom]

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

 

[krw]

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.

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.