One thing that has been baffling me in the last week or so: looking back in time when the first MOSFET input op-amps were introduced, I noticed that they used predominantly p-channel devices in the first stage. On a similar note, in the rare occasions when discrete small-signal MOSFETS were used (i.e. devices like the 3N163), again, there seems to be a leaning towards p-channel devices.
Taking into account the fact that in general, p-channel MOSFETs have somewhat inferior characteristics, I was wondering why this was the case. Just about the only advantage of p-channel devices that I can think of is the lower 1/f noise (even then, it's hideous...) Was there something about the silicon process that made them preferable?
Even with modern devices, such as the excellent LTC6240 op-amp, again, the input stage uses p-channel devices. So is there something in chip manufacturing that favours p-channel in the input stage?
Taking into account the fact that in general, p-channel MOSFETs have somewhat inferior characteristics, I was wondering why this was the case. Just about the only advantage of p-channel devices that I can think of is the lower 1/f noise (even then, it's hideous...) Was there something about the silicon process that made them preferable?
Even with modern devices, such as the excellent LTC6240 op-amp, again, the input stage uses p-channel devices. So is there something in chip manufacturing that favours p-channel in the input stage?
