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Discrete Opamps

Started by dano12, March 23, 2009, 10:13:55 AM

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R.G.

#20
March 24, 2009, 01:15:39 PM
Quote from: Ben N on March 24, 2009, 12:41:36 PM
Which happens to also be the gist of the hifi argument for tubes, right?
Yep. Then they laugh at us for deliberately distorting sounds.  :icon_lol:
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

rnfr

#21
March 24, 2009, 05:56:38 PM
there is also a simple one in the bluesbreaker that i have heard is fun to play with.

JDoyle

#22
March 24, 2009, 06:18:31 PM
Quote from: Sir H C on March 23, 2009, 11:55:03 AMMoon is another odd one, follower output stage with current source, and why that transistor in the middle?

If you are talking about the transistor in the middle of the output stage that is there to stabilize the current source and increase its effective output impedence.

Quote from: Sir H C on March 23, 2009, 11:55:03 AMAnd why jFET input?  That will be higher noise and lower gain than bipolar, to me the only reason to think of going discrete.

A JFET with obviously have a higher input impedence and a JFET input is lower noise than a BJT for high impedence sources. And in this situation, gain doesn't matter as it is cumulative over the whole circuit and therefore high. What DOES matter is transconductance, with a lower transconductance on an input stage allowing for a higher slew rate.

Other than that, I am assuming it was chosen for sonic/marketing reasons.

Regards,

Jay Doyle

JDoyle

#23
March 24, 2009, 06:34:57 PM
Quote from: Sir H C on March 23, 2009, 02:28:16 PMPartially, since the output is followers, they are not really current limited the same way that some other op-amp circuits are, and there doesn't seem to be any current limiting in the output stage other than the current source pull down on the third circuit.  You could do the third circuit, include a PNP current source above the follower device and then you would have current limited slewing to that current limit.

The slew rate of an operational amplifier is determined solely by the ability of the input stage to charge and discharge the Miller capacitor of the second stage from collector to base.

The ability of the input stage to charge and discharge that cap is determined by the tail current in the differential amplifier divided by the value of the compensation capacitor.

Nothing else in the circuit has any effect on slew rate.

Regards,

Jay Doyle

Lurco

#24
March 25, 2009, 03:38:33 AM
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