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Old 26th April 2005, 04:17 AM   #11
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Hi Quasi,

Perhaps you would like to try the cascode as the current arrangement appears quite sensitive to supply line artefacts and there goes low level resolution - "depth" , "space" . Think of it as a simple divider with the Zout of the collector in the input stage. The active load takes even more, but if you raise the collector impedance significantly less supply hash gets in. And there's not significant gain prior to the injection point so feedback won't help it.

Evolution. Don't believe me. Simulate it with a voltage source superimposed on the supply. Better still a BC546, two R's and a small C to GND.
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Old 26th April 2005, 04:29 AM   #12
andy_c is offline andy_c  United States
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You're being totally unclear about the specific configuration you're suggesting. A partial schematic is necessary to convey these kinds of ideas effectively.
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Old 26th April 2005, 05:03 AM   #13
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On the thumbnail. Forget the component values (std 1k resistors from quickie) Simple circuit SE resistive output from stage one, next stage gets fraction of supply hash defined by R5 divider with Zc of the first stage. But cascode raises that by orders of magnitude so the R5 fraction is much less , while signal levels remain the same. So stage 2 receives many times less supply hash from the improved divider. The practical circuit uses a current mirror load which effectively increases R5 assuring more supply hash but more gain as well.

You need to envisage the supply line with a sawtooth on it looking to get in and R5 /Zc divider is it . Then it's amplified by all the gain of the high gain Vas stage.

So the current mirror is good if you need more LF gain as the stage 2 miller C will kill it at high freq. But for CLEAN from unreg supplies the cascode AND/OR differential second stage.
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Old 26th April 2005, 05:18 AM   #14
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Perhaps if you picture below as the spray of misinformation that's trying to get in from your output stage commutating supply line to smear your detail, blur your images, and....


Thats from a single 1K sinewave at the output.
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Old 26th April 2005, 07:27 AM   #15
quasi is offline quasi  Australia
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Default Hey Greg (A'guru)

Ah yes I see. My cascode was a zener locked one and I found this unsatisfactory from a noise point of view. So your idea brings up new (and interesting) discussion. Probably outside the scope of Raszvan's thread though.

Hey Rasvan, if you decide to build the amp you would be able to implement Greg's idea easily by cutting some tracks and adding / changing some components. But maybe you should just build it first and see what you think, then tweak later.

Cheers
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Old 26th April 2005, 11:20 PM   #16
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Yes Quasi,

Zener locked cascodes, in my experience, always give higher (THD+noise) than resistive divider and C bypass to ground.

A good test if you have a 2 channel (stereo) amplifier using a common unregulated supply is to drive the other channel with signal and look/listen to what's coming out of your undriven/shorted input channel. Crosstalk - but generally it's coming due to Power Supply intrusion. Go from there for improvements as, if it can be stimulated interchannel, it can probably occur intrachannel.
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