Bride of Zen with cascode and current source. - diyAudio
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Old 24th August 2003, 12:06 PM   #1
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Question Bride of Zen with cascode and current source.

I'm about to build a ZenV4 and want a preamp to go with it.
Since it will be an integrated amp I will have a supply voltage
of approximatley 50V (like the Zen) at my disposal. After having
read Mr. Pass articles on cascoding and DIY OP amps I was
tempted to add som circuitry to the BoZ in order to, hopefully,
improve its performace. So my question to all of you is: Is the
following a good ide? I suppose not since I haven't seen
any attempts to modify the BoZ in a similar matter... But why is
that?

/Niclas
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Old 24th August 2003, 12:37 PM   #2
bocka is offline bocka  Germany
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This circuit does not work. It has no gain. Output voltage is always 0V
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Old 24th August 2003, 12:53 PM   #3
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Hi,

Quote:
This circuit does not work. It has no gain. Output voltage is always 0V
If this is the case my simulator is _way_ off...
Could you please explain your reasoning?

Below is the simulation from LTSpice/SwitcherCADIII with the
above circuit:
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Old 24th August 2003, 01:02 PM   #4
Bricolo is offline Bricolo  France
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"I haven't seen any attempts to modify the BoZ in a similar matter"


Yes, that's strange. Cascode is considered as a good thing (unlike NFB that some people like and others dislike)
I was also expecting to see more cascoding modifications, on pass amps. Especially since Mr Pass himself wrote an article about cascoding.


Is there any disadvantage to cascoding (except the lower woltage swing) that could dissuade diyers to try this?
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Old 24th August 2003, 07:15 PM   #5
bocka is offline bocka  Germany
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Do not believe everything a simulator tells you. If Q3 works as current source with an (nearly) infinite output impedance it's
obvious that there isn't any voltage deviation on R7 and in this way any voltage gain.

If Q3 saturates I can't see any reason why implementing this MOSFET.

In other words: This circuit works not as expected.
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Old 25th August 2003, 06:39 PM   #6
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As bocka says, the gain should be near 0, as the gain will
be the ratio of the output impedance load divided by the
Source resistance of Q1, which with a constant current source
will be very high.

Drop the current source, Ground the Source of Q1 and enjoy
the cascoding provided by Q2.
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Old 25th August 2003, 07:15 PM   #7
PMA is offline PMA  Europe
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Quote:
Originally posted by Nelson Pass
As bocka says, the gain should be near 0
Sometimes the silence would be very appreciated .
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Old 26th August 2003, 07:27 AM   #8
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Thank you bocka and Mr.Pass for pointing out the error.
I didn't think the MOSFET Q3, which can't supply a whole lot of gain, would work that well as a CCS. The purpose was just to try to better the 100 Ohms to ground in the original BoZ.
Now I find I've lost all my faith in Spice simulations!
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Old 26th August 2003, 08:21 PM   #9
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Spice is just a tool, a fine one at that, but like any tool can
be well or ill used. It is not a substitute for understanding.
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Old 26th August 2003, 09:21 PM   #10
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Quote:
Spice is just a tool, a fine one at that, but like any tool can be well or ill used. It is not a substitute for understanding.
How very true.
It was som ten years since I read my last electronics course so I have som catching up to do...

May I ask whether you have constructed the BoZ with a cascode and if so how it compares to the original?

/Niclas
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