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Old 21st November 2010, 10:42 AM   #101
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IMHO, negative version will perform much better, since base of Q4 is "feeded" through Q3. In positive version, base of Q2 is feeded through 100R resistor.

According to Walt Jung in Figure 1b, 40dB rejection is possible. No matter if anyone stack 2, 3,...10 transistors, if pass transistor's base is feeded with noise, you have "garbage in, garbage out" situation.

Swap R1 and Q1 positions.
Or feed base of Q1 through resistor, but then you need heavy R/C filtering, which will add complexity.
Or add JFET.
Or.....do anything.
But 100R resistor there is wery bad choice. No way to get better than -40dB (in theory).

Another important thing is capacitor. You want low leakage type there. Film caps are the best, low ESR solid-polymers (OSCON,....) are the worst.

Just my personal opinion.

Last edited by stormsonic; 21st November 2010 at 10:46 AM.
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Old 21st November 2010, 10:51 AM   #102
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Another example can be found HERE.
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Old 21st November 2010, 07:08 PM   #103
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by stormsonic View Post
Swap R1 and Q1 positions.
Or feed base of Q1 through resistor, but then you need heavy R/C filtering, which will add complexity.
I see nothing wrong with the operation of R1 Q1 as shown in Kean's schematic.
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Old 21st November 2010, 07:28 PM   #104
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Storm, the article doesn't say anything about my configuration, only stuff about opamps and zener diodes.

There is no difference between the positive and negative versions; the bases of the outputs are "fed" by the drivers, and the 100R resistors give ~6mA bias for the drivers, to reduce output impedance. There may be a difference if Early effect is different for PNP's and NPN's, but if so even Andy_C's models haven't revealed this.

Let me draw the schematic a different way.

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Old 21st November 2010, 07:51 PM   #105
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Storm has a good point about low-leakage caps. Unfortunately the rubycon ZL datasheet is down, so I can't check.

Also, some models DO suggest that input rejection for the positive reg would be less because of the PNP transistor, by ~10db. SPICE has trouble modeling Early effect, so we can't know how drastic the changes are in real life.

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Old 21st November 2010, 10:32 PM   #106
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I take that back, the datasheet shows the 2SA1930 has worse Early behavior than the 2SC5171. This would forecast worse input rejection for the positive regulator.

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Old 21st November 2010, 11:07 PM   #107
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Low current leakage lytic cap

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Last edited by stormsonic; 21st November 2010 at 11:14 PM.
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Old 21st November 2010, 11:44 PM   #108
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Storm, let's make sure we've got our facts straight before you write my name on anything of yours (awkward moment, lol). We only need low enough current to not turn on D7. Across 1k, this means less than 300uA. 300uA at 16V mean 53.3kohms. As long as the cap's leakage is say 80kohms, we're okay. What say you? I don't think ANY reasonable lytic will have such high leakage current.

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Old 21st November 2010, 11:51 PM   #109
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keantoken,

SPICE doesn't have any trouble modeling the Early Effect, but the transistor model may not have the correct parameter for Vaf.

Rick
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Old 22nd November 2010, 01:35 AM   #110
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Kean,

Can you also add some ESR and ESL into C1 in your simulation?

Bill
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