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Old 17th September 2011, 06:43 AM   #1
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Question Comment on the schematic

Hi !
I found this schematic and i want to create an amplifier preparing the schematic.
So seek help, and that means little comment she ate good, and so on?
thanks
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Old 17th September 2011, 04:30 PM   #2
djoffe is offline djoffe  United States
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Default Looks pretty reasonable...a few things though

I haven't simmed it, or looked up transistors...just a quick look...there's probably 1 thing missing, and 1 or 2 things that should be modified...
1. Missing-RF ingress filter on the front end.
2. arrangement of bias transistor and pot should not be carried into the real design, though ok for sim...if the POT wiper goes open, e.g. sits on a bit of dirt, then the Vbe multiplier voltage goes up very high, and cross-conduction kills the amp.
3. Q13 will die if the amp is clipped against the negative rail for any length of time...even square wave tests are probably enough to cause mayhem..Q5 and Q14 might then die from collateral damage.
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Old 17th September 2011, 04:59 PM   #3
simonov is offline simonov  Bulgaria
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It's wise to use a current limiting resistor in Q13 collector. The author of the schematic had tried to combine two types of VAS. The emitter follower (Q13) will effectively enhance the beta of VAS adding more local negative feedback. This results in better linearity. By cascoding Q5 and Q14 this effect is further increased, resulting in even better VAS linearity. This however had forced the designer to increase the dominant pole compensation in order to maintain the stability.
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Old 17th September 2011, 06:12 PM   #4
CBS240 is offline CBS240  United States
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Hi

It appears Q5 & Q14 are mislabled. IMHO Q13 may not be necessary if Q5 is a high beta device such as a 2N5089 (Hfe~500) It does not matter that its Vceo is 25V, due to cascode. C3 seems large for Cdom. Perhaps TPC or TMC could be used? Switch C6 & C8 around. The drivers output is lower Z than VAS and the pull off capacitors should reflect that. Degeneration in the LTP might be a little light.

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Old 17th September 2011, 06:58 PM   #5
Mooly is offline Mooly  United Kingdom
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The circuit appears to be little more than a copy of Doug Selfs original (1994/5 ish from memory) "Blameless Class B" with a few component value changes to make it look different and the addition of Q14.

That design worked well and did all that was claimed of it.
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Old 17th September 2011, 07:13 PM   #6
simonov is offline simonov  Bulgaria
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C3 should not be reduced in this design. The open loop phase margin will suffer, causing potential instability.
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Old 17th September 2011, 08:32 PM   #7
jaycee is offline jaycee  United Kingdom
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330p for a miller capacitor is ridiculously high. If it needs to be that high to be stable, there is something fundamentally wrong.
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