Both exhibit a noticeable variation of current when the Vce of the output device varies. A nice improvement is a cascode output, however there's a price to pay in terms of reducing voltage excursion.analag said:Gentlemen tell me which is better.
Reducing noise on the one on the left is relatively easy, just putting a cap acroos the diodes; on the RH circuit, this is not possibleanalag said:Gentlemen tell me which is better.
In terms of current control one is significantly better than the other... The imposition of the so called infinite resistance is better realized.
As I recall, in Walter Jung's recent article on current sources, he found the two-diode source better than the BJT source.
Svart said:I think Analag was specifically talking about the function of the circuit, probably not the parts count or anything like that. the difference in size between a couple diodes and a BJT in size is negligible.
Samuel Groner said:There are three basic limitations regarding output impedance for the left one:
* Early effect modulates the hFE of Q1, causing variable base current.
It's late here, the coin didn't drop here anymore why Early effect modulates the hFE of Q1, but not so for Q2.
Mechanisms are what gives insight, but in addition it's interesting to get the feeling one should spend that additional BJT or not.
clintrubber said:Samuel Groner said:There are three basic limitations regarding output impedance for the left one:
* Early effect modulates the hFE of Q1, causing variable base current.
Samuel Groner said:It's late here, the coin didn't drop here anymore why Early effect modulates the hFE of Q1, but not so for Q2.
I don't quite follow why you think hFE Early effect should not apply to Q2? The local feedback does just address the Vbe, but not the hFE modulation.
I second this. It's a very interesting thing to read. Here are the links to the article:pstamler said:As I recall, in Walter Jung's recent article on current sources, he found the two-diode source better than the BJT source, but other configurations (including LEDs, zeners, cascoded sources, etc.) were a good deal better than either.
Peace,
Paul
tv said:Hijack: that's an interesting buffer -maybe it could be presented as a "ruby" buffer? (short for "rubirio" 8) )
.....but IMHO it would be even better if:
-Q3 Q4 collectors would be connected to Q6 emitter
-Q1 Q2 collectros would be connected to Q8 emitter
-Q2 Q6 Q4 Q8 would have B-E resistors. (now.. what exact values??..)
yay or nay?
what about adding a parallel "D.Self 1uF cap" for "even better better"?A resistor from base to base of the output devices wouldn't hurt. Size it to provide some turnoff current without raising Q5/Q7 dissipation unduly---maybe 750 ohms?
adding Q2 Q4 B-E resistors would do that, no? Providing sort-of-fake CCS's for Q1/Q3 loading...One thing that bothers me a bit is the very low and ill-determined currents in Q1 and Q3. If they were much smaller geometry devices this would be o.k. but as it is, with no more than the base currents of Q2 and Q4 they are going to be operating way down on the gain-bandwidth curve. Use of integrated darlingtons like the MPSA13/MPSA62 etc. would work, since the internal equivalents of Q1/Q3 are tiny.
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