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Old 31st August 2005, 08:33 PM   #1
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Default Purpose of this capacitor in discrete amplifier?

http://www.ampslab.com/c200_1.htm

What is the purpose as well as necessity of C6 in this or similar circuit? Unfortunately it is not mentioned in the summary so I assume it should be obvious.
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Old 31st August 2005, 08:39 PM   #2
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It's supposed to act as a filter for the bias voltage (created by R9 and D1) into the constant current sources Q3 and Q5.

Rune
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Old 31st August 2005, 09:29 PM   #3
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Quote:
Originally posted by runebivrin
It's supposed to act as a filter for the bias voltage (created by R9 and D1) into the constant current sources Q3 and Q5.

Rune
Indeed, it wins at RF, compared to the diode impedance. But will there be RF ?

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Old 31st August 2005, 09:56 PM   #4
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Quote:
Originally posted by Guido Tent


Indeed, it wins at RF, compared to the diode impedance. But will there be RF ?

cheers
Personally I doubt it has practical value, but it probably won't hurt either.

Rune
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Old 31st August 2005, 10:30 PM   #5
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Default Zener diodes and LEDs produce noise...the capacitor may be to

filter this noise...as voltage there is stable, because of LED zener effect.

regards,

Carlos
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Old 31st August 2005, 10:43 PM   #6
ilimzn is offline ilimzn  Croatia
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Probably only makes for a relatively small difference, just keep in mind that a LED is not a regular Si diode, and indeed it's dynamic resistance is not as low. Although not directly applicable to this circuit, putting non-inductive caps in parallel with LEDs used as references can actually help at HF, I have found this is almost mandatory in cascodes. In circuits with zener references, the cap can help with noise filtering (zeners make rather good noise generators). With LEDs and in general, forward biassed diodes, this is far less of an issue.
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Old 31st August 2005, 10:50 PM   #7
Eva is offline Eva  Spain
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C6 should improve power supply rejection ratio, altough Vbe fluctuations of Q3 and Q5 with Vce changes will still be a source of error.
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Old 31st August 2005, 11:57 PM   #8
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Yes the presence of C6 across the LED impedance, depending on it's value will lower that impedance relative to R9 so improve the stability of the current source and the power supply rejection from the -ve supply AT HF!

Of course C7 will do exactly the opposite at the +ve supply. You can see/quantify the effect by doing a sim of the cct with a sig gen in each supply lead and doing a bode plot (magnitude dB) to the output. Simply change the C values to quantify the effect.

Since PSRR is an input referred quantity and the graph is output, simply subtract the gain of the amplifier (example 29 dB), so the PSRR over most of the audio band in the example is 102dB.

Cheers,
Greg
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File Type: png psrr.png (11.2 KB, 365 views)
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Old 1st September 2005, 03:00 AM   #9
CBS240 is offline CBS240  United States
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What if you replaced D1 with a bjt current source, base to emitter of Q3, emitter to -Vs, and collector to base of Q3 & Q5, lose R10 and adjust R6 & R15 to get the same current. (0.6V) Then would there be a need for C6? How about PSRR?

Looks like a pretty straight forward circuit though. Of course if you are changing stuff, might as well replace R3 & R4 with a mirror and replace R11 with a forward biased diode.



Not to sound dumb, but what is the purpose of C5?
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Old 1st September 2005, 03:33 AM   #10
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Hi CBS,

C5 is a film cap to counter the non-ideal character of the electro.

Regarding the use of C6 if you improved the CCS to a two transistor one, the answer can be seen in my example thumbnail. The right value of C6, used with a 2 transistor current source is responsible for the kickdown (improvement) in the PSRR to 80 dB (+29 dB gain = 109 dB). A lesser or greater value was less effective. It's a good frequency to have extra PSRR - in a freq band where real power can be often demanded.

Cheers,
Greg
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