Purpose of this capacitor in discrete amplifier?

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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.
 
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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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. :D :rolleyes:



Not to sound dumb, but what is the purpose of C5?
 
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
 
Ah yes, I guess because the signal is going through the electrolitic and electrolitics are noisy. Since were talking about Hf noise the film cap is very small in value relative to the electrolitic. I believe I understand. It looks like this principle applies to the PS filter caps also. Hmm, I guess this is related to wires generateng Rf noise acting as antaneas without any shielding. Since the capacitor is in series with a resistor(feedback resistor), it obviously makes a LP filter. I have a "bench amp" I am playing with using a DC blocking electrolitic like this. Since this is my circuit, the way I see it, improvement is always good even if sometimes it requires major changes. I guess this is why it has so far remained a "bench amp".:D It works quite well but chalk up one more improvement yet to make.....:cool:
 
It is wonderfull what a single capacitor can do positive or even negative to

An amplifier, and this turns our hobbie, profession for others, so interesting.

One single wrong capacitor value, or in a wrong place, can destroy the audio quality, reading we see the enormous consequences when we forget something.

I think this is fascinating and keeping me "on" for so long time (45 years)...everyday something brand new to learn.

This forum is wonderfull, and there are people that made it better, Eva is one of those that put a lot of brigth in our forum, the know how is clear as she can see every detail alike some microscope.

Goooood!

regards,

Carlos
 
Paralleling capacitors of different kind and whose values differ more than an order of magnitude is something that is seldom seen outside audio circuits. That's because a resonant system is always created by doing so, and Q may be high, but audio people just don't care about that, they don't even measure the result, they act following strange pseudo-scientific criteria.

It's said that it's good to paralell different kinds of capacitors so everybody does it without thinking. Actually, it's a good way to create high Q resonant systems. Damn, I've even created a several Mhz resonator by just paralelling two 10uF 10V ceramic capacitors (with very low impedance at HF) with some PCB leakage inductance inbetween. It was in an SMPS and I was absolutely puzzled when I saw the ringing. I had to solder them together to solve it.
 
And, if anyone's interested, here's the PSRR of the same design I exampled earlier - but with just resistive tail resistors on the input stage instead of 2BJT CC sources!

Once again adjust down by 29dB for the amplifier gain. That's 71dB total and over 30 times inferior to the CC source! Also,
102 dB < 0.001% in THD terms while 71 dB is 0.03%. How much of your THD would you like coming from the PS commutation hash?

Cheers,
Greg
 

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Well, how about I couch it differently.

Consider a tune with heavy mid bass and some very delicate top detail. The mid bass is causing significant supply modulation and generating n harmonics on the supply which are injected though the PSRR to produce an elevated noise floor consisting of a haze of generated harmonics blurring the detail and depth/size of soundstage. Softens any harshness by burying it - but something's obviously missing with a 71 dB noise floor.

Your preferred sound? :D

Cheers,
Greg
 
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amplifierguru said:
The mid bass is causing significant supply modulation and generating n harmonics on the supply which are injected though the PSRR to produce an elevated noise floor consisting of a haze of generated harmonics blurring the detail and depth/size of soundstage.


I am not sure Guru. Think about a line stage linked through the junction capacitance with the prior current mirror load factor that when under driven causes massive unsolicited inter-phase and time-dependent modulation that reverse travels up through just the minority carriers and the vertical SCR structure in a typically fused semiconductor tri-layers back into the feedback node where it finally meets with the stronger-than-necessary bass that a typical drumer will NOT produce as you and I can attest through our ears and our less-than-ideal hearing which is further impacted by a intermittent power supply that modulated the alternator through the power grid.

Do you think it will have any negative impact on how the electrons tranverse in an oxygen-free copper and aluminum oxidized wire?
 
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