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Old 14th January 2013, 02:20 AM   #11
naf is offline naf  Indonesia
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Quote:
Originally Posted by Bigun View Post
I used the version with one transition and a diode, mounting only the one transistor against the body of the output device package. I believe the Vbe does better when it's not mounted on a large heatsink - too much thermal lag, better to be mounted on the power device itself.
Dear bigun, how to calculate 47r on your TGM5 hagerman Vbe?
if i use, say bd139 is still 47r?

thank you.
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Old 14th January 2013, 04:38 AM   #12
Bonsai is online now Bonsai  Taiwan
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I have tried both the simple bias spreader and the CFP. I prefer the CFP, and especially so if your TIS current is high (few 10's of mA), or is likely to vary with signal level and/or output load.

1. CFP instability - easy to fix - place a comp cap between the base and the collector of the sense transistor. A few nF will suffice, but I have used much higher values.
2. Temp compensation - easier said than done! Most practiitoners recomend you place the bias spreader in thermal contact with the drivers and mount the drivers on their own heatsink. For a triple, I found this unsatifactory, and mechanically complicated.
3. My solution was to use a CFP and then include a NTC resistor circuit between the top of the spreader and the base of the sense transistor. This works really well, and holds the Iq stable over a broad temperature range. You can read about my experience on page 37 of the e-Amp article on my website. pre-drivers, drivers and outputs all mounted on the same heatsink which is nice and convenient.
4. Temp comp for straight EF2 output stages is generaly a lot easier - so no need for NTC's or anything like that.
5. Bypass cap: this is really just to ensure the spreader circuit is seeing DC, or very low frequency. I have used values of between 10uF and 47uF. If you do not install a bypass cap, the spreader voltage can increase at high frequencies as the spreader loop gain drops off - I would suggest you always fit it.
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Last edited by Bonsai; 14th January 2013 at 04:45 AM.
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Old 14th January 2013, 08:01 AM   #13
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A B-C miller cap is not the best way to compensate a CFP, though it may still prevent oscillation. A 10nF cap across the B-E of the slave transistor is a better way. I learned this from my Kmultipliers. The value of this cap changes with driver and slave bias currents but In my experience 10nF is right for this range. Iq shift should not be a problem for a Vbe multiplier.

If a CFP is used I would avoid using less than 100nF film bypass capacitance, as values around 10nF and under are the most likely to cause oscillation. It is generally stable into anything over 10nF (or maybe 1nF, I don't remember). And this is total capacitance, so for instance 1nF+10uF bypass would be okay.

Last edited by keantoken; 14th January 2013 at 08:04 AM.
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Old 14th January 2013, 08:14 AM   #14
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Dave, you are right but without the transistor the compensation resistor value can increase quite a bit. If the VAS sees significant load capacitance this can be undesirable. However this should not be a problem if the bias generator is decoupled.

I think the gist of it is, any compensated Vbe multiplier will work well enough barring specific fault and tempco requirements. But if the bypass cap makes an audible difference then the quality of the bias generator circuit should too. So those concerned with sonics probably want to try out different things.
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Old 14th January 2013, 09:19 AM   #15
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"The value of this cap changes with driver and slave bias currents ""

How so?
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Old 14th January 2013, 09:43 AM   #16
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Because the cap is across the B-E junction, it causes integrative current gain as a function of transconductance for the slave transistor. Since transconductance is proportional to Iq, corners and GBWP are also proportional to Iq. Thus Gm needs to be compensated with degeneration or other methods when a CFP will be used across a wide current range. If at any point the Ib of the slave dominates the Ic of the master, the master's Gm will need stabilized as well.

I meant the value chosen for the cap, not the cap's actual value, in case it was unclear.
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Old 14th January 2013, 10:28 AM   #17
Bonsai is online now Bonsai  Taiwan
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"I meant the value chosen for the cap, not the cap's actual value, in case it was unclear."

Ok, that clears that one up!

You seem to imply tha GBWP is moving around a lot. If the spreader is decoupled well, it should only be dealing with low frequencies, assuming all other things are correct.
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Old 14th January 2013, 10:45 AM   #18
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An amplifier is usually band-limited to the audio range by an input LP filter, but it is still possible for it to oscillate into a reactive load. The same is true for triple EF output stages, and CFP Vbe multipliers. The problem is not one of external signals causing oscillation; it is the inherent instability of the circuit in question regardless of which signals it can "see".

However now I remember something else important about CFP Vbe multipliers. Any active voltage regulator with lower output impedance than the Gm of its output device will exhibit a virtual output inductance. This virtual inductor can have high Q. For this reason even a stable CFP Vbe multiplier can cause an amp to oscillate because it forms an RF resonator with capacitances in the VAS output area. This is one major caveat emptor with all active voltage-output circuits. Apart from slowing the CFP down so much that virtual inductance is very large, in which case its usefulness as a Vbe multiplier may suffer, the only solution is an RC snubber across its output.

So while a CFP Vbe multiplier may seem like an expedient option, it has many potential liabilities and if you don't understand it thoroughly you may scrap a good design unaware that the only problem was the unstable or resonating bias generator.

Last edited by keantoken; 14th January 2013 at 10:49 AM.
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Old 14th January 2013, 12:43 PM   #19
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It looks inductive because the loop gain drops off at HF, and hence it's terminal to terminal output impedance increases. The solution is simple, and analogous to what you would do with a three terminal reg, or a shunt reg like a TL431: bypass the output with a suitably sized capacitor. Internal loop comp of the CFP - either through a cap b-c on the sense transistor or the method you propose of course goes without saying.
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Old 14th January 2013, 01:17 PM   #20
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You are right about the large capacitor bypass. I forgot that I was recalling this from years ago before I learned how to compensate a CFP. Without compensation it would sometimes oscillate into the lytic just like an overreactive amplifier into a loudspeaker cable.

One valid way of reducing Vbe multiplier impedance is by bypassing the C-B resistor with a capacitor. Because the base node is relatively high impedance, the same size cap will cover much lower frequencies. This is super-effective for a CFP, but alas, it is also one of the things that will make it unstable. If not small enough or insufficiently large, it causes a second pole in the middle of the inductive region and maddening instability is back.

Using a feedback bias generator is nice and fun until you end up having to treat it like a high-speed opamp. It can be especially frustrating if you don't understand all these relationships. And this is a lot of unnecessary work if a precision bias generator was never needed in the first place.
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