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Old 13th February 2003, 04:46 PM   #21
mlloyd1 is offline mlloyd1  United States
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My experience is that cascoding the CCS primarily boosts the output impedance (obviously over a limited range); I agree with capslock.

The question then becomes: of what benefit is this characteristic in your particular application?

I think most often, not vey much (Oops, did I mean to say that)

jcarr, hugh: very useful comments! thanks!

How rugged (as in damage resistant) are these VERY low capacitance devices in real world amplifier applications? Any historical experience to share? Generally speaking, low capacitance implies small size which doesn't always generate warm fuzzy feelings of robustness. I've even seen a significant number of mfr schematics that parallel devices in the VAS, which brings the capacitance back up. Yes, I know Sanyo has a fancy proprietary process for fabricating these devices.

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Old 13th February 2003, 05:46 PM   #22
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"Does anybody else have an opinion on cascoding CCSes?"

A constant current source is slow by definition. It is meant to provide a constant dc current. Ft is not directly relevant.

An ideal CCS has infinite Z. A single BJT source will have a dc resistance caused by changes in beta with Vce (because Ie = Ib+Ic). This can be reduced by increasing the bias voltage across the emitter resistor. The ac Z is this dc resistance in parallel with Ccb and Cce and the miller effect of Ccb. If the base voltage bias circuit has zero Z there is no miller effect. The capacitances can be mitigated with a series R if you have the voltage headroom.

The dc resistance effect and the capacitive effects can be mitigated by keeping Vce constant. This is what a cascode tries to do. However, you get the effect of the Ccb and Cce of the cascode transistor. However, the miller effect is virtually eliminated. The cascode also maintains a constant power dissipation of the CCS transistor so thermal drift is mitigated too.
To realize all these benefits, each transistor must have an independent base bias circuit.
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Old 13th February 2003, 06:18 PM   #23
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These Sanyo devices have a respectable current and power rating, so the chip would not be very small.

My concern about the slow CCS is twofold:
- Parasitic C_cb and C_ce would cause lag, limiting the bandwidth available from the VAS and maybe even causing oscillation.
- Above f_T the CCS certainly no longer is a CCS, so impedance of the VAS stage drops, causing additional droop above f_T, which causes another pole in the open loop gain which will force us to use a much more severe frequency compensation. Just as an example, consider using one of the Sanyo NPN devices with about 1 GHz f_T as a VAS. As a CCS transistor, we use the PNP BF470. This is an old video transistor that has very low output capacitance (1p8 pF) but also a low 60 MHz f_T. Gain is pretty pointy, so at the current chosen, f_T may well be only 10 MHz. This will force us to compensate the whole amp for a unity gain bandwidth of 5 MHz (or 250 kHz total bandwidth at a gain of 19) where using the fast VAS transistor and fast output devices, a unity gain bandwidth of 50 to 100 MHz should have been easily achievable. This is admittedly an extreme example, but it serves to contradict the notion that the speed of CCS transistors is completely irrelevant.
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Old 13th February 2003, 06:19 PM   #24
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Quote:
Originally posted by traderbam

To realize all these benefits, each transistor must have an independent base bias circuit.
The base circuit is just a low impedance voltage source. So if you use heavy capacitive buffering, one should be sufficient for all CCS, shouldn't it?
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Old 13th February 2003, 09:44 PM   #25
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Yes, if the bias circuits are extremely good then you can use one. But then you wouldn't need to cascade in this case (other than for thermal stability reasons). So I would use two just to make the most of the cascade: it doesn't take many parts. That's my opinion.
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Old 3rd May 2005, 06:15 AM   #26
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Default Sanyo here to get!

Hello all.

All the Sanyo transistors you are talking about can be optained from these:

www.heldt-electronic.de

Push "Aktive Bauteile" in left menu to see the catalogue.

JørgenP in Denmark Phonenumber is +4544945244
They only have a giant catalogue in paper but they get you all types.

By the way I am ordering Sanyo transistors now from Heldt electronic so let´s see how it gets!

Best regards
Kim
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Old 3rd May 2005, 07:54 AM   #27
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Ha chasing that elusive 1pF Vas transistor with 500MHz and Beta of 500 all peaking at 10mA for a Vas! Sifting through obsolete devices. Think of the PSRR when I slug it to ground instead of miller. Brings back memories. Got all that extra GBW from the dominant pole stage, hope the other stages are up to it.

Commercially you need to spec devices that support you're design in 10 years.

Of course output stage distortions under reactive loads are likely to swamp the early distortion and the sum of the squares is?

Feels good though.
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Old 3rd May 2005, 09:44 AM   #28
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Hey

Amplifierguru could you please explain what you mean with your first lines in your message? an exsample?

What kind off transistor are you talking about?

In which way will PSSR be affected?

Best regards
Kim
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Old 3rd May 2005, 10:16 AM   #29
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Hi everyone,

how about using a Mosfet for VAS[drawback it has huge Cgs & Cds]. Since Nelson Pass has also used it in its designs.

regards,
kanwar
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Old 3rd May 2005, 08:44 PM   #30
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mosfets have terribly low trnsconduction compared to bjts.
I would rather think about RF bjt. Have you ever used RF bjt?
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