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.
mlloyd1
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.
mlloyd1
"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.
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.
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.
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.
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
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
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.
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.
Hey
kimschips - just as added miller Cdom capacitance is used to slug down the GBW of the Vas to establish the dominant pole which also degrades PSRR to the supply that the Vas input is referenced to,... so too with the intrinsic Ccb of the device which sets the upper limit. intuitivey a device with 1pF is better than one with 2pF.
kimschips - just as added miller Cdom capacitance is used to slug down the GBW of the Vas to establish the dominant pole which also degrades PSRR to the supply that the Vas input is referenced to,... so too with the intrinsic Ccb of the device which sets the upper limit. intuitivey a device with 1pF is better than one with 2pF.
If the rest of the circuit is made for it, what is wrong with designing the VAS Iq around 1-2mA? Then small, Lo Noise devices like FJV992 (& FJV1845) can come into play, particularly the 'E' gain series, Beta 400-800! Fairly linear gain up to 6 or 7mA. I am playing with a circuit that uses reg. +/- 60V with this device as the VAS sourced with the FJV1845 mostly out of curiosity, well when I am at home...I work out of state. 
I'm in Utah right now...nice scenery. A bit different from east Tennessee.
Anyway for most Applications under 10mA, also current sources, I think these could be useful devices and they are cheap and small. (SOT-23) As a VAS, I'm not totally convinced, but it does sound pretty good. I have yet to do "experiments" though to see what it will do in this application. Any thoughts?
http://www.fairchildsemi.com/ds/FJ/FJV992.pdf
I'm in Utah right now...nice scenery. A bit different from east Tennessee.Anyway for most Applications under 10mA, also current sources, I think these could be useful devices and they are cheap and small. (SOT-23) As a VAS, I'm not totally convinced, but it does sound pretty good. I have yet to do "experiments" though to see what it will do in this application. Any thoughts?
http://www.fairchildsemi.com/ds/FJ/FJV992.pdf
Hey
Now if we are in for smd:
www.Infinion.com
BFN 18-19
BFN 24-26
BFN 38-39
Rohm 2SC3906 2SA1514
BCP53-56
Best regards
Now if we are in for smd:
www.Infinion.com
BFN 18-19
BFN 24-26
BFN 38-39
Rohm 2SC3906 2SA1514
BCP53-56
Best regards
VAS for +/- 15V supplies
I would appreciate recommendations for a 1 watt complementary pair of VAS transistors for a +/- 15 V power supply. I was hoping that the lower voltage would yield some better specs.
I am currently using BD549C and BD559C in a VAS and Diamond Buffer, but they are only 500mw.
I would appreciate recommendations for a 1 watt complementary pair of VAS transistors for a +/- 15 V power supply. I was hoping that the lower voltage would yield some better specs.
I am currently using BD549C and BD559C in a VAS and Diamond Buffer, but they are only 500mw.
Hey Linesource
Here is some off my favorite transistors:
2SA1540E
2SC3955E
2SA1405E
2SC3599E
2SC3601E
2SA1407E
I think you like them if low capitance, very linear hfe and 1.2W disspation without heatsink is desired
Anyone got a schematic or idea for a preamplifier with nonfeedback, 6dB gain and symmetrical circuit?
And off course very god soundquality.
Best regards
Kim
Here is some off my favorite transistors:
2SA1540E
2SC3955E
2SA1405E
2SC3599E
2SC3601E
2SA1407E
I think you like them if low capitance, very linear hfe and 1.2W disspation without heatsink is desired
Anyone got a schematic or idea for a preamplifier with nonfeedback, 6dB gain and symmetrical circuit?
And off course very god soundquality.
Best regards
Kim
got new transistors!
Hello
As I said erlier I write when I got new transistors
2SA1540E
2SC3955E
From http://www.heldt-electronic.de/
Just mail them your order and send them money by Iban and switchcode.
2SA1405E
2SC3599E
From Jørgen P in denmark TLF +4544945244
Don´t know about paying them from another country, should be not be any problem.
Heldt send me MJE350 instead off 2SA1407E yuk!
Could not get 2SC3601E
Jørgen P could only get 2SC3601E and not 2SA1407E
2SA1407 can optains in another Hfe ranking
Best regards
Kim
Hello
As I said erlier I write when I got new transistors
2SA1540E
2SC3955E
From http://www.heldt-electronic.de/
Just mail them your order and send them money by Iban and switchcode.
2SA1405E
2SC3599E
From Jørgen P in denmark TLF +4544945244
Don´t know about paying them from another country, should be not be any problem.
Heldt send me MJE350 instead off 2SA1407E yuk!
Could not get 2SC3601E
Jørgen P could only get 2SC3601E and not 2SA1407E
2SA1407 can optains in another Hfe ranking
Best regards
Kim
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