Cascode Output Stages

[ppl]

I like Cascoded output stages because of the fast transient response i hear and yes i agree with Mr Pass completely that cascoding can make the transistor more Linear because of constant Vce plus the Advantage of substantially lower miller capacitance yes this quality is also impotent in Emitter Follower's. I have found Cascoded output stages to be as prone to instability as the more popular CFP type BTW is not the Stasis circuit come sort of gene splicing of a casscoded and a multi-Transistor CFP output Topology ????? attached is a Diamond Buffer cascoded type of low power line level output stage.

[PigletsDad]

I came up with a similar bootstrapped diamond buffer variant about a year ago. See http://www.battletonphoenix.co.uk/buffer.pdf

[ppl]

Originally posted by PigletsDad
I came up with a similar bootstrapped diamond buffer variant about a year ago. See http://www.battletonphoenix.co.uk/buffer.pdf

Yes you did! How was it's sound compared to conventional Diamond Buffer? I Amuse that with 4 volts on the Emitter's of Q2 & Q6 that the LED's you used were Blue? Have you tried different reference voltages? I found the 2 Blue LED's almost perfect.

BTW Cascoding also substantially reduces the amount of high frequency EMI polution on the AC Mains even better than two cascaded regulation stages.

[jcx]

Originally posted by pietjers
Grey,

The circuit in the link is an example of what you mention in your post.
http://www.audio-circuit.dk/Schemati...d_Stasis_1.pdf

I was thinking: wouldn’t it be more accurate to drive the upper part of the Cascode from the INPUT instead of the OUTPUT. One of the main goals of this construction to keep the Vce of the output device constant. Because the voltage at the output has added distortion, time lag and a small voltage attenuation, wouldn’t it be better to drive both upper and lower parts of the Cascode from the SAME input voltage? It’s also very important that both parts of the Cascode (upper and lower) are identical. So if you use a darlington or triple darlington for the lower part, use the same darlington or triple darlington for the upper part.

A few years ago I was working on a tube Cascode stage and measured the how constant the voltage of the lower part of the Cascode was held. It was clear then that the best result were obtained when the upper part got it’s signal from the input.

Peter

depends on which characteristics are most important

often the drive impedance of the BJT or even MOSFET output device is a very nonlinear load to the VAS - cascoding the output Q with bootstrapped Vref from the source/emitter gives higher input Z and less nonlinearity for the VAS to drive - SS outputs are quite different form tubes with their much higher gate Z

[Bonsai]

my reply is a little late, but jcx is correct - its more totem pole rather than cascode since Vce varies on both devices in sync with the output devices.

[ppl]

It is my understanding that there are 2 types of Totem pole output stage topologies used lot in the early 1970's. With the Dynaco Stereo 400 long with early SAE and Sumo products representing what is often referred to as a Bootstrapped configuration using Resistive voltage dividers along with a pair of Large Value Electrolytic capacitor supplying the AC drive and thus performing the Bootstrap operation, if we then remove the capacitor and replace the Resistive voltage divider with a Current source and Stable Voltage Reference then we have what is referred to as a cascode configuration with the Voltage reference floating along with the Output wave form as is the case with the resistive network however in the second case the stable voltage reference keeps the DC Vce of the lower cascode transistors constant, This type of cascoded output stage topology is what is shown in Mr. Pass's Technical white paper on the subject previously mentioned. Any way that is my understanding of the different configurations.

[Lumba Ogir]

Peter,
in my view: cascoding originally was meant to extend the bandwidth in RF applications already in the tube era by decreasing the (sonically highly deteriorating) Miller capacitances in voltage amplification stages, but it provides significant improvements in numerous other ways and places, like the output stage. There is no justified reason to desist from using it.
The cascoding device should be applied in grounded base configuration with (by definition) zero signal voltage at its base* and keeping Vds or Vce constant by a stable low impedance reference voltage.
* The cascoding device should be bipolar or (discretely composed) Darlington.

[pietjers]

Thanks again for the interesting replies.

It is a bit surprising that with al the technical benefits that cascode output stages have, almost all designs (still) are normal CFP or EF.
Or is the performance on paper better than in real life?

What are the sonic benefits? Has anyone experience in this area?

Peter

[Lumba Ogir]

Peter,

It is a bit surprising that with al the technical benefits that cascode output stages have, almost all designs (still) are normal CFP or EF.

It is not surprising, people prefer stereotype topologies. One may have considerable difficulties seeing why those amplifiers would sound better now than 30 years ago. Of course, they can not.

What are the sonic benefits?

Briefly, a less distorted sound. It is due to reduced burden of the amplifying devices by limiting the operating range and placing them in their more linear region. Improved power supply ripple rejection, lower thermal related distortions and noise, the provided insulation are additional reasons, there are more. Cascoding often allows use of devices with better properties.
The most remarkable compound is the CFP, combining these would certainly result in unbeatable linearity in the output stage, where usually high distortion levels occur. Look out for oscillations!

[h_a]

Cascoding for the outputstage is not very practical in real life.

If you want best linearity, you use the transistor in a very small voltage range, thus small voltage swing. Consequently you have greatly reduced output power.

It's the same for the cascoded power jfet; how much power can you get from power jfets, cascoded to about 5 volts to keep dissipation down? C'mon!

If you enlarge the outputswing, you get reduced linearity. Next thing is that especially mosfets like high Drain-Source voltages for linearity, so usually it's better to drop the cascode and get the larger voltage swing.

But of course feel free to build and see for yourself!

By the way, read D. Selfs book, the outputstage does not produce so much distortion compared with the VAS or the input. While most here will not agree with his high-feedback attitude, he shows undebatable measurements.

Have fun, Hannes