Why collector/drain output stages are so rare???

[rlim]

Originally posted by Eva
The huge pitfall of collector/drain output stages is the complete lack of quiescent current and cross-conduction control when large high frequency AC signals are amplified.

Can you elaborate on this? If the designer intended it to be a class AB push-pull output stage, then up to a certain amplitude, either the p or the n will not have current while the other provides all the current to the load.

[Eva]

Originally posted by rlim


Can you elaborate on this? If the designer intended it to be a class AB push-pull output stage, then up to a certain amplitude, either the p or the n will not have current while the other provides all the current to the load.

That's true only if you consider an output stage with ideal devices having infinite bandwidth and constant Vbe and Vgs turn-on thresolds, but it's not true at all for real transistors whose turn-on thresholds are dependent on Vce and Vds and which take a certain time to start and stop conducting. Also, output stage has to be intentionally slowed down with gate and base stopper resistors to get a stable amplifier, which further contributes to create current tails and unexpected cross-conduction. Simulation software WON'T predict these effects in most cases because transistor models are not that accurate.

[jneutron]

Originally posted by Eva


That's true only if you consider an output stage with ideal devices having infinite bandwidth and constant Vbe and Vgs turn-on thresolds, but it's not true at all for real transistors whose turn-on thresholds are dependent on Vce and Vds and which take a certain time to start and stop conducting. Also, output stage has to be intentionally slowed down with gate and base stopper resistors to get a stable amplifier, which further contributes to create current tails and unexpected cross-conduction. Simulation software WON'T predict these effects in most cases because transistor models are not that accurate.

My my, somebody's been busy...

Veeeerry nice..

Toimal vbe..dey be da teekit..

Your tails... be they 100 usec level or 100 msec...ie, silicon thickness or case thickness.


Cheers, John

[rlim]

Eva,
The phenomenon that you're talking about.....does it occur with emitter/source follower outputs?

Thanks.

[Wavebourn]

Originally posted by rlim
Eva,
The phenomenon that you're talking about.....does it occur with emitter/source follower outputs?

Thanks.

Try to measure current consumptions on different frequencies.

[Ultima Thule]

Originally posted by Eva
The huge pitfall of collector/drain output stages is the complete lack of quiescent current and cross-conduction control when large high frequency AC signals are amplified. Even popular circuits like SKA suffer very badly from that phenomena..

Hi Eva,

have you tested SKA, if not would you still say that SKA suffers from cross conduction?

Cheers Michael

[lineup]

Originally posted by Leolabs

Why collector/drain output stages are so rare???

Is it because of stability issue???
.

First, common drain outputs are not so rare,
but not at all as common as common emitter.

And you are a bit right in 'stability guess'
- I think generally common drain can be a bit more instable
as well as They are Often VERY FAST amplifiers.
So when building such amp, you may need to do some things to stabilize.
But this goes for most all amplifiers - even the standard type.

--------------------------

Common collector output has got Voltage gain (usually considerably more than 1.0) in output stage.
And this gain is effected by the load.

While common emitter has got less than 1.0 ( something like x0.90-0.99 in a best case ).

This makes common emitter amplifiers a bit less sensitive to loads.
In my opinion.

---------------------------

We can not say, that common collector or common emitter is better.
They can do good, both.

For example in PreAmplifiers, without a buffer follower stage,
the feedback is taken directly from common collector output.
This is not unusual for low power Class A pre amplifiers.


I have a number of bookmarked links to Common Emitter ( Drain ) amplifiers.
They are a challenge and sure are very interesting to design, to build and explore.


lineup

[Ultima Thule]

Lineup,

now you are really mixing up some things here...

Hint: Common drain -> Source follower -> certainly not so rare!

Cheers Michael

[forr]

Hi,

Some examples :
Bipolar collectors output :
Wim de Jager, Erik van der Ven, Ed van Tuyl, ELECTRONICS WORLD, December 1999 pp. 982 – 987
Mosfet drain output :
see the Transnova schematics at the Hafler site.

[lineup]

Originally posted by Ultima Thule
Lineup,
now you are really mixing up some things here...

Anybody that does not understand my post message - probably has hang up on single words and expression.

Messages, posts are made up by sentences.
Sentences are made up by words and expressions (combination of words and terms).
Words are made up with letters, characters of alphabet.

If I write:
- Helo, Ultima Thule.
the usual reply would be:
- Hello, Lineup.

Because this exchange of thoughts, messages is a context,
that shows the receiver got the message, the words
- even though was a detail in message not correct.

Some, often people with negative attitude, will try to look for the details
like bad spelling, bad improper words or terms.
Because they like to take away attention from message
or wont accept to take message.
Maybe they even have another view, that was proven not very good
and instead of accepting - they try to get out of Context .. out of Topic
by focus interest on side notes .. on details.
Details that are not so important
for those who understand the intentions and what the sender is saying.
this is not true communication - it is not good communication


But i can agree so much, that it is very good we use same terms for same things in amps
thanks for correcting my terminology
- i have no whatsoever Degree from Any School in Electronic Engineering.


In my plain world, something that is COMMON, share same thing
in my mind this is the output joint.
And when Collectors are joined together at one point
this is for me 'common collector'.
But this is for those using electronic terms and language
is not the same as my definition and I have to use some other expression.

So exchange all places where I say 'common collector' with
the topic starter 'collector/drain output'
and so you will be able to take part in discussion, Ultima Thule.
As you will know what I mean.

Hope everything is clear now.


No Regards
lineup