Amplifier without vas stage

[bogdan_borko]

Whot do you think about this design of mine.
It has no VAS stage, working in class AB.



(just simulation for now)


IS there any other design without vas and do you have the schematic?

[bogdan_borko]

I know that distortion predictions in simulator are not so true, so what do you measure in simulations when designig an amplifier?

thanks!

[traderbam]

Hi Bogdan,
I like the novelty of your design. By my reckoning your design has no single VAS stage but instead has two separate VAS stages formed by Q7+Q9 and Q8+Q10.

One issue with it is that if the two VAS stages do not behave in a perfectly complementary way then their outputs will "fight" and you will see cross conduction. It is as if you have designed two voltage amplifiers and then put them in parallel. To make them behave in a truely complementary way will be extremely difficult.

[darkfenriz]

Check Greg Ball's "Simple killer" amp, it uses similar differential to common source output stage implementation.

You should now find ways to control bias and thermal track output stage of this design, but most probably you know how to do it

[lineup]

Originally posted by bogdan_borko
Whot do you think about this design of mine.
It has no VAS stage, working in class AB.
(just simulation for now)
IS there any other design without vas and do you have the schematic?

This type of so called Current Feedback Amplifiers can be very fast.
Can easily get into oscillation, if not proper care is taken to avoid this.

Another issue is to try to get the idle current at correct and stable level.

Usually the compensation cap is not put across the feedback resistor
for this type of amplifier ( Emitter Feedback Amps is a better name ).
Instead, in your circuit should be a cap across R5+R10 and R6+R11
or alternatively from Q5 emitter to V+ and Q6 emitter and V-.

[banana]

Shouldn't feedback be taken to the gate of Q1 Q2 instead?

It looks like the circuit will reverse signal phase.

[bogdan_borko]

I`m planing to use Q5 and Q6 for thermal compensation, but ther`s a bias problem. Whould it be safe to adjust bias, by changing input stage jfet current?

you didn`t answered to my second question...

thanks guys.

[lineup]

.
Sure there are several other Amplifiers
that uses the output complementary pair to do the voltage amplification.

Your circuit has one input stage + one VAS output stage.
In reality it is not the VAS you have removed
it is the last output stage .. the Follower you have removed!

Because in your circuit the VAS is the final stage .. but using slower more powerful output transistors.



There is a clever and interesting old boy in Texas USA
that has made a lot of different amplifier experiments.
One such try out is: ZEN v5 ... The Complementary ZEN
Before he had mostly used only Single End N-MOSFET output stages.
This time he investigated Push Pull.


Zen v5 takes your way to remove one stage .. a step further:
Nelson has here removed even the input stage!
Leaving us with ONE Powerful VAS stage
that acts also as input and output stage.

Links:
Papa! I want to have Zen V5
Pass D.I.Y. Amplifiers

Attachment shows the most beautiful and unique amplifier schematic
.. one of a kind ...

lineup

[sreten]

Hi,

Two stage amplifiers do not have a seperate Vas but tend to be class A.

The JLH class A is an example, the Death of Zen another.

Its a bit difficult to build an amplifier that does not amplify voltage,
and whatever does is the voltage amplification stage.

/sreten.

[bogdan_borko]

The output stage is the last stage as far as I know, operating in common source or in comm. drain doesn`t matter. WHEN I say VAS stage I think a classic voltage amplif. stage that is often i the middle.
IMO ,the thing that is good in this design is that otput mosfets are loaded from low impedanse emmiter followers and we still have 3 amp stages! In the classic design would be: Input stage+VAS+Follower, and it is also 3 stage amplifier, but this time output mosfet are loaded from high impedance VAS.

I hope you`ll understand what I`m trying to say.
Cheers!