Cascoded -MosFet- Zen without feedback

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Jump to Latest
T

Tyimo

Member
Joined 2002
Hi!
I moved the theme and open this thread beacause I didn't want to distub the conversation in the original Pass JFET Power Amplifier thread.

So, I will try again to ask and maybe some expert could be more focused to my "problem". :)

I would like to build the ZenV8, but here in Europe-Hungary I cann't buy the Lovoltech JFets yet. So, I was planing to build the ZV8 circuit as it is ,but with only Mosfets instead of the Lovoltech JFet.
I asked Mr. Pass how about to use Mosfets instead of Jfet in the circuit. He answered:

"Absolutely.Just Cascode a Zen . My original design for the F3 was precisely that, but did not use feedback."

Vix built successfully a version of the Cascoded Zen, but he used some negativ feedback. I would like to build mine without feedback, like Mr. Pass did with his prototype circuit.

So, my old question is still:
How could we build a NNFB-Cascoded- Zen???

or, I have to ask Mr. Pass self: could you post here your prototype's schematic?

Greets:

Tyimo
 
jh6you

jh6you

Disabled Account
Joined 2002
Hi, Tyimo

Sometimes we are flexible in thinking. When we see a number of 0.01, we easily claim that it is 0. When we have a feedback line with very small (ignorable) feedback, we could say NO feedback. If you post a circuit as you think, many will help you to get a nice result . . . I believe so . . . :D

Regards
jh
 
T

Tyimo

Member
Joined 2002
Hi JH!

O.K. For starting point here is Vix's modificated ZV8 version that works already but with a little feedback.

I hope He doesn't mind that I post here his schematic!

Greets:

Tyimo
 

Attachments

  • cascoded zen by vix.jpg
    cascoded zen by vix.jpg
    39.5 KB · Views: 650
jh6you

jh6you

Disabled Account
Joined 2002
I think that R3 and P1 are there to set a DC bias point at the Drain of the upper MOSFET with respect to the operating Gate voltage of the lower MOSFET, and might be necessary as having a role to set up the gain of the amp.

I will interprete your "In-" as a ground.

Regards
jh
 
flg

flg

Member
Joined 2005
Sure, they are necessary for setting gain, as a feedback function. To operate with no F.B./open loop, you need to use other methods for setting gain.
Raising the values very high basically is asking for very high gain. You reach a point were ther is no more... This allows the input to yank the gate node around, basically overcoming the feedback I and operating open loop. By using the rail voltage as the source for your bias, there is no variability based on output voltage. No summing with the output feedback I. And, full open loop operation.
The size of R1 can be used to set gain. The bigger it is, the more it kills the gain. But, in the range you might use to get reasonable gain, you will also be making the device operate more linear. As usuall, it is a compromise of the good and bad characteristics and the best balance is the final measure of performance...
I realize, I'm not the best at describing this stuff but, I'm thinking I'm correct about that issue. Another issue I'm still thinking about is the cascode modulation thing. If you want to drop in a MOSFET rather than the JFET, I think you are trying to make a crude oscilator but I'm still thinking about it...
 
C

Circlomanen

Member
Joined 2004
What would happen with the in the original ZV8 amp, if I would simply put an IRFP044 MosFet to the place of the Lovoltech JFet ?!?

Greets:

Tyimo
Click to expand...
Nothing at all. You need to get a couple of volts on the gate of the mosfet referenced to the sourcepin, before it starts conducting.
The J-Fets are depleationmode, so they have to have a negative voltage on their gate referenced to their source to limit currentflow through the device.

Johannes.
 
Vix

Vix

Member
Joined 2003
Hi Tyimo!

I don'y want to discourage you, but I don't understand you you insist so much on a non-feedback variant, with a mosfet?

I am saying this because I have an impression that the attention is shifting toward the new Jfet devices, and that's where you could expect the biggest benefit.

While all of us are free to experiment as much as we wish (ss long as we are not violating Nelson's copyrights), I am afraid that by trying to make a non-feedback variant you invite more trouble (instability) than benefit.

Personally, I was curious to see how the circuit will a mosfet (and it worked), but that is not going to be the final version. I will mount the Jfets as soon as I get them. (I am planning to order them).

Then, my suggestion is: Make a feedback variant, enjoy it for a while, and in the meantime try to obtain some Jfet devices. Then you will be able to enjoy the non-feedback variant with Jfet, exactly as Nelson described.

Moreover, I am impatienly waiting for Mr. Pass to publish other Zen variants..(or other amplifier circuits featuring Jfets)

Sorry for harsh words,

regards,

Vix
 
T

Tyimo

Member
Joined 2002
Hi Vix!

I don't want to discourage you, but I don't understand you you insist so much on a non-feedback variant, with a mosfet?
I am saying this because I have an impression that the attention is shifting toward the new Jfet devices, and that's where you could expect the biggest benefit.
Click to expand...

You are right!! I give up!! I will rather go to listen music on my ZenLite!!!!! :)))

But some more last question:
-what is it for the C2 1000uF elko and the R4 100ohm resistor?
-from Who will you order the JFets?

Greets:

Tyimo
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Top Bottom