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Old 2nd April 2012, 08:41 AM   #1
Bonsai is offline Bonsai  Taiwan
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Default 182MHz Oscillations :-0

I was doing some debugging this week end. Here's what a cascode can do if your layout and cascode base circuit are not taken care of. I have left a small write up on my experience on my web site - last page.
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Old 2nd April 2012, 09:33 AM   #2
wahab is offline wahab  Algeria
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Modern transistors are awsomely fast....
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Old 2nd April 2012, 09:39 AM   #3
PMA is offline PMA  Europe
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Quote:
Originally Posted by Bonsai View Post
I was doing some debugging this week end. Here's what a cascode can do if your layout and cascode base circuit are not taken care of. I have left a small write up on my experience on my web site - last page.
Do not cascode your VAS. Most of the "improvements" dictated by race for lowest THD and based on simulations only in fact lead to VHF local oscillations ....
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Old 2nd April 2012, 09:44 AM   #4
Bonsai is offline Bonsai  Taiwan
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Hello PMA - the picture above was for the front end LTP cascode . . .

My new design does not cascode the VAS. I agree its not needed unless you are shooting for sub 1ppm - but that is not my aim. What's clear is if you want to cascode, you need a good widebandwidth scope :-)
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Old 2nd April 2012, 09:46 AM   #5
Bonsai is offline Bonsai  Taiwan
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Quote:
Originally Posted by wahab View Post
Modern transistors are awsomely fast....
Yes. Couple that to 30MHz output stage bipolars or even faster mosfets and you can have an RF transmitter if you are not careful.
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Old 2nd April 2012, 09:52 AM   #6
PMA is offline PMA  Europe
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Quote:
Originally Posted by Bonsai View Post
Hello PMA - the picture above was for the front end LTP cascode . . .
Hi Bonsai,

was it a BJT input or FET input? I got something similar with a BJT input ... And I could measure it only with a spectrum analyzer ..
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Old 2nd April 2012, 10:05 AM   #7
Bonsai is offline Bonsai  Taiwan
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BJT input fully balanced with cascode. If you go to my website on the last page (Audio Amplifier Design) its right at the top of the page Audio Amplifier Design and Circuits | hifisonix.com.

I was using a 200MHz analog scope and it looked like noise until I hooked up Rigol digital scope.
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Old 2nd April 2012, 10:18 AM   #8
PMA is offline PMA  Europe
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Quote:
Originally Posted by Bonsai View Post
BJT input fully balanced with cascode. If you go to my website on the last page (Audio Amplifier Design) its right at the top of the page Audio Amplifier Design and Circuits | hifisonix.com.

I was using a 200MHz analog scope and it looked like noise until I hooked up Rigol digital scope.
That's it ... the BJT input fully balanced with cascode - same problem. I will visit your page. Many designs have similar problems inherent without being disclosed ..

Thanks,
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Old 2nd April 2012, 10:51 AM   #9
Bonsai is offline Bonsai  Taiwan
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I have just noticed I drew the PNP cascodes in the negative half of the FE upside down. I'll correct this later tonight.
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Old 2nd April 2012, 11:22 AM   #10
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Hi Andrew,

Before further discussing your oscillation problem, I've a more fundamental question:
Why connecting the bases of the cascode trannies to a fixed voltage (12V) anyhow?
Wouldn't it be better (and much simpler!) to let them follow the input voltage, as shown below. Moreover, in this case you get the base stoppers (R11&R14) for free.

Cheers,
E.
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