On Tuesday, November 6, 2012 5:18:14 AM UTC-5, clifford wright wrote:
Has anyone else tried these circuits?

I have built 2 examples, a 9 MHZ AGC controlled amplifier and a 20 dB wideband amp for a noise cancelling system.

BIG problems! The diode string in the source of the FET always biases the J310 OFF.

According to the spec sheet curves:
The j310 gets turned off with a bias of 3.7vdc,

Quiescent of 3ma that is called for gets set at 3.3vdc bias.

Meaning the resistor in series with the diodes is going to drop .54vdc at 3ma.

so it is going to fly on the Vdrop of the diodes Vdrop that changes with temp also? Might work in a spice sim but not in real life.






Result, no gain or a loss!!!

It took me an hour or two to twig what was going on. The trouble is that I cannot see how it can possibly work when the gate is referenced to ground. It is equivalent biasing off a valve!

Not only that, but the AGC circuit published at the same time had extreme instability in the front end of the AGC amplifier. I am a VERY experienced electronics person (60 years) and was unable to find any obvious reason for this.


Yeah this is a cascode amplifier, great for eliminating miller effect, and gain that is the gain of the bipolar times the gain of the fet. Another reason they swamp the gate with 2200 ohms with all that gain. well except in your case.

SO Now I have actually began to lose faith in one of my favourite engineers.

Or, have I done something really stupid in my old age.

FYI the circuits were built "dead bug" style with extensive shielding and full decoupling.

Lastly Since they are using diode drops for bias I would have used a TL431 precision and temp compensated regulator, would be alot easier to set that 3ma quiescent current.

73 OM
de n8zu

On Friday, December 14, 2012 4:08:58 PM UTC-8, raypsi wrote:
On Tuesday, November 6, 2012 5:18:14 AM UTC-5, clifford wright wrote:

Has anyone else tried these circuits?



I have built 2 examples, a 9 MHZ AGC controlled amplifier and a 20 dB wideband amp for a noise cancelling system.



BIG problems! The diode string in the source of the FET always biases the J310 OFF.



According to the spec sheet curves:

The j310 gets turned off with a bias of 3.7vdc,

The datasheet for the J310 specifies a pinch-off voltage range
of -2.0 to -6.5V. Completely normal production variation may be
quite different than the spec sheet curve. So it sounds like
Clifford has J310s that have a relatively small pinch-off voltage,
perhaps even out of spec, and the ~2V drop of the diode chain is
biasing the FET off.

Data sheet for the Fairchild parts: http://www.fairchildsemi.com/ds/J3/J310.pdf

Dana K6JQ

I had the same problem and did not understood why.
I replaced the J310 with J309 and it works properly.
Regards