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.
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.
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.

On Tue, 6 Nov 2012 02:18:14 -0800 (PST), clifford wright
wrote:

Has anyone else tried these circuits?

No, but I saw that in theory they might work.

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.
Result, no gain or a loss!!!

The spread in FETs is rather high so you might have had the bad luck that the
ones you used have a cut-off Vgs lower than ~ 1.8V. One issue is strange though:
maximum gain occurs not at the limit of V(AGC) of 4V but at a higher voltage.
At 8V at the base, one stage (J310 +2N3409) provides for some 16 dB (voltage
gain) then but about 10 dB of it is due to transformation from 50 to 2200 ohm at
the input LC.

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.

It could be useful to try the circuit with a FET with much higher cut-off gate
voltage. An example is BF246C and BF247C. It should work with the diodes in
series with the source resistor.

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.

I wonder why you didn't try to get an oldie like the Plessey SL312. Look it up,
and see that it's perfect for the job - it provides some 30 dB gain without an
input up-transformation and with a larger AGC range.

Regards,
Jan

On Thursday, November 8, 2012 6:37:09 PM UTC-8, Arid ace wrote:

I wonder why you didn't try to get an oldie like the Plessey SL312. Look it up,

and see that it's perfect for the job - it provides some 30 dB gain without an

input up-transformation and with a larger AGC range.

Is the SL312 still available? Texas Instruments' TL026 is still new from
DigiKey and Mouser, even through-hole parts. Av of 55 and 50dB of AGC
range.

Cheers,
Dana K6JQ

On Thu, 22 Nov 2012 02:27:13 -0800 (PST), Dana wrote:

On Thursday, November 8, 2012 6:37:09 PM UTC-8, Arid ace wrote:

I wonder why you didn't try to get an oldie like the Plessey SL312. Look it up,

and see that it's perfect for the job - it provides some 30 dB gain without an

input up-transformation and with a larger AGC range.

Is the SL312 still available? Texas Instruments' TL026 is still new from
DigiKey and Mouser, even through-hole parts. Av of 55 and 50dB of AGC
range.

Cheers,
Dana K6JQ

My error - the IC mentioned should be SL612C and nowadays it's available from
China via eBay:
http://www.ebay.com/itm/120922014405... 4.m1497.l2649
It has a much wider AGC range than the TL026, up to 70 dB IOW higher than the
stage gain.

Jan

On Tuesday, December 4, 2012 6:54:15 PM UTC-8, Arid ace wrote:

My error - the IC mentioned should be SL612C and nowadays it's available from

China via eBay:

http://www.ebay.com/itm/120922014405... 4.m1497.l2649

It has a much wider AGC range than the TL026, up to 70 dB IOW higher than the

stage gain.

Let's see. SL612, $8+ each. TL026 $1.50 each.
-3dB bandwidth - SL612 up to maybe 15MHz. TL026 up to 50MHz.
SL612 AGC range 70dB, gain of 34dB. TL026 AGC range 50dB, gain of 38dB.
SL612 obsolete. TL026 in production.

An AGC range far in excess of stage gain isn't much use if the stage
is overloading anyway.

Why again would I consider the SL612? :-)

Dana K6JQ

On Thu, 6 Dec 2012 00:45:17 -0800 (PST), Dana wrote:

On Tuesday, December 4, 2012 6:54:15 PM UTC-8, Arid ace wrote:

My error - the IC mentioned should be SL612C and nowadays it's available from

China via eBay:

http://www.ebay.com/itm/120922014405... 4.m1497.l2649

It has a much wider AGC range than the TL026, up to 70 dB IOW higher than the

stage gain.

Let's see. SL612, $8+ each. TL026 $1.50 each.
-3dB bandwidth - SL612 up to maybe 15MHz. TL026 up to 50MHz.
SL612 AGC range 70dB, gain of 34dB. TL026 AGC range 50dB, gain of 38dB.
SL612 obsolete. TL026 in production.

An AGC range far in excess of stage gain isn't much use if the stage
is overloading anyway.

Why again would I consider the SL612? :-)

Dana K6JQ

The data sheet of the TL026 doesn't mention the maximum input signal that can be
processed. For the SL612 that is 250 mV rms. Then there's the supply current:
less than 5 mA for the SL612 but for the TL026 it's 6 times as much. Not to
mention the issue of stability.

For an IF higher than 10 MHz, the SL611 has -3 dB at 80 MHz and for the SL610
that is 120 MHz. On SW, an IF amp with low NF allows for a passive mixer as
long as the overall NF stays below ~10 dB. The SL612 has a 3 dB NF and a passive
mixer like the VAY1 a loss of ~ 5.5 dB so even with some input filter losses,
one can design a radio with high dynamic range, low power consumption and NF
10dB. Who would need a TL026? :-D

Jan

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