agc circuit

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Re: agc circuit

Post by Admin on Sat Mar 30, 2013 7:59 am

demej00 wrote:Thanks for response and ideas. Will try them out.

Additional comments:

An AGC amplifier will change the gain of a stage - great, but there must also be an errer for there to be a correction. If you apply ACG to stages BEFORE the ACG detector you will always have an error, BUT you can also apply a little AGC to the AF stages AFTER the detector and get a seemingly perfect ACG effect :-)

Anothet thing is that the silicon diode has a lagrythmic function. If you apply RF to back-to-back diodes through a 1K resistors then the diodes will conduct and shunt the RF. BUT they will still allow amplitude changes at a log rate, so this can also be used to make a LOG-Amp that does not need ACG.

Food for thought?

Very best regards from Harry - SM0VPO

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Re: agc circuit

Post by demej00 on Fri Mar 29, 2013 10:54 pm

Thanks for response and ideas. Will try them out.

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Re: agc circuit

Post by Admin on Tue Mar 19, 2013 9:29 pm

demej00 wrote:I am interested in adding agc functionality to my 40 meter receiver and saw this circuit:
http://www.hqew.net/circuit-diagram/Simple-AGC-for-radio-receiver_10845.html

which is a little like your IF Amp AGC. The one above has no details and I would like to know if you believe it will work. Doesn't it need voltage on the collector? Would yours work better for 7 meg frequency?

Interesting! In the circuit link there should be some form of voltage on the collector, but by using an FET the voltage shouold not be necessary.

I once did a lot of work on AGC and found that by simply clamping the BASE bias voltage of IF amplifier transistors to ground the gain can be controlled. In fact the amplifier can be switched off totally by clamping with a single transistor (as in the link).

You can also add DC bias to the clamping transistor to add manual IF gain as well add a little to increase the AGC sensitivity. The last IF stage can simply be rectified with a germanium diode and used to turn ON the clamp transistors.

Note, you should use one clamp transistor for each stage but you can feed them from the same detector signal.

I also found that if the clamp stages have too much DC gain then the AGC can oscillate. You can also use "Zero-bias Schotky" diodes to get a better sensitivity.

Best regards from Harry - SM0VPO

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Re: agc circuit

Post by DragonForce on Fri Mar 15, 2013 10:01 am

Why not try it and share your results with us?

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agc circuit

Post by demej00 on Thu Mar 14, 2013 2:57 pm

I am new to building radios but have completed a couple of simple projects.
I am interested in adding agc functionality to my 40 meter receiver and saw this circuit:
http://www.hqew.net/circuit-diagram/Simple-AGC-for-radio-receiver_10845.html

which is a little like your IF Amp AGC. The one above has no details and I would like to know if you believe it will work. Doesn't it need voltage on the collector? Would yours work better for 7 meg frequency?

Thanks for any help.

demej00

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