The AGC addition on the cascade/cascode IF amplifier

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Re: The AGC addition on the cascade/cascode IF amplifier

Post by John on Mon Apr 29, 2013 11:05 am

I have not tried that circuit out yet, but I see it as more of a variable attenuator than a true AGC circuit; it appears that when the IF output reaches a certain level the agc transistor turns on and shunts some of the IF input to ground, lowering the effective IF input to the amplifier. My only wory is that the amplifier is always working at maximum gain' with the input attenuated; so the circuit may be noisier than it has to be. I have been looking for a circuit with voltage controlled negative feedback.

I have recently experimented with a similar circuit using the cascode configuration, I use a resistor in the emitter circuit of the bottom transistor, but rather than bypassing this resistor with a fixed capacitor, I use a capacitor in series with the emitter-collector junction of an additional transistor, and vary the bias into the base of this additional transistor to provide variable emitter degeneration. I have also tried it with a simple common emitter amplifier and with a rather high 10K emitter resistor and variable bypassing, i get an AGC range of 38dB. Unfortunately the AGC works in the opposite direction, with the highest agc voltage producing maximum gain. This could be fixed with an inverting op amp I suppose.

Harry's circuit has the advantage of simplicity and the fact that it is easy to 'bolt-on'.

I dont know if anyone else agrees, but I would like to see a simple superhet DSB transceiver on the homepages using only discrete transistors, with none of those expensive crystal filters, or 'seen it all before' NE612 designs.

John

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Re: The AGC addition on the cascade/cascode IF amplifier

Post by radix3 on Sun Apr 28, 2013 3:35 pm

As no one replies, I post my own ideas concerning the operation of this nice additive AGC circuit:

1) Due to the fact that, the time constant of the RC parallel circuit is much much greater than the period of the input signal (assuming 455 KHz) the diode circuit will work pretty like a peak detector. The time constant also is considerably higher than the audio frequency range so the output of the detector circuit will be a DC voltage.
2) The level of the DC voltage appeared at the output of the detector will depend on the level of the IF signal, the values of the elements of the AC voltage divider formed by the 22k, 10nF and 10K circuit elements before the diode.
3) The diode will turn on when the voltage level exceeds a certain threshold which is determined when the voltage drop across the diode is about 0.6-0.7 Volts.  
4) As a result the transistor will turn on if the voltage level appears on its base-emitter junction (base itself is enough as emitter is connected to GND)  is around 0.7V.
5) We have no AC signal onthe collector.
6) However, when the collector - emitter junction turns on a current will  flow over the 47K resistor through it. This phenomenon can also be compared to the diode band switching mechanism in the UHF/VHF tuner circuits where an RF current flows through the band switching diodes when a small DC current is allowed to flow through the diode.
7) This means that one can, flow a RF current from the collector - emitter junction just like in the case of band switching diodes. So if the collector is connected to the collector of the AGC transistor through a DC blocking capacitor as done in the schematic, an IF current will flow from the input of the cascode amplifier which will lead to a decrease in the level of the IF signal on the input of our amplifier. This will lead to the clamping of the IF signal on the output of the amplifier.
Cool The level of clamping will depend on the turn-on resistance of the AGC transistor which determines the level of IF current flowing across it. This determines the input level of the signal to the amplifier. 

This was a short summary on the operation of the AGC circuit from my point of view. Any corrections by Harry or colleagues will be welcomed.

Best regards,

Ozgur

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Re: The AGC addition on the cascade/cascode IF amplifier

Post by radix3 on Fri Apr 26, 2013 5:17 pm

Thanks Ivan but I do not see any related replies there.

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Re: The AGC addition on the cascade/cascode IF amplifier

Post by Ivan on Fri Apr 12, 2013 5:50 am

I am sure Harry (admin) has already replied in the original thread. Arrow

BR from Ivan

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Re: The AGC addition on the cascade/cascode IF amplifier

Post by radix3 on Thu Apr 11, 2013 10:49 pm

No replies?

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The AGC addition on the cascade/cascode IF amplifier

Post by radix3 on Mon Apr 01, 2013 4:55 pm

Hello,

I would like to ask a question about the cascode/cascade IF amplifier. The AGC circuit appears interesting to me which is different from the ones in conventional IF circuits. However, I could not grab the connections from the input and output terminals of the AGC circuit. Is this, a feedback type connection (ie. connection from IF out terminal of the amplifier and feeding back the output of the AGC to the input of the amplififer? Or just connected to the output of the amplifier and the output of the AGC is for example connected to the demodualator circuit.

Sorry for a bit complicated question but thanks for your kind consideration.

Regards,

EDIT: This is an identical post to one in the thread "Cascade/Cascode IF Amplifier question". I have removed that post since we have a new thread started here.

DragonForce, board moderator.


Last edited by DragonForce on Sun Apr 07, 2013 6:38 pm; edited 1 time in total (Reason for editing : Moderation.)

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