High Performance AGC Amplifier

26 Jan

I’ve been looking for a high performance AGC amplifier. Some criteria are:

  • Wide control range with linear volts/dB.
  • Strong input signal handling capability
  • Decent noise performance

For a while, I thought the HyCAS amplifier developed Hayward et al was it. However, some LTSpice modelling followed by bench analysis indicated that the amp suffered from very non-linear gain control as well as completely collapsing with strong input signals when gain is set a minimum. This is because the circuit reduces gain by starving the input JFET stage of drain voltage. With maximum gain reduction, the FET has virtually no voltage to work with. That’s a no-no.

I developed an enhanced version of the HyCas that linearizes the gain control over a 70 dB range and suffers no loss of input dynamic range. It does this by varying the JFET  source voltage, driving the FET current towards pinchoff and decreasing the current. So far, it’s only been run as a LTSpice model, so it needs validation. Therefore, I present the circuit solely for your edification until I build it and test it. The amplifier in the schematic above is tuned for 9 mHz and has about 14 dB gain, but it can be re-tuned for most any frequency in the HF range.  The resistor across the tuned circuit is to limit the gain and prevent oscillations. The diodes on the control voltage, together with the input resistor,  linearize the control range, otherwise it gets way too sensitive at the bottom end of the control range. This is very important in AGC applications, as severe non-linearities can cause motorboating and other anomolies. The linearization of the control voltage also makes it much more useful to develop an S-meter from this voltage.

If  you decide to build it, keep in mind that it was developed for a U309 (J309 is identical), so it will perform differently for a more common J310. Also, JFETs can vary widely with respect to their pinchoff voltages, so some tweaks to the linearizer might be required.


2/1/2018 Update:

I got an LTSpice model for the J310 and confirmed it’s a different beast in this circuit. I ordered a bunch of ‘309’s and will report when I get a chance to try them in a real circuit.





4 Responses to “High Performance AGC Amplifier”

  1. (Thomas) Richard Davies February 15, 2018 at 3:27 am #

    One of the difficulties of AGC amplifiers is that you still need plenty of output even when the gain is at its minimum. This is very different from an amplifier used for say a volume control. The secret is usually to have several stages, each with only a modest gain reduction. I’m very happy to exchange emails on this subject, but my life is fairly busy just now so I can’t get too involved just yet.

    Well done on your work and on taking the time to publish.

    Richard. M0AQF

    • w3jdr February 15, 2018 at 7:11 am #

      Thanks for the reply.
      I’m having a little difficulty understanding the intent of your first sentence. If you were thinking of audio AGC, which is applies after the IF gain/Detector process, I agree with you. On the other hand, IF AGC, especially if applied early (but not too early) in the IF gain cascade only has to contend with relatively small signals. Having said that, this amplifier is pretty robust.
      I’ve built similar amps, but this one uses a different method to apply the AGC voltage. This one has only been tested in LTSpice, so the actual bench performance is yet to be proven, especially the linearizer, which will probably require tweaking from JFET to JFET.


  2. Albert Peter February 23, 2018 at 11:17 am #

    Hi Joe,
    Nice work on extending the hybrid Cascode. I bread-boarded the circuit using a J310 I had on hand (have J309 on order). It works great with one exception – the diodes appear to be unnecessary, in fact with them as shown, the DC control voltage appears to be shunted to ground.

    I was not able to measure the full gain reduction capability, but I was able to accomplish at least 50dB reduction with the trend line continuing.

    Next task is to integrate with a detector and put into the uBitx. I intend to use audio from the top of the audio pot into a high gain amplifier followed by an envelope detector for the DC. Later, I may substitute a log amp to acquire the DC control.

    Any thoughts on the diodes? Also, would you possibly share the Spice model?

    • w3jdr February 23, 2018 at 11:44 am #

      Please send me an email at joeATroccisDOTcom.
      For the record here,the LTSpice sim does show that the J310 is a different beast as far as the AGC curve, and I even have doubts that the ‘309 will behave in the real world as it does in the sim. My latest thinking is to sample the audio in software to get a peak value and use that to run an I2C digipot that develops the AGC voltage according to a lookup table.This is potentially a very fast settling loop, as the feedback would be exact, with little/no overshooting or undershooting. If you use this in the uBITX, you’ll have to figure out how to retrofit it within the existing BiDi amplifier. I’m working on that as we speak.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: