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.
W3JDR Bright Ideas 