So I built the similar one like the circuit below, but found poor gain when load with 50 Ohm in the output side. I found the BGA416 is the cascode type LNA and have 2 BJT inside it. The ADI/TI chip is so expensive now and I also heard it is a little noisy than the other type. I will try the core of the Steber tuned preamp. So I am seeking for the more wider bandwidth circuit. The sensitivity were poor above the 10m band due to the J310. As for my current circuit,there have a bandpass filter in the front end ,following with the switchable 20db common emitter ampliifer, then the J310 and the quadrature detector. The J310 were loaded with 200 Ohm and gain 6db on and below the 10m band. ![]() ![]() Hi there, I agree with UR5FFR about using a common gate JFET stage, it's simple and offers good input-output isolation, another idea may be starting with the tuned preamplifier circuit presented in the February 2018 issue of QEX and found here the preamp is pretty simple/straightforward and offers good performances, willing to improve it, an idea may be adding a common gate JFET stage between the antenna and the tuned circuit input, such a stage should offer pretty LOW gain, it will only be used as a buffer to both isolate the antenna and offer a low input impedance suitable for feeding the stage using a standard coax being the stage wideband, it will be better keeping the gain low to avoid overload from strong "off band" signals, the following varactor tuned circuit may be left as-is, or if aiming for multiband operations, modified to include a band switch to allow the circuit to operate over the whole desired frequencies range, the preamplifier may be modified too, if desired, an idea in such a case may be replacing the whole preamp (starting from R3 in the schematic) with an AD605 IC such an IC offers a 40MHz bandwidth, which is more than enough to cover the whole HF range, plus it offers gain control, as is the gain control ranges from -14dB to +34dB, but adding a -10dB attenuator, it will be possible to bring the preamp to a -24dB to +24dB range which should be more than enough to deal with strong or weak signals, plus it would then be easy to add an AGC control to the AD605, in such a case the operator may still have a "gain" control knob, but it would just act on the max gain, while the effective gain (up to the max set by the operator) would then be controlled by the AGC That being said, I'd avoid using a wideband RF preamplifier, in my opinion if you're seeking gain, better adding it where it's needed, that is after a tuned circuit, so that the preamp won't be overloaded by offband (off the DESIRED band) signals and offer much better performances My 2 cents Forgot, if going to build the Steber tuned preamp (either using the original preamp or another one), it will be advisable to put a voltage regulator and some bypass capacitors on the line going to R2/VR1 to ensure the voltage driving the varactor will be stable and that it won't be modulated by RF leakageĭear All, ManY Thanks for your kindly help! I have tried the common gate amplifier with J310 and found good isolation but gain dropping for about 6db when working on 6m band and above.
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