2x J310 Push-Pull Low-noise HF pre-Amplifier (500 KHZ – 54 MHZ) Built & Tested

BY-PASS RELAY (The pre-amp switches in-circuit as long there is DC feeding either from the Bias-T or from the DC input connector)
BIAS-T through the output connector going to the receiver
GAS PLASMA DISCHARGE TUBE OVERVOLTAGE PROTECTION
NEW EARS FOR YOUR HF RECEIVER
The design is successfully tested with around 15 dB gain from 500 KHZ to 30 MHZ and still has 8-10 dB gain on more than 55MHZ.

The 30 KHZ to 30 MHZ version has much lower gain above 15 MHZ up to 30 MHZ.

The outupt P1dB is +15dBm and it’s noise figure is around 3 dB. The input and the output impedance is 50 Ohms.
This amplifier can be used as a low-noise pre-amplifier in receivers or as a general purpose wideband gain stage.

The two J310 J-FETs are biased at about 25mA each. The input transformer T1 splits the input signal to feed both of the J-FETS in a balanced configuration, while T2 adds the two amplified signals together. This technique improves the 2nd order performance significantly, since all even harmonics cancel out. R1 and L1 form a negative feedback which affects amplifier’s gain and frequency response.

MODIFICATION FOR 50 KHZ TO 30 MHZ (STILL USUABLE FROM 30 KHZ), FOR THE USERS OF LF BANDS
You need to change T1 and T2. Now, for both transformers is needed to be wound on BN73-202 dual hole cores. T1 has a primary winding of 2 turns and its secondary winding 12 turns, again center tapped. T2 now needs to have a 12 turns primary winding (center tapped) and a secondary winding of 2 turns. See the spectrogramms below. By sacrifying the upper frequency response (30-55 MHZ), you can now get a new one at 30 KHZ to 30 MHZ. This is without tweaking the feedback network.