RF amplifier and filter for 2.5MHz

Posted on Feb 6, 2014

By use of transformers one can apply both voltage feedback and current feedback. There was an article in Ham Radio november 1979 by DJ2LR Ulrich Rohde giving details about this circuit. For the 2. 5MHz to audio converter the transformation ratio from drain to source is 2:1. The output transformer, TR3 in fig. 1, is 9 turns trifilar wound on a 16 mm

RF amplifier and filter for 2.5MHz
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toroid. Two of the windings are connected in series at the drain side. The drain is loaded by one 100 ohm resistor to ground and one 100 ohm resistor to the bandpass filter. For details, see the circuit diagram of fig. 1. The input impedance of the filter is about 100 ohms within the passband but it varies a little. Outside the passband the input impedance of the filter is lower. The impedance at the drain of the transistor is very low due to the feedback. The impedance feeding the filter is therefore 100 ohms regardless of what impedance is connected to the input of the amplifier. The third order input intercept point, IP3 of the amplifier is about 40 dBm. The 1dB compression point is at +15 dBm when clipping occurs at the drain. The RF voltage at the drain is then 20V which makes the power absorbed in the 100 ohm resistor 0. 4W. Two input signals of +8 dBm give third order IM at -55dBc while two signals at 0 dBm gives IM3 at -81dBc The input transformer, TR5 in fig. 1. steps up the voltage by a factor of 2. 5 from the antenna input to the gate. The source current is routed through a small winding to provide a load on the transformer that makes the input impedance 50 ohms. The input transformer is loaded by the input capacitance of the transistor, the inductance of the transformer balances for a low Q resonance at 2. 5 MHz. By reducing the number of turns on the transformer and adding a capacitor at the input, C116 in...

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