VHF Preamplifier for 50-70MHz

Biasing

Using a 2SC2498, the datasheet suggests biasing around 4mA should give the lowest noise:

See Common Emitter Simple Bias with Grounded Emitter Calc for the python code I used to calculate this.

Modelling the Feedback Amplifier

Using 2SC2498 Feedback RF Amplifier Calculator Study (in Python), focusing on 70MHz.

This models the transistor using the Hybrid-PI methodology plus addition of layers for intrinsic and extrinsic properties. Then adds the feedback network.

Note: If instability occurs below 10MHz then $C_f$ can be increased.

Schematic

Prototype

(Very)Ugly construction:

Notes:

• L1 represents the stray inductance of the feedback components’ lead lengths (10mm $\approx$ 10nH).
• Keep the transistor’s grounded emitter lead as short as possible.

Results

Although I was targetting 50-70MHz, it is pretty broadband.

Summary

Frequency	i/p swr	o/p swr	gain
7MHz	1.5:1	1.3:1	14.3dB
14MHz	1.4:1	1.2:1	14.5dB
28MHz	1.4:1	1.4:1	14.4dB
50MHz	1.3:1	1.6:1	14.0dB
70MHz	1.2:1	1.8:1	13.5dB
145MHz	1.2:1	2.2:1	10.8dB

NanoVNA Sweep