# Using antenna instead of an AC source on the circuit diagram

Here's an LNA circuit I found on google:

Can I replace the AC source with an antenna?

• What frequency are you interested in amplifying? If it's AM, a simple common Emitter without fancy extras is fine. If it's FM, if the transistor Ft is rated in the GHz (RF transistor), just putting a 100-200 ohm resistor in parallel with the Base (before the Base), will be close enough to 50 ohm impedence (Base + resistor together). Rc can be as high as you'd like (it's close enough to the transistor not to experience TL effects). The load should indeed be between 50-200 ohm (it might see TL effect). But please simulate everything in Falstad and in LTSpice. Commented Aug 10, 2022 at 9:14
• I'd just suggest, some of these LNA are application and transistor specific. With a different transistor you might get stability concerns - it might begin oscillating. So I believe that with a good RF BJT for an FM radio, best to start with a Common Emitter without many extras. Commented Aug 10, 2022 at 9:20
• Finally, this circuit has an explicit feedback loop, so I'd be quite wary of it (more oscillation concerns). The same circuit can be a good starting point without all the inductors, without the feedback loop, and with only 2 decoupling capacitors, and 1 between DC voltage source and ground against noise. Commented Aug 10, 2022 at 9:31

## 3 Answers

For a drawing, you certainly can, but for simulation, you will need to use an AC source and a suitable impedance.

Can I replace the ac source with an antenna?

Sure you can.

I'd expect reasonable results but, remember the circuit you showed is optimum in a specific area of the spectrum (looks like about 3 GHz to me) and, you need to ensure that your antenna is designed to produce a 50 Ω (ish) impedance in that area. By "ish" I mean anything from 25 Ω to 100 Ω. A little bit of series reactance won't be too troublesome either.

You can use an antenna as your signal source, only insert the needed connector. I assume you are going to use the amp in the frequency range the amp is designed for.

There are some caveats:

The circuit can be designed for a source which has internal impedance = pure resistive 50 ohms and those obvious matching elements L1, CB1 and (maybe unbelievable, but definitely true) also the parts in the output side are selected for certain operating frequency in a way that the amplifier becomes unstable with wrong source impedance.

NOTE: I have not calculated how critical this circuit is in that sense, but I have seen oscillators which are by design unstable amplifiers which have no obvious feedback circuit, the feedback is inside the transistor and, of course, there's no actual signal source connected.

The feedback (=through RB1) shown in your schematic is for DC operating point i.e. for stabilizing a few milliamperes of idle collector current - I guess it's 6 mA.

If the circuit is an unshielded mess on a breadboard it probably will not work. Even the breadboard will spoil the function. In addition an antenna near an unshielded amplifier can catch enough feedback for oscillation. A properly designed circuit board and shielding are as essential as other parts of the design.

ADD after reading from comments you want a preamp for a FM radio:

If you explore some schematics of FM radios from the era when 88-105 MHz front ends were commonly made of discrete transistors you'll find that a common base transistor amp in front of the mixer stage was common. That's because it fitted well for the purpose (=take its input from a whip antenna, gave enough gain for substantially higher S/N -ratio than what's got by having the mixer as the first stage) and did not have potential stability problems caused by tuned circuits, antenna and the internal CB capacitance of the transistor together.