# Transistor amplifier configuration for transmitter antenna

#### GhostLoveScore

Nov 27, 2016
71
This question is about what is the most efficient way to transfer power to the antenna from the amplifier.

Let's say I have two stages of common emitter amplifiers right before the antenna. I have enough voltage gain, but I need current gain and I need to transmit that power from the antenna.

My first thought is that I could build emitter follower and just connect the antenna to emitter. But I don't know if that will work.

What should I do?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
The first step is to match the amplifier's impedance to that of the antenna.

#### GhostLoveScore

Nov 27, 2016
71
If I were to use dipole antenna, it has 73 ohms impedance. In theory. That means I need amplifier with output impedance of 73 ohms. Is it that simple, just amplify the voltage as much as I want and then use buffer amplifier to match impedances?

How high is voltage from Colpitts oscillator with 12V Vcc? In LTSpice simulation it's almost rail to rail. But what about in real life? All along I was thinking it's few millivolts. 12V peak to peak seems very very large.

Last edited:

#### Colin Mitchell

Aug 31, 2014
1,416
You have to show the circuit before we can even start to comment.

#### GhostLoveScore

Nov 27, 2016
71
Just about any circuit will work. For example, this one

I am not asking this for a specific circuit, I just want to know generally is it in the range of mV or volts.

I am asking this because if it's already at rail to rail voltage, adding voltage amplifier won't do a thing.

#### duke37

Jan 9, 2011
5,364
Valve transmitters use a PI match circuit between the valve anode and the 50Ω load. This allows the valve to feed into a high impedance for efficiency.

Transistor output usually uses a grounded emitter amplifier and needs to provide the transistor with a low impedance load. Normally, an LC circuit is used. If you do not need efficiency and have a very low power then a simple emitter follower may be sufficient.

#### Arouse1973

Dec 18, 2013
5,178
If I were to use dipole antenna, it has 73 ohms impedance. In theory. That means I need amplifier with output impedance of 73 ohms. Is it that simple, just amplify the voltage as much as I want and then use buffer amplifier to match impedances?

How high is voltage from Colpitts oscillator with 12V Vcc? In LTSpice simulation it's almost rail to rail. But what about in real life? All along I was thinking it's few millivolts. 12V peak to peak seems very very large.

Just to clarify, a half-wave dipole has a radiation resistance of 73 Ohms not an impedance of 73 Ohms. Dipoles of different wavelengths will have different radiation resistances.
Thanks

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