Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Transistors

Status
Not open for further replies.

Overclocked

Member
What is Fco? What is Ft? I see Ft in the RF transistor section of catalogs, what does it mean?

If I were to design a RF amp to give a 5dB Gain to a signal,@2.4Ghz would I need a class C Amp (a tank circuit on the collector) to get rid of any noise? Would it matter if the signal coming in is FM? Or a square wave? Or a sine wave?

Or would I just need to design a small amp?
 
Don't know what Fco is but Ft is the transition frequency of the transistor. It is not advisable to operate a transistor near Ft because it won't have gain.

5 dB gain is not much, less than double the amplitude. Class C is used to increase the power of CW signals, or FM signals but since you are using 5 dB gain I suspect your signal is very small and class A amplification would be more suitable.

At 2.4 gHz you should consider the waveform to be sine because you don't have a way to tell if it is square and probably don't have the bandwidth to make a square wave.

In a class A amplifier it will not matter if the signal is AM or FM or PM, you could not find enough Q to affect the signal in any case.

You should use surface mount components to keep runs short and use 50 ohm input and output impedances. You should search for some 5 gHz transistors to make an amp at 2.4gHz.
 
Ive found transistors that have an Ft of 22Ghz or so, with about 13dB Gain at that freq.

The Amp is based on an idea I had for a passive wifi repeater. I figured, add a 5dB amp in to compenstate for losses when transfering to one antenna to the other.

TV repeaters work on the same princible right? Thats the thing too, Does wifi use PWM or FM (sine)

Well Id figure since wifi has 11 Channels,

* Channel 1 - 2,412 MHz;
* Channel 2 - 2,417 MHz;
* Channel 3 - 2,422 MHz;
* Channel 4 - 2,427 MHz;
* Channel 5 - 2,432 MHz;
* Channel 6 - 2,437 MHz;
* Channel 7 - 2,442 MHz;
* Channel 8 - 2,447 MHz;
* Channel 9 - 2,452 MHz;
* Channel 10 - 2,457 MHz;
* Channel 11 - 2,462 MHz

(taken from wiki)

So doing a simple subtraction, I found that each channel increases by 5 Mhz. So does that mean that those given frequency's are the Fc (carrier signal)? I forgot how much BW FM takes up.

Would I still need a Tank circuit on the collector for a peak of 2.4Ghz since it will gently roll off ?
 
Ft = Transition frequency, where by definition the gain is 0dB.

WiFi repeater? Not sure what you are trying to do but be aware that this thing will recieve and transmit on the same frequency.
If it has any gain it will howl. ie the input will hear the output, amplify it, the input will hear a bigger output, amplify it...... get the idea?
At a quick guess, a TV repeater will change frequency between the receive and transmit.

I guess WiFi is FM, and the bandwidth must be less than the channel spacing, (5Mhz in this case). The bandwidth of an FM signal depends on the modulating frequency and the deviation.

If you want to proceed with this project, an easy way to get some gain at these frequencies is to use a MMIC, they are broad banded and need a minimum of external components (but carefull PCB layout).

Try googling for Mini Circuits (the manufacturer) and MAR1 (a MMIC, probably obsolete, but a good starting point).

JimB
 
JimB said:
At a quick guess, a TV repeater will change frequency between the receive and transmit.

Yes, TV repeaters change the channel frequency, usually by as far as possible! - this gives the best performance.

The local TV repeater station where I work was changed after it was built (for political reasons only), the ouput channels weren't changed, but the input ones were - giving the situation where it transmits only a single channel aways from it's input channels. Because of this it's performance and reliablity are really poor.

WiFi repeaters do basically the same thing, they receive on one channel, and transmit on another - the instructions usually specify the minimum number of channels they have to be apart - I seem to recall it was 5 channels on the one I used? (although the input was via Ethernet, not RF).
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top