Fm Transmitter, how much audio gain???

[youngboss]

hi everyone
im just designing a simple fm transmitter using three 3904 BJT
first BJT will be my audio amplifier coming off of my mic
and im just wondering
i know how to calculate the gain
but how much gain do i have to put up before going in to my RF oscillator from my Audio amplifier??
and also how much Gain should my RF amplifier have when the input is coming from my RF oscillator??
right now im using 50Mhz Carrier frequency off of my oscillator
and using 5v from my voltage regulator as my Vcc
any help would be great thanks

 

[MikeMl]

How are you FM modulating the transmitter? Varactor diode across the tuned circuit? How much deviation do you need? What is the bandwidth of the receiver? Is it designed for narrow (+-5kHz)or wide FM (+-75kHz)?

 

[audioguru]

A 49MHz frequency is (was?) usually used for narrow band baby monitors and narrow band home wireless phones.
The gain of the mic preamp depends on the distance from the baby to the mic and the gain of the audio amp in the radio.

 

[youngboss]

sooo more gain i put up on my first amplifier, and the amplifier after my tuned LC circuit would help me get greater distance???
also making my antenna longer would help me get longer distance also???
are there any calculation i can do to predict the distance?? or is that just way~~~ too difficult at a newbie stage

and yes auidio guru you are soo right~~ im using a baby monitor as my receiver
im trying to understand FM transmitter fully before working on the receiver part

and another question i have is how is this acting as a amplifier??? dont i need a emitter resistor to pull current down?
View attachment 68236
sorry for stupid questions cuz I AM stupid at the moment who's trying to get better

 

[audioguru]

A single transistor can be the preamp for the electret microphone but it will not have too much gain because its low input impedance loads down the mic which reduces the output level from the mic.

An ordinary FM oscillator used for FM broadcast band transmitters will produce wideband FM which will produce severe distortion on a narrow-band baby monitor. You need to limit the audio voltage swing that feeds the oscillator somehow.

The RF amplifier transistor you show is turned on by the base resistor. The amount of current it conducts is determined by the hFE of the transistor.
A resistor to ground at the base is used when the transistor has a series emitter resistor to ground which would control the current.

A whip antenna works best when it is the correct length. For 49MHz the correct length is about 163cm. It should be parallel to the receiver's 163cm antenna.
A baby monitor is cheap so it is not sensitive. The distance is also determined by its sensitivity and the amount of power from the transmitter.

 

[youngboss]

im sorry i still dont fully understand the RF amplifier
im i trying to tune the LC to match my impedance of the antenna inorder to get my maximum output?????

 

[audioguru]

The LC does not match the impedance of the antenna. Additional LC parts will do that.
Instead the LC is tuned for maximum output power. Make a simple field strength indicator circuit with an LED.

 

[youngboss]

i have my hands on a spectrum analyzer
is there a way to figure out with a spectrum analyzer?