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# help to understand simple FM walkie talkie circuit diagram

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##### Member level 1
the red part is just a power amplifier with Q1 that amplify the signal. And I suppose the part where there's Q2, that's a power Amplifier too. Am I right ?

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Regarding C4, the DC component. That's the main value of sine wave.

#### BigBoss

C4 and R4 are not parallel ..!!
C4 is a decopuling capacitor against small low frequency ripples to make them inefficient..
R4 is a biasing resistor for a A-Class small signal audio amplifier.
They are not parallel..

#### davenn

the red part is just a power amplifier with Q1 that amplify the signal. And I suppose the part where there's Q2, that's a power Amplifier too. Am I right ?

Regarding C4, the DC component. That's the main value of sine wave.

##### Member level 1
Hi all!

I am not trying to diversify his mind to the wrong side. May be I wrong. But I was just trying to help him. If I said something wrong then I do sorry.
Happy learning to chris and happy teaching to experts.

Don't worry. It happen.

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C4 and R4 are not parallel ..!!
C4 is a decopuling capacitor against small low frequency ripples to make them inefficient..
R4 is a biasing resistor for a A-Class small signal audio amplifier.
They are not parallel..

I see. This transmitter have a FM range from 88 to 95 MHz. And this range relate to VHF range. As we can see in this link:

About the audio amplifier where we find Q1. It contains an active filter RC. Am I right ?

##### Member level 1
In any case. Thanks to all for your replies. And If i had questions, i would pose them.

#### Eshal

@BigBoss I have question here. How did you recognize it is a a-class power amplifier? Did you find the Q-point? Or is there any shortcut to recognize it?

Thank you.

#### davenn

The RF oscillator is the same thing that the local oscillator ?

They are the same in that they are both RF oscillators
but generally a local oscillator is referring to the oscillator(s) in a receiver. there may be more than one depending on the number of down conversions

for example you mite have a receiver that is receiving on 400.000 MHz and it has a 10.7MHz first IF freq ... a common first IF
that means you need a local oscillator running at 389.3 MHz. The incoming 400 is mixed with the 389.3 and you get 10.7MHz IF which may be amplified and filtered before
going on to the next stage....

Dave

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@BigBoss I have question here. How did you recognize it is a a-class power amplifier? Did you find the Q-point? Or is there any shortcut to recognize it?

Thank you.

A Class-A Output device(s) conduct through 360 degrees of input cycle (that is, it never switches off) - A single output device is possible as can be seen in the microphone preamp section of the circuit above.
The device conducts for the entire waveform. This means that in its most basic design, the power devices must conduct a continuous current which exceeds the maximum peak load current

cheers
Dave

#### chri\$

##### Member level 1
Thanks for your reply Dave.Yes the oscillator may have two ranges. Then the mixer make 2 arithmetics. One operation 400+389.3 and 400-389.3 But we want 10,7 MHz. So, IF take the value 10,7 MHz. And about (400+389.3) is going to delete by the filter.

- - - Updated - - -

They are the same in that they are both RF oscillators
but generally a local oscillator is referring to the oscillator(s) in a receiver. there may be more than one depending on the number of down conversions

for example you mite have a receiver that is receiving on 400.000 MHz and it has a 10.7MHz first IF freq ... a common first IF
that means you need a local oscillator running at 389.3 MHz. The incoming 400 is mixed with the 389.3 and you get 10.7MHz IF which may be amplified and filtered before
going on to the next stage....

Dave

- - - Updated - - -

A Class-A Output device(s) conduct through 360 degrees of input cycle (that is, it never switches off) - A single output device is possible as can be seen in the microphone preamp section of the circuit above.
The device conducts for the entire waveform. This means that in its most basic design, the power devices must conduct a continuous current which exceeds the maximum peak load current

cheers
Dave

I agree it's A class amplifier. Because the signal input (microphone) which is a small signal go to the base ( the A class Amplifier is used for small signal). And the DC from the power supply go to the collector (with the load resistance R6). So the output go out from the emitter. That's the common emitter configuration used by the class A amplifier. Based on my research. my source: http://www.electronics-tutorials.ws/amplifier/amp_1.html

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