Formula for a resistor and a capacitor, which is used for pre-emphasis in parallel of audio line. Lets try 50uS Pre-enrapis. I hope it is used around Asia.
North America uses 75us and Europe, Australia and Asia use 50us.
In my FM transmitter the 470 ohm emitter resistor for the preamp transistor is bypassed with 150nF for North America (470 x 150nF=) 70.5us. Pretty close to 75us.
The 470 ohm resistor is bypassed with 100nF for Australia and Asia (470 x 100nF=) 47us. Very close to 50us.
The OP said "preempasis" (boosing the level of the high frequency content) yet the discussion has centered on de-emphasis (reducing the level of the high frequency content).
North America uses 75us and Europe, Australia and Asia use 50us.
In my FM transmitter the 470 ohm emitter resistor for the preamp transistor is bypassed with 150nF for North America (470 x 150nF=) 70.5us. Pretty close to 75us.
The 470 ohm resistor is bypassed with 100nF for Australia and Asia (470 x 100nF=) 47us. Very close to 50us.
Wow! How Simple solution!
-Then...in normal situation, may i use 10k with 5nf to get exact 50uS pre-emphasis?
- But in your Mod 4, i think i can't use this 10k at emitter of pre amp becaues of biasing factor, can i?
- I think in your pre-emphasis, 100pf is also very important. Without this capacitor, the audio is not clear. What is this actually?
-Then...in normal situation, may i use 10k with 5nf to get exact 50uS pre-emphasis?
- But in your Mod 4, i think i can't use this 10k at emitter of pre amp becaues of biasing factor, can i?
The transistor will not work properly with its emitter resistor much too high at 10k ohms. It is almost cutoff.
If the biasing is changed so it works better then the transistor will have no gain.
First design the transistor circuit so it amplifies well without distortion then add a capacitor parallel with the emitter resistor to boost high audio frequencies.
I think in your pre-emphasis, 100pf is also very important. Without this capacitor, the audio is not clear. What is this actually?
The 100pF capacitor from base to emitter prevents the audio preamp transistor from amplifying the very strong radio signal which would cause it to have severe distortion.