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shahzad-h

Darlington TR

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Hi Shahzad,
Look at the datasheet for the 2N3904 to see its typical spec's with 1mA collector current.
1) HFE= 230.
2) Vbe= 0.68V. Yours is much too low.

Your 900 and 85k resistors aren't standard values.
The current in R1 and R2 should be about 10 times the transistor's base current.

Recalculate the R1 and R2 voltage divider for the proper amount of base voltage and your circuit will be fine. You can download a simulation program to test it if you want.

When you post a reply, click on Additional Options then click on Browse and double click on the file on your pc of your sketched schematic that you want to attach.

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it amplifies 5mv 1Khz signal to 500mv

Without having about a 10uF capacitor as a bypass across the 1k emitter resistor, the voltage gain is only about 3.6. With the bypass capacitor, the output is very distorted at high levels.
Sorry, I can't upload a pic of the distortion.

In ur FM transmmiter

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Look at crossover distortion in Google.
Your single transistor cannot produce crossover distortion.
Crossover distortion is caused by two transistors as emitter-followers in the output of a power amplifier and without enough bias current so they operate in class-B. The distortion occurs when the signal crosses over from one transistor to the other transistor but they don't have bias current to function smoothly.

post-1706-14279142807606_thumb.png

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Hi audioguru
sorry I worngly write cross instead of gross i reduced the gross over distortion by introducing negative feedback I replace the 100uF bypass capacitor with 1.5uF and  I m attaching Two images first with distortion and second with reduced distortion and show me how much distortion is generating now

Thanks

post-18768-14279142809728_thumb.jpg

post-18768-14279142809872_thumb.jpg

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You reduced the value of your bypass capacitor (for the emitter resistor?). Therefore you introduced negative feedback only at low frequencies. Your harmonics pic shows that distortion for higher frequencies remains the same as with a much larger bypass capacitor.

An unbypassed emitter resistor introduces negative feedback for all frequencies and total distortion is reduced.

I cannot see how much distortion your circuit has without seeing its fundamental frequency's output level and seeing all harmonics. Harmonic distortion is all harmonics added together as a percentage of the output level.

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Hi audioguru
Another problem occure that input  and output are not out of phase image shows this clearly and when i removed bypass and increase  the frequency  the out put is no longer sine wave image shows

post-18768-14279142836476_thumb.jpg

post-18768-14279142836626_thumb.jpg

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The transistor is turned on too much so clips the bottom of the waveform at high levels.
There isn't any phase-shift visible at 3kHz, and also none is seen at 300kHz.
I don't know how to scale levels so I can't show the phase-shift with the emitter resistor bypassed.

post-1706-14279142837928_thumb.png

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HI audioguru
i have a question that indata sheet of any transistor these values are given
Ic=10ma,VCE=10V HFEmin=100,HFEmax=300. so these Ic  and VCE are saturation values    OR    quiescent values

Thanks

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