Darlington TR

[shahzad-h]

HI

 

[audioguru2]

CAN I USE

 

[shahzad-h]

HI audioguru

Thanks for ur fast reply

Ok Before biasing darlington I biased ordinary transistor 2N3904

 

[shahzad-h]

HI audioguru
I don't know how to attach the ckt ??? there fore i m giving u assumptions that i made FOR

 

[audioguru2]

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.

 

[shahzad-h]

HI audioguru
I rebiased the ckt with HFE=230 it works perfectly ;D ;D i checked in circuitmaker software

 

[audioguru2]

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

 

[audioguru2]

Here is a pic of transistor distortion without having negative feedback from an unbypassed emitter resistor:
EDIT: Nope. I am not allowed to upload any more because my upload folder is full!

 

[shahzad-h]

Hi audioguru

Sorry I forgot to write the values of capacitors I uses two

 

[audioguru2]

Let me try again to upload a pic of transistor distortion:
EDIT: Nope, my upload folder is still full so I can't attach anything.

 

[shahzad-h]

Hi audioguru
I m sending U the waveform of amplifier the yellow colour is input and green is output
Thanks :-*

View attachment 38924

 

[audioguru2]

Your input and ouput levels are very low. Turn them up and you will see gross distortion because your trsnsitor doesn't have any negative feedback:

View attachment 38925

 

[shahzad-h]

hi audioguru
ur talking about this cross over distortion which i encircle

View attachment 38926

 

[audioguru2]

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.

View attachment 38927

 

[shahzad-h]

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

View attachment 38932

View attachment 38933

 

[audioguru2]

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.

 

[shahzad-h]

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

View attachment 38938

View attachment 38939

 

[audioguru2]

Your sim shows the same 1kHz frequency as before, but the phase-shift is different. Your sim is not done properly.

 

[shahzad-h]

HI audioguru
can u plz make my ckt in ur sim the values are
Rc=3.6K,RE=1K,R1=90k,R2=24k
C1,C2=10uF
Thanks