shahzad-h said:
in his design he told that datasheet of 2N3904 gives HFE =70 at ICQ=1ma but we are using HFE= 230 at ICQ=1ma.
The hFE of a 2N3904 at 1.0Ma collector current is
a minimum of 70, the text doesn't have a typical hFE but a graph shows a typical hFE of 230.
he takes VCEQ=0.5VCC as i did so power dissipation by 2N3904 will Pd=ICQ*VCEQ=5mw will be within its maximum rating
With VceQ of only 0.5V then the transistor can't do anything useful. Its power dissipation is only 0.5mW (not 5mW) if you need it to operate at such a low power.
if we apply the same rule to 2N2222 transistor it datasheet gives this values Ic=150ma,VEC=10v,HFE=100min HFE=max so if we take this Ic as ICQ and VCE as VCEQ than 1.5W it is out of rang of 2N2222 800mw
The power dissipation rating of a transistor is its
continuous max rating. A 2N2222 has a max power dissipation rating of 500mW (not 800mW) when it doesn't have a heatsink and the ambient is not warmer than 25 degrees C.
The datasheet shows that hFE measurements are made with very short duration pulses so that the transistor doesn't heat too much which would change its hFE and so it doesn't melt.
You can pulse a transistor briefly with a high voltage and current, if the average power dissipation is within its ratings.
