In the first circuit you should make it 5 volt instead of 3.3 v ans vis versa ( the lables)

The proteus file link seems to be broken......

The file is uploaded again. Thanks

The circuit has a flaw. It doesn't matter what characteristics you chose.

I'm talking about BC547 in the upper part of the .
figure.....

see when the Vb=0V, even then the output would be 5V as the path opens at Vce.

secondly when Vb=3.3V again it's wrong because no current limiting resistor is added there. therefore Ib is the short circuit current which will damage the second transistor....

Thirdly, I have read that the Physics of PNP transistor inherently makes them slow.. therefore it would be better to use two NPN rather than one PNP and one NPN.

Regards....

There is no flaw in the circuit, as I have used it and my friend Gary who posted before you is also using it.

"see when the Vb=0V, even then the output would be 5V as the path opens at Vce."

There will never be 0V at Base of BC547. A value of 0.3 to 05 Volts will always remain there.

"secondly when Vb=3.3V again it's wrong because no current limiting resistor is added there. therefore Ib is the short circuit current which will damage the second transistor...."

Yes you are right, without the base resistor the transistor will be damaged. But that is only applicable when then driving circuit is not the emitter of a transistor. Q2 provides current which is already limited by its base resistor. So not in any way the base of Q1 will get high current.

"Thirdly, I have read that the Physics of PNP transistor inherently makes them slow.. therefore it would be better to use two NPN rather than one PNP and one NPN."

My friend you are missing a point here. The reason of using the combination of NPN & PNP is to have the exact logic level as of input. For e.g Q1 gets logic level '0' than it outputs '1'. That '1' is given to Q2 which converts it again into '0'.

If you use both NPN transistors, the logic levels will be inverted.

The PROTEUS file is provided for you to check it thoroughly. And you can verify all the points discussed here.

Thanks for your reply....

The label at R1(1) should be changed from 5V to 3.3V....... This made me confused when I made the first comment......

"The reason of using the combination of NPN & PNP is to have the exact logic level as of input. For e.g Q1 gets logic level '0' than it outputs '1'. That '1' is given to Q2 which converts it again into '0'."

Yes there is no problem in this logic, except for the physics I mentioned before..... What I mean to say that you can connect 2 NPN in cascade fashion.... The first one will invert and the second one will again invert producing the original signal.... e.g A will become A` and A` will again become A.......

Thanks for your correction. Actually these resistor values are dependent upon the characteristics of the transistors you are using.

For Q4 I used an 2N2222 and for Q3 an 2N2907.

Gary

The 3.3 to 5 volt circuit does not work. A high on the 3.3 input causes Q4 to overheats and dies. By adding a 220 ohm resis between C of Q4 and B of Q3 fix that problem. The 5v output does not go to ground. Changing R4 to 1K fix that problem. I also change R5 to 1K. It works good for me now.