How does this transistor circuit work?

[danesh]

hi,
This a logic switch circuit my friend proposed to me .. but i don really understand the circuit can anyone explain hw this circuit works. I need to knw the logic at o/p port when output at U1A is high and low. Thanks

 

[IanP]

Re: Transistor circuit

First of all, to make this circuit working properly you will need to swap transistors: Q1 should be PNP and Q2 NPN ..
Also, you don't need R4 resistor in this circuit ..

With these changes implemented, the o/p will be H (almost +12V) as both transistors are "ON" .. Q2 saturated by the AND-gate (output of U1 =H as V2 and V3 =H), Q1 saturated by Q2 ..
If output of U1 is L (V2 or V3 is L) then the o/p will be floating, as Q1 is OFF .. and there is nothing else to define the stage of this point ..

Regards,
IanP

 

[a_ronagh]

Re: Transistor circuit

Hi
After the changes that IanP mentioned above,If you add a resistor of about 5 kΩ
between the O/P port and ground,when the transistor Q1 is off,output will be low and it will not float.