Today we’ll be discussing about the TTL(Transistor Transistor Logic ) circuit with totempole output. This particular circuit comes under the section of logic families. There are a lot of circuits that deal under this category `out of which this is the important one from GATE point of view as well .
Circuit Diagram :
Now let us dig into the working of the above circuit , here we have 3 transistors connected back to back that allow current flow into different stages of this circuit. As mentioned in the circuit diagram there are three stages which are responsible for the output of this circuit .
- Multi-emitter stage ( consisting of transistor T1)
- Phase Splitter stage (consisting of transistor T2)
- Totempole Output ( consisting of transistor T3 and T4)
From terminal A when logic 0 is given T1 is in Active region , T2 comes in saturation region , T3 comes in Cutoff mode and finally T4 is in saturation region , Now since T4 is in saturation , the transistor is ON and we get output logic as 1 . Why is T4 ON and T3 OFF ? This is because of the diode that is not allowing the current to flow into T3 , due to which one of the two output transistors keep OFF and we obtain the output .
When logic 1 is given from the terminal A , T1 is in Reverse active region , T2 in saturation region , T3 in saturation region and T4 is in Cutoff mode . This time T3 is ON and T4 is OFF , due to which we will get Y output as 0 .
How is the diode preventing the other Transistor from getting turned ON ?
Let us understand this with an example . Suppose we are getting 0.9v are the base of transistor T4 , The cut-in voltage for transistor is suppose 0.7v(in general ) , and the Cut-in voltage for the diode is also 0.7v . Now out of 0.9 for the transistor to be ON , it takes 0.7v , remaining 0.2v is available across the diode , due to which the diode will keep OFF,and hence the diode does not let current flow into transistor T3.
Advantages of Totempole Output :
- High speed of Operation
- Low Power dissipation
- High Fanout
Thankyou guys for reading about this ! Will come up soon with some more circuits . Keep learning and exploring the wonderful world of electronics . Also refer to my post about OPAMP and its uses here http://circuitcooks.com/opamp-and-its-uses/