IAN LANG ELECTRONICS

Integrated Circuits

Transistor-Transistor Logic or TTL depends on the switching function of an n-p-n transistor. The basis of this is that when a voltage is applied to the base of the transistor, if the voltage is sufficiently high the transistor will conduct as hard as it can. Consider the drawing of the simple Not gate below:

As you can see, a voltage divider has been formed between the transistor and the resistor at its collector (the uppermost pin). S1 is open and so no voltage is applied to the base, hence no current flows through the base either, and where no current flows through the base, none flows between collector and emitter (the latter being the lowermost pin. This allows the full supply voltage to be passed to the output, here represented by a multimeter. If we close switch S1 then the following happens:

Closing the switch allows a voltage to be applied to the base. The base current now causes a conduction between collector and emitter and this sinks the current to ground. Output as measured by the multmeter is now negligible and in fact when this low can be considered as good as zero. In this way we have created an input that inverts the output- in fact a Not gate.

Now, suppose we were to elaborate on this and create a circuit like the following:

It does look somewhat more complex, but if we break it down into constituent parts we will see that it really is as simple as the not gate we have just looked at.
First of all on the right hand side we have the voltage divider formed from the transistor and resistor and which we tested above; the only difference being that we have removed S1 and the resistor in series with it. On the left hand side we have a device that you may not have come across before, a transistor with two emitters, and this is the heart of a TTL system. Let us delve in to how it works.

Input is achieved through the emitters. If either input is low (that is at logic 0 or no voltage) then the transistor on the left will conduct. The centre transistor will have no voltage applied to its gate and will not conduct, in turn the rightmost transistor will not conduct either and the voltage at the junction of its collector and the resistor before it will be at the supply level, here 5 volts.
If however both A and B are high (that is at logic 1) both emitters of the leftmost transistor are reverse biased and this will cause a rise in the base potential allowing current to flow through the forward biased collector and to the base of the centre transistor, thus turning it on. As the rightmost transistor 's base is attached to the emitter of the centre transistor, this goes on too, sinking current to ground and causing a zero voltage at the output. Let's draw a truth table:

Those of you who have read the section on truth tables will find this very familiar; it is in fact the truth table for a Nand gate. That is in fact what we have constructed and the Nand is the base gate of a TTL system.
The advantage of using a TTL system like this is that the switching times are very fast and we can use it in high frequency operation. You may have guessed what the disadvantage is: it uses power all the time and the consumption is high.