TTL is the short form of Transistor Transistor Logic. As the name suggests they refers to the digital integrated circuits that employ logic gates consisting primarily of bipolar transistors. The most basic TTL circuit is an inverter. It is a single transistor with its emitter grounded and its collector tied to VCC with a pull-up resistor, and the output is taken from its collector. When the input is high (logic 1), the transistor is driven to saturation and the output voltage i.e. the drop across the collector and emitter is negligible and therefore output voltage is low (logic 0).
A two input NAND gate can be realized as shown in Fig.1a. When, at least any one of the input is at logic 0, the multiple emitter base junctions of transistor TA are forward biased whereas the base collector is reverse biased. The transistor TA would be ON forcing a current away from the base of TB and thereby TB is OFF. Almost all V CC would be dropping across an OFF transistor and the output voltage would be high (logic l). For a case when both Input1 and Input 2 are at VCC , both the base emitter junctions are reverse biased and the base collector junction of the transistor TA is forward biased therefore the transistor TB is on making the output at logic 0, or near ground .
However, most TTL circuits use a totem pole output circuit instead of the pull-up resistor as shown in Fig.1b. It has a VCC -side transistor (TC) sitting on top of the GND-side output transistor (TD). The emitter of the TC is connected to the collector of TD by a diode. The output is taken from the collector of transistor TD. TA is a multiple emitter transistor having only one collector and base but with multiple emitters. The multiple base emitter junction behaves just like an independent diodes. Applying a logic '1' input voltage to both emitter inputs of TA reverse-biases both base-emitter junctions, causing current to flow through R A into the base of TB, which is driven into saturation. When TB starts conducting, the stored base charge of TC dissipates through the TB collector, driving TC into cut-off. On the other hand, current flows into the base of TD , causing it to saturate and its collector emitter voltage is 0.2 V and the output is equal to 0.2 V, i.e. at logic 0. In addition, since TC is in cut-off, no current will flow from VCC to the output, keeping it at logic '0'. Since TD is in saturation, its input voltage is ~0.8 V. Therefore the output voltage at the collector of transistor T B is 0.8 V + VCESat (saturation voltage between conductor and emitter of a transistor is equal to ~0.2 V) = 1 V. T B always provides complementary inputs to the bases of TC and TD, such that TC and TD always operate in opposite regions, except during momentary transition between regions. The output impedance is low independent of of the logic state because one transistor (either TC or TD ) is ON.
When at least one of inputs are at 0 V, the multiple emitter base junctions of transistor TA are forward biased whereas the base collector is reverse biased and transistor TB remains off and therefore the output voltage is equal to VCC . Since the base voltage for transistor TC is VCC , this transistor is on and the output is also VCC . And the input to transistor TD is 0 V, hence it remains off.
TTL overcomes the main problem associated with DTL (Diode Transistor Logic), i.e., lack of speed. The input to a TTL circuit is always through the emitter(s) of the input transistor, which exhibits a low input resistance. The base of the input transistor, on the other hand, is connected to the VCC , which causes the input transistor to pass a current of about 1.6 mA when the input voltage to the emitter(s) is logic '0'. Letting a TTL input 'float' (left unconnected) will usually make it go to logic '1'. However, such a state is vulnerable to stray signals, which is why it is good practice to connect TTL inputs to VCC using 1 k pull-up resistors.
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