Principles of Transistor Logic Circuits
What is a transistor?
A transistor is a solid-state semiconductor device (including diodes, triodes, field effect transistors, thyristors, etc.), which has various functions such as detection, rectification, amplification, switching, voltage regulation and signal modulation. As an AC circuit breaker, the transistor can control the output current based on the input voltage. Unlike ordinary mechanical switches, such as relays and switches, transistors use electrical signals to control them on and off, so switching speeds can be very fast, with switching speeds of over 100GHz in the lab. Transistors are generally solid-state electronic devices made of semiconductor materials. The circulation of the current can be changed by adding electrons. This process causes the voltage change to proportionally affect many changes in the output current, thereby multiplying the amplification factor. With the exception of most electronic devices, not all electronic devices contain one or more types of transistors. Some transistors are placed individually or commonly in integrated circuits and vary depending on the state of the application.

According to the performance of the transistor, the logic circuit of the transistor can be formed, which is widely used in the digital integrated circuit.

Different characteristics of the same type of logic circuit (RTL, DTL, TTL):
Digital integrated circuits are logic circuits that perform logical operations and conversions on digital integrated circuits. The basic units of logic circuits are gate circuits and flip-flop circuits. The trigger circuit is mainly composed of various gate circuits and is the basic unit of the digital integrated circuit. According to the working characteristics of the basic unit circuit, it is divided into three types: saturated logic (RTL, DTL, TTL), anti-saturation logic (STTL), and unsaturated logic (ECL). This article mainly introduces three logic circuits of RTL, DTL and TTL.

The first is Resistor Transistor Coupled Logic (RTL), which is a NOR gate circuit. When the input signal is high level, the output is low level, the output is low level vol=0.2V, and the output is high level vol=1V when using step connection, the circuit has slow speed, low load capacity, anti-interference ability poor characteristics. The circuit is shown in Figure 1:

The second is a diode-transistor coupled logic circuit (DTL), which is a NAND gate circuit. As long as the input signal is low, the output is high. The output is low only when all inputs are high. For the RTL circuit, its load capacity and anti-interference ability have improved, but the circuit speed is still very slow.

The third type is the TTL NAND gate we use. As shown in the figure, since the input stage and the output stage are composed of transistors, it is called a transistor-transistor logic transistor, or TTL circuit for short. In fact, there are many kinds of TTL gate circuits, such as NOT gate, NAND gate, NOR gate, NAND gate and OC output NAND gate. Although there are many types, the basic working principles are similar. So, let's introduce a classic TTL NAND gate circuit next.

And because there are two polarities of carriers involved in conduction in the transistor, this circuit is a bipolar circuit.

Input stage: T1 is a multi-emitter transistor, which can be regarded as a diode, as shown in Figure 4. So it can be seen from the figure that the input stage is an AND gate circuit: C= D1·D2·D3. Only when D1, D2, and D3 are all 1, C will output 1, and the rest of C are 0.

Intermediate stage: consists of transistor T2 and resistors R2 and R3. During the turn-on process of the circuit, the amplification effect of T2 is used to provide a larger base current for the output tube T3, which accelerates the conduction of the output tube. Therefore, the role of the intermediate stage is to increase the turn-on speed of the output tube and improve the performance of the circuit.

Output stage: consists of transistors T3, T4, T5 and resistor R5. As shown in Figure 3, in Figure 3, the T5 triode is not gate circuit, and in Figure 3, T3 and T5 are the output stages in the TTL NAND gate circuit. As can be seen from the figure, the output stage realizes the logic negation operation by the triode T5. However, in the output stage circuit, the active load composed of transistors T4, T3 and R4 is used to replace R4 in the triode non-gate circuit, in order to make the output stage have a strong load capacity. Among them, T4 can play a protective role in the reverse breakdown of the triode.

TTL level principle:

The TTL level signal is most used because the data is usually expressed in binary, +5V is equivalent to logic "1", and 0V is equivalent to logic "0", which is called TTL (Transistor-Transistor Logic transistor transistor logic circuit). Ping) signaling system, which is a standard technique for communication between parts within a device controlled by a computer processor.

First, the data transfer inside the device controlled by the computer processor has low power requirements and low heat loss, in addition, the TTL level signals are directly connected to the integrated circuit, eliminating the need for expensive line driver and receiver circuits. In addition, since the data transmission inside the device controlled by the computer processor is carried out at high speed, the operation of the TTL interface can just meet this requirement, so the TTL level is a good choice for the control device of the computer processor.

In most cases, TTL type communication uses parallel data transmission, and this transmission method is not suitable for distances exceeding 10 feet. This is due to both reliability and cost reasons. Because there are phase skew and asymmetry problems in parallel interfaces, these problems have an impact on reliability.

TTL output high level>2.4V, output low level<0.4V. At room temperature, the general output high level is 3.5V, and the output low level is 0.2V. Minimum input high level and low level: input high level>-2.0V, input low level<=0.8V, noise tolerance is 0.4V.

The TTL circuit is a current control device. The speed of the TTL circuit is fast, the transmission delay time is short (5-10ns), but the power consumption is large.

The output low level of the TTL device should be less than 0.8V, and the high level should be greater than 2.4V. Input, below 1.2V is considered as 0, higher than 2.0 is considered as 1.
Other common TTL applications are four-tube unit TTL NAND gates, STTL and LSTTL circuits, LSTTL, etc.

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