Emitting pole coupling logic circuit


Emitter Coupled Logic is a logic circuit that is coupled to each other with a plurality of transistors to couple with an emitter follower. Abbreviation ECL circuit. The ECL circuit developed in the end of the 1950s and is the basic circuit of the bipolar integrated circuit. It is a current type switching circuit, the transistor in the circuit is operated in an unsaturated state. The ECL circuit features: the switch speed is fast (1 nanosecond). A few times faster than the usual transistor transistor logic circuit switching speed. It can easily constitute the logic function of the expansion circuit, and the number of components can be saved. The disadvantage is that the circuit power consumption is large, and the level threshold voltage is drifted with temperature. The ECL circuit is mainly used to constitute an ultra-high-speed integrated circuit such as a high speed, a giant computer, and the like.

Main features

1. Working speed

ECL circuit transmission time is 1-2ns, switching time is very short, this is mainly due to:

(1) The transistors operate in a non-saturated state, thereby eliminating the charge storage phenomenon.

(2) Logic swing is small, the difference in low level is only 0.83V, and the resistance of R C1 , r c2 It is very low, the time required for level is shortened, and the switch speed is fast.

2. Strong load capacity

Because the ECL circuit is made from t 7 , two emitters respectively The output is output as an output stage, and the emitter output is low, so the load capacity is strong, and the door circuit can obtain a large fan-out coefficient.

3. Strong logic function

Except for or or non-inverting output, you can also directly connect the output of the two doors together, used to get two The logic of the output signal or this is the so-called "line or" logic.

The disadvantage of the ECL circuit is that the power consumption is large, and due to the small logic amplitude, its anti-interference ability is also poor.

Main Advantages

ECL circuit has the following advantages:

Emitting pole coupling logic circuit

(1) ECL circuit is the fastest working speed in various digital integrated circuits . When the transistor in the circuit is turned on, it is entered a saturation, which is fundamentally eliminating the charge storage effect due to saturation conductivity.

At the same time, since the resistance resistance value in the circuit is small, the logic swing is low, thereby effectively shortening the rise time and falling time of each of the circuit node potentials. At present, the transmission delay time of the ECL door circuit has been shortened to 0.1 ns.

(2) Because the output is an emitter output structure, the output internal resistance is very low, and the load capacity is strong.

(3) Since the size of the I C1-5 is near equal, the power supply current changes in the circuit switching layer, and the switch noise inside the circuit is very low.

(4) The ECL circuit has a complementary output, and it is also possible to directly implement line or logic functions directly, so it is very convenient and flexible.

Main Disadvantages

ECL circuit has been prominent, which is mainly in:

(1) power consumption.

Since the resistance resistance value in the circuit is small, it works in the unsaturated state in the triode, so power consumption is very large. In a certain sense, the high speed of the ECL circuit is exchanged with the cost of multi-consuming power. Moreover, the power consumption has also severely limited the improvement of integration.

(2) The stability of the output level is poor.

is in an unsaturated state in the circuit in the circuit, and the output level is directly compressed with T 7 , T 8 The output level is sensitive to the change in circuit parameters and changes in the ambient temperature.

(3) The noise tolerance is relatively low.

ECL circuit logical swing only 0.8V, DC noise tolerance is only 200mV, so the anti-interference ability is poor.

Traditional ECL is operated in V cc1 = 0V, V EE = - 5.2V for operating voltage.

If the + 5V power supply is used, the V CC1 = V CC2 receives a positive power supply and VEE is connected to zero, so that the level is usually It is called PECL; if the + 3.3V power supply is powered, it is called LVPECL.

Pay attention to the power decoupling circuit to avoid noise interference when using the PECL circuit to avoid noise. The output uses AC coupling or DC coupling, and the form of the load network will present different needs.

DC coupled interface circuit There are two working modes: one, corresponding to the close-range transmission, using the transmitting end to the ground partial resistance, the receiving end plus the end resistance mode; Second, In the case where the relay transmission is transmitted, the mode of matching load of the shutoff level V TT and 50Ω is supplied by the receiving end. For AC coupled interface circuits, there is also a standard working mode, that is, the transmitting end is added to the ground resistance, and the coupling capacitor is placed close to the transmitting end, and the receiving end provides a common mode level V BB And 50Ω matched load mode.

Connection Technology

Emit pole coupling logic transmission delay is small, and the signal transmission speed is fast, and thus the signal path on the circuit board is in the form of transmission line. If the transmission latency of the signal path is shorter than the signal, the reflection phenomenon of the rise time is masked, and the upper rush or ringing is not seen. For high speed circuits, the edge speed of the signal is accelerated, and the length of the path must be shortened, otherwise the integrity of the signal cannot be maintained. The ECL circuit is a high-speed circuit, and the method of solving the reflection problem is that the passage of the transmitted signal must impedance match. A system is often composed of a lot of boards, and the cable between the boards tends to be longer than the cable between the devices on the board, so impedance matches, eliminating reflection is more important.

Application range

The maximum feature of the emitter coupling logic circuit is high, the output impedance is low, the noise is low, so it is very suitable for high-speed data communication, high speed modulus conversion, high speed count etc. The emitter coupling logic circuit is working in a negative voltage because the noise suppression performance is good when the negative voltage is working, and thus can be used to constitute a large system. The emitter coupling logic circuit can also be operated in positive voltages (+ 5.2V), V EE , which can be connected to the T circuit, which is specifically designed by a special interface circuit. .

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