Monday, December 27, 2010

【 Weak current College 】 TTL and CMOS IC 】

Currently the most widely used digital circuit is TTL circuits and CMOS circuit.

1 TTL circuits
TTL circuits to bipolar transistor as a switch component, also known as bipolar integrated circuits. Bipolar digital integrated circuit is the use of electron and hole two different polarity of carrier for electric conduction of devices.
It has high speed (switch speed), the advantages of driving ability is strong, but its power is large, the relatively low level of integration.
According to application area, it is divided into 54 series and 74 series, former military, general industrial devices and consumer electronics products with the latter. 74 series digital integrated circuits are common standards on the international circuit. Its variety is divided into six categories: 74 × × (standard), 74S×× (Schottky), 74LS×× (low-power Schottky), 74AS×× (Advanced Schottky), 74ALS×× (advanced low-power Schottky), 74F×× (high-speed), its logical function exactly the same.

2, CMOS circuit
MOS circuit also known as field effect IC, belonging to the unipolar type digital integrated circuits. Unipolar-digital IC only use a polarity of the charge carrier (hole or electron) electric conduction.
Its main advantage is the input impedance is high, the low power consumption, strong anti-interference ability and is suitable for large-scale integration. In particular, its leading product CMOS IC have particular advantages, such as static power is almost zero, the output logic level for VDD or VSS, rising and falling time is in the same order of magnitude, and CMOS IC products has become one of the IC the mainstream.
Its varieties including 4000 series of CMOS circuit and 74 series high speed CMOS circuit. 74 series high speed CMOS circuit is divided into three main categories: HC-CMOS work level; work for HCT TTL level (it can be used interchangeably with 74LS series); HCU for unbuffered level CMOS circuit. 74 series high speed CMOS circuit logic functionsand pin arrangement and corresponding 74LS series of the same variety, high work speed, power consumption is significantly reduced.
74 series can be said is that we always in contact with the most chips, 74 series is divided into a number, but we usually use the most is the following: 74LS, 74HC, 74HCT three

Input level output level
74LSTTL level TTL
In addition, with the launch of BiCMOS integrated circuit, it combines bipolar and MOS integrated circuit, common bipolar gate circuits strengths is gradually disappear, some have dominated TTL series product is gradually withdraw from the market. CMOS Gate circuits continuous improvement process, is moving in a high-speed, low power, high drive capability, low power supply voltage. BiCMOS integrated circuit input gate circuits using CMOS process, the output uses the double pole push-pull output mode, with the advantages of CMOS and bipolar strengths, has become an integrated circuit.

3, CMOS IC's performance and characteristics

Low power consumption
CMOS IC using FET, and are complementary structures, two series of FET is always in a tube via another tube stop State, circuit power dissipation is theoretically zero. In fact, because of leakage current, CMOS circuit there are micro power dissipation. Single door circuit power consumption typical values only 20mW, dynamic power consumption (operating frequency in 1MHz) are only a few mW.
Operating voltage range
CMOS IC power supply power supply simple, small, basically not required voltage. Homemade CC4000 series of integrated circuits, 3 ~ 18V voltages to work properly.
Logic-swing big
CMOS LSI Logic high level "1", a logical low level "0" respectively close to power high-voltage VDD and power low potential VSS. When VDD = 15V, VSS = 0V output 15V logic swing approximation. Therefore, CMOS IC's voltage utilization factor in all types of integrated circuits is relatively high in the index.
Strong anti-interference ability
CMOS integrated circuit voltage noise of typical values for the power supply voltage of 45% to ensure value to the supply voltage of 30%.
With the power supply voltage increase noise voltage proportional to the absolute value will increase. For VDD = 15V power supply voltage (when VSS = 0V), the circuit will have 7V or noise.
Input impedance is high
CMOS integrated circuit to the input of the General protection diode and series resistance constitutes a protected network, than most FET input resistance is a little small, but in the normal operating voltage range, these protection diodes are reverse bias, DC input impedance depends on these diode leakage current, typically, equivalent input impedance up to 103 ~ 1011 Ω CMOS integrated circuit, little power consumption drive circuit.
Good temperature stability
Since the power consumption of the CMOS integrated circuit is low, the internal heat less, and CMOS circuit line structure and electrical parameter has symmetry, in temperature environment changes, certain parameters to the function of automatic compensation, CMOS IC's excellent temperature characteristics. General ceramic metal package circuit, working temperature:-55 to + 125 ° c; plastic encapsulated circuit working temperature range is-45 ~ + 85 ℃.
Fan-out ability
Fan-out is circuit outputs can drive the number of input. As CMOS integrated circuit input impedance is extremely high, so the output capacity from the circuit input capacitance limits, however, CMOS IC's used to drive the same type, such as not consider speedDegree, typically driven 50 more input.
Strong resistance to radiation
CMOS integrated circuit in the basic device is a MOS transistor conductive belonging to majority carrier. Various x-ray, radiation on its conductive properties of both limited and therefore especially suitable for the manufacture of aerospace and nuclear experiment equipment.
Good controllabilityr>CMOS IC output waveform rise and fall time can control the output of the rise and fall time of typical values for circuit propagation delay time of 125% ~ 140%.
Interface convenient
Because CMOS integrated circuit input impedance is high and output swing, easy-to-be other circuits driven, easy to drive other types of circuits or devices.
TTL-Transistor-TransistorLogic Transistor-transistor logic
MOS-Metal-OxideSemiconductor metal oxide semiconductor transistor
CMOS-complementary metal oxide ComplementaryMetal-OxideSemiconductor semiconductor transistor

Q: why BJT faster than CMOS?
A: many people only know the BJT than CMOS but do not know why.
Mainly affected by the impact of migration. To NPN tube and NMOS, for example, the rate of migration in the BJT is body mobility, approximately 1350cm2/vs. NMOS is semiconductor surface mobility, approximately 400-600cm2/vs. So the BJT of MOS transconductance than the speed of the MOS. This is also the NPN (NMOS) than PNP (PMOS) fast.
NPN PNP is also faster than because carrier mobility, the base area in NPN fewer children is electronic, the mobility of large (about 1350); PNP base area little child is hole (480 or so). So the same structure and dimensions of the pipe, faster than the NPN PNP. So in bipolar process and is mainly for NPN, PNP tube are compatible on the basis of the protest. MOS Technology are N-well PSUB process, this technology can do parasitic PNP, NPN pipe is to be done is P trap NSUB process.
BJT is called bipolar, because both exist in the base area and there is electronic, the hole are two carriers involved in conducting, and MOS devices in the shape layer only a carrier participation in conductive.
But not because the two carriers conducting general migration rate. And the situation may be the opposite. Because carriers of mobility is temperature and concentration. The higher the concentration of doped semiconductors, migration rates. And less in BJT, son of mobility play a major role.
NPN PNP pipe faster than the pipe because the base child NPN fewer children is electronic, PNP is a hole, electronic transfer rate than the hole. NMOS is also faster than PMOS.
And faster than NMOS NPN because NPN is in devices, its carrier's mobility is a semiconductor in mobility; NMOS devices, its surface is carrier mobility is surface mobility (because the shape layer is at the gate oxide formation of under the surface). And semiconductors in mobility is larger than the surface mobility.

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