Difference between CMOS and TTL

What is TTL?

TTL (Transistor-Transistor-Logic) circuits are composed of bipolar transistors and belong to bipolar circuits. Its logic level range is relatively narrow, usually between 0V and 5V, with a common voltage of 5V. TTL circuits have fast switching speed and strong driving capability, but their power consumption is relatively high, usually between 1 and 5 milliamps per gate. TTL circuits have weak anti-interference ability and small noise tolerance, but operate at high frequencies and have short transmission delay times.

Bipolar transistor BJT (triode) integrated logic constitutes a chip or device.

Why is 2V recognized as an input high level?

TTL NAND gate

Take the NAND gate as an example: when A and B are both 2V input, T1, 2, and 5 are completely turned on, T4 is turned off, and the output is low;

Why is 0.8V considered as input low level?

Hign and low of TTL

And if one of the inputs of A and B is a low level of 0.8V, the voltage at the emitter point E of T1 is pulled down, T1, 2, and 5 are turned off, T4 is turned on, and a high level is output.

D1 and D2 play a clamping role, limiting the current to protect the device.

Common types:

The 74 series that we commonly use now are TTL devices, and his level is judged to be the TTL level as the standard. In addition, there are the following types: high-speed H-TTL, low-power L-TTL, Schottky S-TTL, low-power Schottky LS-TTL

What is CMOS?

CMOS (Complementary Metal-Oxide-Semiconductor Transistor) circuits consist of enhanced PMOS and NMOS transistors and belong to unipolar circuits. Its logic level range is wide, usually between 5V and 15V, with a common voltage of 12V. The static power consumption of CMOS circuits is very low. When there is no input signal to the logic gate, the transistor is in an off state, and almost no current flows. There is only a brief transient power consumption when the input signal changes. In addition, CMOS circuits have strong anti-interference ability and large noise tolerance, making them suitable for large-scale integration.

Complementary symmetry is because both N-type MOS and P-type MOS appear in the circuit in the device.

Input voltage characteristics: 

• Supply voltage: 3-18V

• Low: 0.3*VCC

• High: 0.7*VCC
  

Output Voltage Characteristics:

• Low: 0.1*VCC

• High: 0.9*VCC

Common series:

74HC High Speed CMOS, 74HCT TTL Compatible High Speed CMOS, 74BCT Bipolar Compatible TTL

(BiCMOS: the field effect transistor is used as the pre-stage and the intermediate stage; the triode is used as the output stage)

CMOS vs TTL comparison:

①TTL is a bipolar current control device, CMOS is a unipolar voltage control device

②The power supply voltage is different, TTL commonly used 5V power supply, CMOS: 5V, 3.3V, 2.4V, 1.8V

③TTL has weak anti-interference ability and small noise tolerance, mainly because the CMOS logic level range is wide, the distance between the high-level threshold interval and the low-level interval is large, and the anti-interference ability is strong; but the CMOS device has a large input impedance and is easy to capture interference. , the noise is large. However, the output impedance of CMOS is low, in the range of KΩ; the unused input pins of CMOS devices must be grounded or fixed at a fixed level (that is, pay attention to electrostatic protection), while the pins of TTL devices are left floating and default to high level.

④TTL power consumption is large, CMOS power consumption is low, static power consumption is almost no, and level switching has large current.

⑤TTL devices work fast, with a transmission delay of 5~10ns; cmos transmission delay of 25~50ns.

Mutual drive analysis

Comparing the input and output levels of the two, it is obvious that TTL cannot directly drive CMOS devices, while the latter can directly drive the former; therefore, TTL driving CMOS requires a pull-up resistor on the periphery, and the VCC of TTL is connected to a resistor, so the high-level driving ability is small.