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CMOS |
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CMOS Introduction CMOS Amplifier Amplifier characteristics Gain Output dynamic range Bandwidth and rise time Settling time and aberrations Slew rate Noise Efficiency Linearity Electronic amplifiers Two-Stage Amplifier Design Of OP-AMP Design Of Two Stage OP-AMP |
Complementary Metal Oxide Semiconductor
CMOS logic is a newer technology, based on the use of complementary MOS transistors to perform logic functions with almost no current required. This makes these gates very useful in battery-powered applications. The fact that they will work with supply voltages as low as 3 volts and as high as 15 volts is also very helpful.
CMOS gates are all based on the fundamental inverter circuit shown to the left. Note that both transistors are enhancement-mode MOSFETs; one N-channel with its source grounded, and one P-channel with its source connected to +V. Their gates are connected together to form the input, and their drains are connected together to form the output.
The two MOSFETs are
designed to have matching characteristics. Thus, they are complementary to
each other. When off, their resistance is effectively infinite; when on,
their channel resistance is about 200
When input A is grounded (logic 0), the N-channel MOSFET is unbiased, and therefore has no channel enhanced within itself. It is an open circuit, and therefore leaves the output line disconnected from ground. At the same time, the P-channel MOSFET is forward biased, so it has a channel enhanced within itself. This channel has a
resistance of about
200
When input A is at +V (logic 1), the P-channel MOSFET is off and the N-channel MOSFET is on, thus pulling the output down to ground (logic 0). Thus, this circuit correctly performs logic inversion, and at the same time provides active pull-up and pull-down, according to the output state.
This concept can be expanded into NOR and NAND structures by combining inverters in a partially series, partially parallel structure. The circuit to the right is a practical example of a CMOS 2-input NOR gate.
In this circuit, if both inputs are low, both P-channel MOSFETs will be turned on, thus providing a connection to +V. Both N-channel MOSFETs will be off, so there will be no ground connection. However, if either input goes high, that P-channel MOSFET will turn off and disconnect the output from +V, while that N-channel MOSFET will turn on, thus grounding the output.
The structure can be inverted, as shown to the left. Here we have a two-input NAND gate, where a logic 0 at either input will force the output to logic 1, but it takes both inputs at logic 1 to allow the output to go to logic 0.
This structure is less limited than the bipolar equivalent would be, but there are still some practical limits. One of these is the combined resistance of the MOSFETs in series.
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Since the gate is essentially an open circuit it draws no current, and the
output voltage will be equal to either ground or to the power supply
voltage, depending on which transistor is conducting.
