In contrast, the complementary inverter needs significant voltage thresholds (but still < VDD/2) since both transistors switch close to middle (i.e., here the voltage threshold is desired). That is why, BJT are more suitable for this configuration since their base-emitter voltage VBE (0.7 V) is a relatively small threshold. The voltage follower needs small voltage thresholds of the transistors since they determine the difference between the input and output voltage (i.e., here the voltage threshold is something undesired). So, the CMOS inverter provides voltage levels almost equal to the supply rails. and they should be connected in a CMOS inverter configuration. This means that the voltage drops across the drain-source parts of both transistors should be almost zero. In contrast, digital circuits work close to supply rails their output voltage is either 0 V (ground) or VDD (+5 V). and they can be connected in a CMOS source follower configuration. So, the voltage drops across the drain-source parts of both transistors can be significant. Analog circuits work in the middle range of the power supply (in active mode) their output voltage does not reach supply rails (ground and VDD). It is impossible to make this by the source follower because its gain is less than one. The fact that logic gates are amplifiers makes it possible to build latches by introducing a positive feedback (simply by connecting the output of the cascaded inverters to the input). So the CMOS inverter improves the input signal. As a result, the input signal is amplified and its transitions become steep. In contrast, the CMOS inverter has significant gain during the switching because each of the transistors acts as a "dynamic load" to the other. So, if the input signal has poor (sloping) transitions, the output signal will also be poor.
The output voltage of the CMOS follower is a copy of the input voltage. Follower vs inverterįollowing vs amplification. Let's consider what is the difference between them. but here we are talking about digital circuits (logic gates). and it is widely used in analog amplifiers. He simply asks, "Why do we need to make a follower by two cascaded CMOS inverters (4 transistors in total) when such a simpler circuit (CMOS follower of only 2 transistors) exists?" and to the more specific question here, "Why not a source follower?" I will try to do it in a human friendly manner, without any special terms and definitions that impede the intuitive understanding at this initial stage. to find the answer to the question, "Why are they made exactly this way?". It is a big challenge to reveal the "philosophy" behind these legendary complementary stages.
But they frequently forget to tell us why they are made this way so usually we have to find the explanation ourselves. As a rule, the authors of circuit textbooks willingly show us how circuits are made.
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