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Field Effect Transistors Circuit Analysis EE314 HP PA8000 Fujitsu 68903 Fairchild Clipper C100

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Chapter 12: Field Effect Transistors 1.MOS Small Signal Equivalent 2.Transconductance 3.Common-Source Amplifiers 4.Source Follower 5.Logic gates FET Circuit Analysis

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Small-Signal Equivalent Circuit for FETs Output signal from an amplifier using FET can be effectively modulated by small changes of input signal current. In this way it is possible to make small changes from the Q point. Symbols: The total quantities: i D (t), v GS (t) The dc point values: I DQ, V GSQ The signal i d (t), v gs (t) Characteristic

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Small-Signal Equivalent Circuit -Transconductance Schematic Analysis… (a little bit of math) We know that Also we assume that *

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Small-Signal Equivalent Circuit -Transconductance Schematic We know that Also we assume that Drain current generated by signal *

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Small-Signal Equivalent Circuit -Transconductance We define the transconductance as or so

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Small-Signal Equivalent Circuit -Transconductance Thus the transconductance so

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Small-Signal Equivalent Circuit -Transconductance The transistor has KP=50 A/V 2, V to =2V, L=10 m, and W=400 m Exercise

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Small-Signal Equivalent Circuit Also we assume that Better performance is obtained with higher values of g m. Please notice that g m is proportional to the square root of the Q point drain current. Simply, we can increase g m by choosing a higher value of I DQ.

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More Complex Equivalent Circuits For more accurate analyses of FET transistor we have to add more components to an equivalent circuit. Small capacitance: for high response FET amplifiers Drain resistor: account for the effect of v DS on the drain current Correction for i d Please read section: Transconductance and … pp.591 Example 12.3

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Drain Resistance Calculation so at v GS =4V

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Common-Source Amplifier Schematic Equivalent circuit G D S The dc supply voltage acts as a short circuit for the ac current.

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Common-Source Amplifier Schematic Equivalent circuit ac signal C s -bypass C C 1, C 2 -coupling C G D S The dc supply voltage acts as a short circuit for the ac current.

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Common-Source Amplifier: Gain, R in and R out Equivalent circuit (once more) Voltage gain Input resistance From bias point analysis

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Common-Source Amplifier: Gain, R in and R out To find out the R out we have to: disconnect the load, replace the signal source by short circuit – Thevenin equivalent resistance No source connected to the input if v gs =0 then g m v gs =0 Output resistance Example 12.4

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Source Follower

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Small-Signal Equivalent Circuit –Source Follower Notice that small signal I DS goes up. Why?

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Voltage gain Input resistance Small-Signal Equivalent Circuit –Source Follower Since the output voltage is almost equal to the input – hence the name source follower

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Logic gates – COMS Inverter V in V out 01 10 Logic truth table Switch level equivalent circuits

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Logic gates – COMS NAND gate ABV out 001 011 101 110 Logic truth table

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Logic gates – COMS NOR gate Exercise Draw switch level circuits for different inputs and derive the truth table for this gate

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ABV out 001 010 100 110 Logic truth table Logic gates – COMS NOR gate

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