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Graphical solutions

Graphical solutions. Fig. 5.18. Small signal equivalent circuits. Assuming operation in the saturation region. For small signal case is negligible. Where. The gate current for FET is negligible. g m = transconductance. Small signal equivalent circuit. Where,.

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Graphical solutions

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  1. Graphical solutions

  2. Fig. 5.18

  3. Small signal equivalent circuits Assuming operation in the saturation region For small signal case is negligible Where The gate current for FET is negligible gm = transconductance Small signal equivalent circuit Where,

  4. Dependence of gm on Q-point and device parameters We know that and But from (5.3) Therefore μn- surface mobility of electron Cox- capacitance of gate per unit area

  5. At higher frequencies small capacitance have to be added between device terminals • Also in the saturation region iD versus VDS is considered to be constant. This is not actually the case. The drain current, iD increases slightly as VDS increases. In order to take care of that we must add a drain resistance rd in the small signal model. More complex equivalent circuits

  6. Determine the values of gm and rd the MOSFET characteristics shown below. From equation 5.34, we have, obtain iD = 6.7 mA at VDS = 4 V and iD = 8 mA at VDS = 14 V. Thus, the reciprocal of rd is calculated as: Example Thus, rd = 7.7 KΩ

  7. C1, C2 – Coupling capacitors  short circuit for AC signals and open circuit for DC bias calculation CS – bypass capacitor  small impedance for AC Common source amplifier Voltage Gain (5.38) (5.39) (5.40) (5.41)

  8. Input resistance (5.42) Output resistance (5.43) Example 5.6 (Modified)  = 0 implies 1/rd = 0 or rd =  Common source amplifier contd.. Find the midband voltage gain, input resistance, and output resistance of the amp. Thevenin Equivalent Circuit Bias circuit

  9. or Common source amplifier contd..

  10. Section 5.6 Recall the similarity with Emitter Follower To draw the small-signal ac equivalent circuit, we have to first draw the following circuit like remembering that: • Drain is grounded • Source is not grounded • R1 Gate to ground (drain) • R2  Gate to ground (drain) • RS  Source to ground (drain) • gmvgs & rd drain to source The Source Follower Thus, Small-signal ac equivalent circuit

  11. The Source Follower contd… How to calculate the output resistance ?

  12. Current gain: The current gain Ai = Av Rin/RL , like before. Fig. 5.33

  13. Example contd….

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