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MALVINO

MALVINO. Electronic. PRINCIPLES. AC Models. Chapter 9. X C. R. R. R. SHORT. OPEN. The coupling capacitor. {. Good coupling: X C < 0.1 R. 1. For ac analysis, the capacitor is a short. 2. For dc analysis, the capacitor is open. +30 V. 5 k W. 1 M W. b dc = 100. 100 k W.

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MALVINO

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  1. MALVINO Electronic PRINCIPLES

  2. AC Models Chapter 9

  3. XC R R R SHORT OPEN The coupling capacitor { Good coupling: XC < 0.1 R 1. For ac analysis, the capacitor is a short. 2. For dc analysis, the capacitor is open.

  4. +30 V 5 kW 1 MW bdc = 100 100 kW 100 mV A base-biased amplifier with capacitive coupling A dc analysis reveals IB = 30 mA, IC = 3 mA and VC = 15 V.

  5. IB 30 mA t IC 3 mA t VC 15 V t

  6. + 15 V 0 +0.7 V 0 +30 V 5 kW 1 MW bdc = 100 100 kW 100 mV The base-biased amplifier with voltage waveforms

  7. A = 200 The voltage gain of an amplifier is the ac output divided by the ac input. vin vout vout A = vin

  8. The bypass capacitor Ac ground R XC Good bypassing: XC < 0.1 R

  9. +6.04 V +1.8 V 0 +1.1 V 0 A VDB amplifier with voltage waveforms +10 V 3.6 kW 10 kW 100 kW 2.2 kW 1 kW 100 mV

  10. +5.32 V 0 0 -0.7 V 0 A TSEB amplifier with voltage waveforms +10 V 3.6 kW 100 kW 100 mV 2.7 kW 1 kW -2 V

  11. IE Q VBE Input signal Large-signal operation produces distortion

  12. ie IE IEQ = 10 mA Less than 1 mA VBE Total emitter current: IE = IEQ + ie Small-signal operation: ie(PP) < 0.1IEQ

  13. The dc current gain is given as: IC bdc = IB The ac current gain is given as: ic ib bac = Use capital letters for dc quantities and lowercase letters for ac.

  14. IE VBE The size of the ac emitter current depends on the Q point.

  15. Total emitter current: IE = IEQ + ie Total base-emitter voltage: VBE = VBEQ + vbe The ac resistance of the emitter diode is defined as: vbe re’ = ie

  16. vbe re’ = ie IE Larger ie Smaller ie VBE Note that re’ varies with the operating point. This implies that re’ is a function of the dc emitter current.

  17. A very useful formula: 25 mV re’ = IE

  18. vbe zin(base) = ib iere’ zin(base) = ib The T model of a transistor: ic vbe = iere’ zin(base) ib re’ ie zin(base) = bre’

  19. The p model of a transistor is based on zin(base) = bre’ : ib zin(base) bre’ ic ie

  20. Amplifier analysis • Perform a complete dc analysis • Short all coupling and bypass capacitors • Visualize all dc supply voltages as ac grounds • Replace the transistor by its p or T model • Draw the ac equivalent circuit

  21. Data sheets • The four h parameters are traditional • hfe is the ac current gain • hie is equivalent to input impedance • bac = hfe • re’ = hie/hfe • hre and hoe are not needed for basic design and troubleshooting

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