Chapter 8 BJT SmallSignal Analysis Slide 1 BJT

Chapter 8: BJT Small-Signal Analysis

Slide 1 BJT Small Signal Analysis The re and hybrid models will be used to analyze AC small-signal transistor circuits.

Slide 2 Common-Emitter (CE) Fixed-Bias Configuration The input (Vi) is applied to the base and the output (Vo) is from the collector. The Common-Emitter is characterized as having high input impedance and low output impedance with a high voltage and current gain.

Slide 3 Removing DC effects of VCC and Capacitors

Slide 4 re Model Determine , re, and ro: look in the specification sheet for the transistor or test the transistor using a curve tracer. re: calculate re using dc analysis:

Slide 5 Impedance Calculations Input Impedance: [Formula 8. 1] [Formula 8. 2] Output Impedance: [Formula 8. 3] [Formula 8. 4]

Slide 6 Gain Calculations Voltage Gain (Av): [Formula 8. 5] [Formula 8. 6] Current Gain (Ai): [Formula 8. 7] [Formula 8. 8] Current Gain from Voltage Gain: [Formula 8. 9]

Slide 7 Phase Relationship The phase relationship between input and output is 180 degrees. The negative sign used in the voltage gain formulas indicates the inversion.

Slide 8 CE – Voltage-Divider Bias Configuration

Slide 9 re Model You still need to determine , re, and ro.

Slide 10 Impedance Calculations Input Impedance: [Formula 8. 10] [Formula 8. 11] Output Impedance: [Formula 8. 12] [Formula 8. 13]

Slide 11 Gain Calculations Voltage Gain (Av): [Formula 8. 14] [Formula 8. 15] Current Gain (Ai): [Formula 8. 16] [Formula 8. 17] [Formula 8. 18] Current Gain from Voltage Gain: [Formula 8. 19]

Slide 12 Phase Relationship A CE amplifier configuration will always have a phase relationship between input and output is 180 degrees. This is independent of the DC bias.

Slide 13 Unbypassed RE CE Emitter-Bias Configuration

Slide 14 Again you need to determine , re. re Model

Slide 15 Impedance Calculations Input Impedance: [Formula 8. 20] [Formula 8. 21] [Formula 8. 22] [Formula 8. 23] Output Impedance: [Formula 8. 33]

Gain Calculations Slide 16 Voltage Gain (Av): [Formula 8. 25] [Formula 8. 26] or [Formula 8. 27] Current Gain (Ai): [Formula 8. 28] Current Gain from Voltage Gain: [Formula 8. 29]

Slide 17 Phase Relationship A CE amplifier configuration will always have a phase relationship between input and output is 180 degrees. This is independent of the DC bias.

Slide 18 Bypassed RE CE Emitter-Bias Configuration This is the same circuit as the CE fixed-bias configuration and therefore can be solved using the same re model.

Slide 19 Emitter-Follower Configuration You may recognize this as the Common-Collector configuration. Indeed they are the same circuit. Note the input is on the base and the output is from the emitter.

Slide 20 re Model You still need to determine , re, and ro.

Slide 21 Impedance Calculations Input Impedance: [Formula 8. 37] [Formula 8. 38] [Formula 8. 39] [Formula 8. 40]

Slide 22 Impedance Calculations (cont’d) Output Impedance: [Formula 8. 42] [Formula 8. 43]

Slide 23 Gain Calculations Voltage Gain (Av): [Formula 8. 44] [Formula 8. 45] Current Gain (Ai): [Formula 8. 46] Current Gain from Voltage Gain: [Formula 8. 47]

Slide 24 Phase Relationship A CC amplifier or Emitter Follower configuration has no phase shift between input and output.

Slide 25 Common-Base (CB) Configuration The input (Vi) is applied to the emitter and the output (Vo) is from the collector. The Common-Base is characterized as having low input impedance and high output impedance with a current gain less than 1 and a very high voltage gain.

Slide 26 You will need to determine and re. re Model

Slide 27 Impedance Calculations Input Impedance: [Formula 8. 54] Output Impedance: [Formula 8. 55]

Slide 28 Gain Calculations Voltage Gain (Av): [Formula 8. 56] Current Gain (Ai): [Formula 8. 57]

Slide 29 Phase Relationship A CB amplifier configuration has no phase shift between input and output.

Slide 30 CE Collector Feedback Configuration This is a variation of the CE Fixed-Bias configuration.

Slide 31 You will need to determine and re. re Model

Slide 32 Impedance Calculations Input Impedance: [Formula 8. 58] Output Impedance: [Formula 8. 59]

Slide 33 Gain Calculations Voltage Gain (Av): [Formula 8. 60] Current Gain (Ai): [Formula 8. 61] [Formula 8. 62]

Slide 34 Phase Relationship This is a CE amplifier configuration; therefore there is a 180 -degree phase shift between input and output.

Slide 35 Approximate Hybrid Equivalent Circuit The h-parameters can be derived from the re model: hie = re hfe = hoe = 1/ro hib = re hfb = - The h-parameters are also found in the specification sheet for the transistor.

Slide 36 CE Fixed-Bias Configuration

Slide 37 Hybrid Equivalent Circuit Impedances: [Formula 8. 83] [Formula 8. 84] Gain: [Formula 8. 85] [Formula 8. 86]

Slide 38 CE Voltage-Divider Configuration

Slide 39 Impedances: Hybrid Equivalent Circuit [Formula 8. 87] [Formula 8. 88] Gain: [Formula 8. 89] [Formula 8. 90]

Slide 40 CE Unbypassed Emitter-Bias Configuration

Slide 41 Impedances: Hybrid Equivalent Circuit [Formula 8. 91] [Formula 8. 92] [Formula 8. 93] Gain: [Formula 8. 94] [Formula 8. 95] [Formula 8. 96]

Slide 42 CC or Emitter-Follower Configuration

Slide 43 Impedances: Hybrid Equivalent Circuit [Formula 8. 97] [Formula 8. 98] [Formula 8. 99] Gain: [Formula 8. 100] [Formula 8. 101] [Formula 8. 102]

Slide 44 CB Configuration

Slide 45 Hybrid Equivalent Circuit Impedances: [Formula 8. 103] [Formula 8. 104] Gain: [Formula 8. 105] [Formula 8. 106]

Slide 46 a Summary Table

Slide 46 b Summary Table

Slide 47 Troubleshooting 1. Check the DC bias voltages – if not correct check power supply, resistors, transistor. Also check to ensure that the coupling capacitor between amplifier stages is OK. 2. Check the AC voltages – if not correct check transistor, capacitors and the loading effect of the next stage.

Practical Applications Slide 48 • Audio Mixer • Preamplifier • Random-Noise Generator • Sound Modulated Light Source