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Chapter 9. Semiconductor Amplifier Circuits

Recommended Article : 【Circuit Theory】 Circuit Theory Table of Contents


1. Bias Circuits

2. Large-Signal, Small-Signal Analysis

3. Amplifiers Using BJT Transistors

4. Amplifiers Using FET Transistors



1. Bias Circuits

⑴ Issue : Input signal of the amplification circuit is AC and small in magnitude.

⑵ Apply appropriate DC voltage to the transistor to perform its intended function.

① Generally, design the function in the active region for BJT transistors.

② Generally, design the function in the saturation region for FET transistors.

⑶ Bias circuits for BJT transistors

① Base bias circuit

○ Circuit that adjusts VC and VB with a single power source and 2 resistors.

○ Not commonly used due to low temperature stability.

Figure 1. Base Bias Circuit

② Emitter bias circuit

○ Adjust VC and VB with 2 voltage sources.

○ Disadvantage of needing two voltage sources.

Figure 2. Emitter Bias Circuit

③ Voltage division bias circuit

○ Divide Vsource to supply appropriate voltage to VB.

Figure 3. Voltage Division Bias Circuit

Tevenin equivalent circuit

Figure 4. Voltage Division Bias Circuit Equivalent Circuit

The power below RTH is the Tevenin equivalent power.

④ Practical advice

○ Generally, set VCE to 0.5VCC.

○ Generally, set VE to 0.1VCC.

⑷ Bias circuits for FET transistors

① Voltage division bias circuit

Figure 5. Voltage Division Bias Circuit for FET Transistors



2. Large-Signal, Small-Signal Analysis

⑴ General circuit design

① DC voltage is necessary for bias.

② Use capacitors to block distortion in the output signal caused by DC voltage: blocking effect for DC.

③ Use capacitors with sufficient capacitance to allow AC signals to pass easily: bypass effect for AC.

⑵ Signal analysis

① Small-signal analysis : Interpreting the circuit using the superposition principle by separating DC and AC power supplies.

○ Generally, uppercase for DC quantities, lowercase for AC quantities.

② Large-signal analysis : Interpreting the circuit in the time domain by considering both DC and AC power supplies.

③ Large-signal analysis is mathematically complex, so small-signal analysis is preferred.



3. Amplifiers Using BJT Transistors

⑴ AC equivalent circuit

Model 1. Hybrid π Model

Figure 6. Hybrid π Model

○ Calculate gm, ri using DC analysis-derived control quantities (current, voltage).

○ Reason: AC small signals do not significantly alter circuit control quantities.

○ Even if there is a resistor between emitter (marked as emitter) and ground, gm vbe remains constant.

Model 2. Common emitter re Model

Figure 7. re Model

○ Calculate β × re using DC analysis-derived control quantities (current, voltage).

○ Reason: AC small signals do not significantly alter circuit control quantities.

⑵ Common emitter circuit

Figure 8. Common Emitter Circuit Example

① Definition : Circuit where input is at the base terminal and output is at the collector terminal.

② Operation region determined by DC Vcc.

③ Amplify AC input Vin to output Vout : Phase is inverted.

⑶ Common base circuit

① Definition : Circuit where input is at the emitter terminal and output is at the collector terminal.

⑷ Common collector circuit

① Definition : Circuit where input is at the base terminal and output is at the emitter terminal.



4. Amplifiers Using FET Transistors

⑴ AC equivalent circuit

Figure 9. FET Transistor AC Equivalent Circuit

① λ is intrinsic to the component and is often assumed to be 0 for convenience → ro = ∞

② Calculate gm using DC analysis-derived control quantities (current, voltage).

③ Reason: AC small signals do not significantly alter circuit control quantities.

⑵ Common source circuit

Figure 10. Common Source Circuit

① Definition : Circuit where input is at the gate terminal and output is at the drain terminal.

② DC analysis : Capacitor portion is treated as cut wire. Operation region determined by DC Vcc.

③ AC analysis : Vo varies due to AC input signal Vs.

Figure 11. Common Source Circuit AC Equivalent Circuit

⑶ Common gate circuit

① Definition : Circuit where input is at the source terminal and output is at the drain terminal.

⑷ Common drain circuit

Figure 12. Common Drain Circuit

① Definition : Circuit where input is at the gate terminal and output is at the source terminal.

② DC analysis

③ AC analysis

Figure 13. Common Drain Circuit AC Equivalent Circuit

④ Conclusion

○ Effect similar to reducing loaded resistance while maintaining input signal.

○ Suitable when internal resistance of AC voltage source is high.

○ Refer to source follower.



Input : 2019.12.04 00:01

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