Chapter 9. Semiconductor Amplifier Circuits
Recommended Article : 【Circuit Theory】 Circuit Theory Table of Contents
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
① 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.
② 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