Analog Circuit Basics Series 5: Multi-stage Amplifier Circuit

1. Inter-stage coupling mode
1. Resistor-capacitor coupling---the static operating points of each stage are independent of each other; can effectively transmit AC signals; small size and low cost. But it is not easy to integrate and has poor low-frequency characteristics.
2. Transformer coupling---the static operating points of each stage are independent of each other, which can achieve impedance transformation. Large size, high cost, and cannot use integrated technology; not conducive to the transmission of low-frequency and high-frequency signals.  
3. Direct coupling---good low-frequency characteristics and easy to integrate. The static operating points of each stage are not independent and affect each other. There is a "zero point drift" phenomenon.
     *Zero point drift---when the temperature changes or the power supply voltage changes, the static operating point also changes, causing uo to deviate from the initial value "zero point" and change randomly.
2. Frequency response of single-stage amplifier circuit
1. Mid-frequency band (fL≤f≤fH)

Bode plot---the amplitude-frequency curve is 20lgAusm=constant, and the phase-frequency curve is φ=-180o.

2. Low frequency band (f ≤ fL)

4. Frequency characteristics of a complete basic common-emitter amplifier circuit

3. Frequency response of voltage-dividing stable operating point circuit
1. Estimation of lower limit frequency

2. Estimation of upper frequency limit

4. Frequency response of multi-stage amplifier circuit
1. Frequency response expression

2. Bode plot