Multisim基本共射放大器设计

Question

仿真:设计基本共射放大器,要求供电12V,ICQ=5mA,尽量让Q点在中间区域。设计电路参数,并分析交流放大倍数。请问这个怎么弄啊？😭

Answer 1

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Let's break down how to design a common-emitter amplifier in Multisim with a specified quiescent current (Icq) and power supply (Vcc). We'll aim for a Q-point near the middle of the DC load line for good signal swing.

1. Choose the Transistor:

Start by selecting an NPN transistor like the 2N3904 or 2N2222. These are common, readily available in Multisim, and well-documented.

2. Determine the Collector Resistor (Rc):

• Vce(sat): Assume the transistor's saturation voltage (Vce(sat)) is approximately 0.2V. This is the voltage across the collector-emitter when the transistor is fully on.
• Vcc: You're given a 12V supply.
• Icq: You want a quiescent collector current of 5mA.
• Rc Calculation: For a Q-point near the middle, aim for Vce = Vcc/2. Then, using Ohm's law: Rc = (Vcc - Vce - Vce(sat)) / Icq ≈ (12V - 6V - 0.2V) / 5mA ≈ 1.16kΩ. Choose a standard resistor value close to this, such as 1.2kΩ.

3. Determine the Emitter Resistor (Re):

• Ve: For good stability and bias control, set the emitter voltage (Ve) to be around 10% of Vcc. So, Ve ≈ 1.2V.
• Re Calculation: Re = Ve / Icq = 1.2V / 5mA = 240Ω. Choose a standard value like 220Ω or 270Ω.

4. Determine the Base Bias Resistors (R1 and R2):

• Vbe: Assume the base-emitter voltage (Vbe) is approximately 0.7V for a silicon transistor.
• Vb: The base voltage (Vb) is Ve + Vbe = 1.2V + 0.7V = 1.9V.
• Rule of Thumb for Stability: A good rule of thumb is to make the current through the R1/R2 voltage divider about 10 times Icq's base component (Ib). Ib = Icq / β. Assume a typical β (hFE) value for the 2N3904 of around 100. So, Ib ≈ 5mA / 100 = 50µA. The current through the voltage divider should be around 10 * Ib = 500µA.
• R2 Calculation: R2 = Vb / (10 * Ib) = 1.9V / 500µA = 3.8kΩ. Choose a standard value like 3.9kΩ.
• R1 Calculation: The voltage across R1 is Vcc - Vb = 12V - 1.9V = 10.1V. R1 = (Vcc - Vb) / (10 * Ib + Ib) ≈ 10.1V / 550µA ≈ 18.4kΩ. Choose a standard value like 18kΩ.

5. Coupling and Bypass Capacitors:

• Input Capacitor (Cin): Choose a value large enough to pass the AC signal without significant attenuation. A good starting point is 1µF.
• Output Capacitor (Cout): Similar to Cin, choose a value like 1µF.
• Emitter Bypass Capacitor (Ce): This capacitor bypasses the emitter resistor for AC signals, increasing gain. A good starting point is 10µF.

6. AC Analysis (Voltage Gain):

• Av Calculation (approximate): The voltage gain (Av) of a common-emitter amplifier is approximately Av = -Rc / Re. Using our chosen values: Av ≈ -1.2kΩ / 220Ω ≈ -5.45. The negative sign indicates a 180-degree phase shift.
• Multisim AC Analysis: In Multisim, use the AC analysis tool to plot the frequency response. You'll set the input voltage source to a small AC signal (e.g., 10mV) and sweep the frequency. The plot will show you the gain at different frequencies.

Multisim Implementation:

1. Place the components (transistor, resistors, capacitors) on the workspace.
2. Wire the circuit according to the common-emitter configuration.
3. Connect a 12V DC power supply.
4. Connect an AC voltage source to the input.
5. Connect ground appropriately.
6. Use the DC operating point analysis to verify the quiescent current (Icq) and voltage (Vce). Adjust resistor values slightly if necessary to get closer to the desired Q-point.
7. Use the AC analysis tool to determine the voltage gain.

Code (Netlist Example - This is a simplified example and might need adjustments based on your specific Multisim version):

* Circuit Description: Common Emitter Amplifier

vcc 1 0 dc 12
vin 2 0 ac 10m 0
cin 2 3 1u
r1 1 3 18k
r2 3 0 3.9k
rc 1 4 1.2k
q1 4 3 5 2n3904  ; Node 5 is connected to the emitter bypass capacitor
re 5 0 220
ce 5 0 10u
cout 4 6 1u
rload 6 0 10k  ; Optional load resistor

.control
run
.ac dec 10 1 10Meg  ; AC analysis from 1Hz to 10MHz
plot v(6)/v(2)   ; Plot the voltage gain
.endc

Key Considerations:

• β Variation: Transistor β values can vary significantly. Using a more robust biasing scheme like adding a resistor in the emitter to ground (emitter degeneration) can improve stability.
• Temperature Effects: Transistor parameters are temperature-dependent. Consider the operating temperature range.
• Signal Swing: Ensure sufficient headroom for the output signal to swing without clipping.

This comprehensive guide should help you design and simulate your common-emitter amplifier in Multisim. Remember to double-check your calculations and component values before building the physical circuit. Good luck!