Download Integral Common Emitter - Analog Electronics - Lab Handout and more Exercises Digital & Analog Electronics in PDF only on Docsity!
Lab 5. Audio Amplifier
I. Introduction
Audio amplifier is required in all audio equipments. Figure 1 shows an audio amplifier consisting of a Class AB amplifier (X1 and X2) and an integral common emitter (CE) bipolar amplifier (Q4). The Class AB amplifier is the output stage delivering high power to the load resistor with very small output impedance. It is biased by the VBE multiplier (Q3, R 1 and R 2 ). The common emitter (CE) bipolar amplifier itself has a gain from tens to hundreds. With the help of the bootstrap capacitor C 4 , the open circuit voltage gain is very high, in the order of thousands. When negative feedback (R 5 ) is added, the overall voltage gain of the audio amplifier is determined by the ratio of R 7 to the feedback resistor R 5 , AV = – R 5 /R 7.
Q Q2N
Vcc 30Vdc
RE 1
R 20k
C 100u RE 470
X
TIP
R 1.8k
R 2.7k
Q Q2N
CL 100u
C 100u
C4 100u
Vs FREQ = 1k
VAMPL = 100m
VOFF = 0
R 1.8k
R 100k
X
TIP
input
R 1k
Vb
output
0
RL R7^8 5.1k
0
RE 1
Figure 1. Audio Amplifier with Class AB power stage
II. Objective
In this one-week lab, you are going to simulate the audio amplifier shown in Figure 1 with OrCAD-Capture. Due to the limit of time, all parameters in the circuit are given. First, you need to obtain the DC biasing voltages and currents. Then, you will find out the AC voltage gain, current gain as well as the frequency response of the circuit.
III. Lab Steps
The lab procedures are listed below and all the parameters in the amplifier circuit are given in Table 1.
a. Create a schematic as shown in Figure 1 in OrCAD-Capture. All the parameters are given in Table 1. b. Simulate the circuit with Transient Analysis. Record the base current ( IB ), collect current ( IC ), emitter current ( IE ), base-emitter voltage ( VBE ) and collect-emitter voltage ( VCE ) for each of the four transistors. Calculate the current gain β = IC/IB for each transistor. c. Capture both AC input voltage and output voltage with the peak-peak values. Calculate the AC voltage gain. d. Capture both AC input current and output current with the peak-peak values. Calculate the AC current gain. e. Change the value of R7 to 1 kΩ and 10 kΩ, respectively, and repeat Step c and d. f. Keep the value of R7 as 5.1 kΩ. Capture the frequency response (AC Sweep) of the circuit. g. Change the values of CL to get the 3dB cut-off frequency at 50 Hz.
IV. Lab Report
Report should include at least the following:
- Brief Introduction and Objective
- Schematic of the circuit
- Simulation results: − DC biasing currents: IB1, IC1, IE1, IB2, IC2, IE2, IB3, IC3, IE3, IB4, IC4, IE4. Calculate the current gain β = I (^) C/IB of each transistor. Comments on the relations between these currents and the current gain β
