o Objective: To design, build and analyze a Type I symmetry amplifier and provide power to an 8 ohm speaker. o Materials: • Signal generator • Oscilloscope • Power Supply • Wires • Resistors • Variable resistors • Capacitors • Bread board • Transistors (npn, pnp) o Theory and Understanding - The symmetry power amplifier is usually the last stage in a multistage circuit as it is responsible for doubling the overall output voltage. While the voltage gain is normally 1, this amplifier has a great power gain as well as overall stability. In order to put together the circuit, we need two transistors; n-p-n and p-n-p is cascading form. We then have to perform calculations to get the proper R1 and R2 values to power the 8 ohm speaker. o Preparatory Work 1.) The q-point needs to be above cutoff so we can reduce the crossover distortion. This is also why R1 value is critical in the diode-compensated power amplifier design. 2.) The output capacitor charges during the time interval in which the transistors switch from on to off. During this time, the capacitor charges to half the supply voltage. 3.) The voltage gain of the complementary amplifier is only 1. 4.) With 𝑅𝐿𝑂𝐴𝐷 = 8 Ω, Vcc = 12 V, Ai = 20 and (beta) = 100: 𝑅 𝑅 a. 20 = 𝑅2 2 = 𝑅2 2 //solve to get 400 Ω for R2. (2∗𝑉𝑏𝑒) 𝑏𝑒𝑡𝑎 + 2∗𝑅𝑙𝑜𝑎𝑑 𝑅1 𝑏𝑒𝑡𝑎 𝑅1 + 2∗8 (2∗0.7) b. 𝑉𝑐𝑐 = 𝑅1+ 𝑅2 = 𝑅1+ 400 = 12 𝑉 //solve to get 52.8 Ω for R1. c. 𝑅𝐵 = 𝑅2 || 𝑅2 = 200 Ω d. 𝑅𝑖𝑛 = 𝑅𝐵 || 𝑏𝑒𝑡𝑎 ∗ 𝑅𝐸 = (200 Ω) || (800 Ω) = 160 Ω 1 e. 𝐶𝑖𝑛 ≫ 2∗𝑝𝑖∗(100 𝐻𝑧)(𝑅𝑖𝑛) f. 𝐶𝑜𝑢𝑡 ≫ 1 2∗𝑝𝑖∗(100 𝐻𝑧)(𝑅𝑙𝑜𝑎𝑑) o Data and Analysis a.) As usual, we have to build and simulate the circuit designed in preparatory work within P-Spice. SAUL B. HENDERSON (N00231037) 1 Figure 1: P-Spice schematic of symmetry amplifier. Figure 2: The output voltage of the amplifier circuit. We do not see it now, but eventually, the negative dips in voltage will progressively get larger as the positive dips will gradually get smaller. The input voltage used was 2 volts. SAUL B. HENDERSON (N00231037) 2 Figure 3: The frequency roll off for the voltage gain. b.) After running the circuit on P-Spice, we could put together the circuit. However, we used around 100mV for the input for our circuit as the AC signal generator could not supply the 2 volts needed. So we went with 80 mV. Figure 4: Hardware implementation of the symmetry amplifier. SAUL B. HENDERSON (N00231037) 3 Figure 5: Voltage in versus voltage out. The input is shown on Channel 1 and the output is shown on Channel 2. As shown, there is slight voltage gain, but not much. Freq. (Hz) Original Output Cap Vin 20 50 100 200 500 1000 2000 5000 10000 20000 50000 100000 200000 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.064 0.064 Vout 0.094 0.094 0.094 0.094 0.094 0.108 0.108 0.108 0.108 0.108 0.108 0.094 0.094 Freq. (Hz) Cout = 100uF Vin 20 50 100 200 500 1000 2000 5000 10000 20000 50000 100000 200000 Freq. (Hz) Vout 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.064 0.064 0.064 Cout = 47uF Vin 0.08 0.08 0.108 0.108 0.108 0.108 0.108 0.108 0.108 0.108 0.094 0.094 0.094 20 50 100 200 500 1000 2000 5000 10000 20000 50000 100000 200000 0.025 0.025 0.025 0.025 0.025 0.025 0.025 0.025 0.025 0.025 0.06 0.072 0.072 Vout 0.064 0.064 0.064 0.064 0.064 0.064 0.064 0.064 0.064 0.08 0.096 0.096 0.01 Figure 6: Data on voltage from using three different sizes of capacitors: 220uF, 100uF and 47uF. We have noticed that there is a drop in voltage on both input and output when we decrease the size of the capacitors. However, the voltage gain is still pretty small and we have gotten the same noisy output waveform. o Conclusion SAUL B. HENDERSON (N00231037) 4 It did not take much to calculate the value of the resistors, but it took a lot to get the hardware implementation as there were many times we didn’t get what we wanted. At first we used the TIP29A and TIP31A transistors, but to no avail. Then we used QN3904 and QN3906 in which we have gotten results. Although we have gotten a slight gain in voltage, we could not get sound to the output speaker. SAUL B. HENDERSON (N00231037) 5