engineering electronics

David Rubenstein Final Project Report - iPod Docking Station

ECE 2115 - 31 Bhavin Mehta 23 April 2014

Introduction An audio amplification station was constructed. The system utilized a common emitter (CE) to common collector (CC) cascade of amplifiers to achieve amplification. The CC stage used two darlington paired power transistors for current gain. The whole system was powered by a DC power supply designed for it. The power supply consisted of a 115V-18V transformer, two half wave rectifiers, voltage filters, and voltage regulators. The power supply had an output of 24V. The power supply circuit is shown in Figure 1, and the amplifier circuit is shown in Figure 2.

DC Power Supply The DC supply designed used the 166K18 transformer for the initial step down of voltage. D1 and D2 were the 1N4002 diodes. Each one constitutes a half wave rectifier. CEq1 and CEq2 were both .8mF. Each of the equivalent capacitances was connected from one of the diodes to the center tap which was acting as ground. Multiple electrolytic capacitors were used in parallel to achieve a greater capacitance. The voltage regulators were also connected to the capacitors and diodes to achieve the designed voltage. The LM7812 outputs +12V with respect to the transformer’s center tap (CT), and the LM7912 outputs -12V with respect to the transformer’s CT. The desired voltage for the amplifier was 24V, thus, -12V was used as the ground and +12V was used as the VCC for the amplifier circuit. This way, there are 24V between the outputs of the two voltage regulators. A resistor and LED were also placed between the outputs of the voltage regulators to indicate when the circuit was turned on. R was equal to 3.2kΩ. Assuming there is a 2V drop across the LED, this means that when the power supply is on there is 6.9mA through the LED indicator light. PSPICE does not have the LM7812 or LM7912 so simulating the power supply required zener diode regulators with breakdown voltages of 12V. Figure 1:

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Figure 2: SPICE simulation of power supply

" Figure 3: Unloaded simulation result

" Figure 4: Voltage taken at capacitors without regulation or load

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Common Emitter Stage The transistor used for the CE amplifier was the 2N3904 NPN BJT. RL was assumed to be the two 7kΩ resistors in parallel in the CC stage of the amplifier. This makes RC 350Ω by equation 1. However, a 350Ω resistor was not used in the final design. Instead, it was replaced by a 1kΩ potentiometer for volume control. This potentiometer allowed for volume control. When the potentiometer is set to a very small resistance, The collector is essentially shorted to VCC. Because the output is taken at the collector, this means that the CC amplifier’s input is essentially also shorted to VCC. There will not be any small signal AC voltage at VCC, therefore there will be no sound coming from the speaker. Using Equations 3, 4, and 5, RB was determined. Equation 5 was an assumption given to students. β was determined to be 125 previously, thus it was assumed to still be 125 for these calculations. R1 and R2 could be calculated once VB and the ratio between R1 and R2 were known. VB was determined with equation 8, and the ratio between R1 and R2 was determined with equation 7. Then equation 6 was used to determine R1 and R2. R1 is the resistor connected between VCC and the base terminal. R2 is the resistor connected between the base terminal and ground.

" Equation 1 " Equation 2

" Equation 3 " Equation 4

" Equation 5 " Equation 6

" Equation 7 " Equation 8

Table 1: Bias results (RC fixed at 350Ω)

Figure 5: Amplifier circuit in SPICE

Measured Simulated Calculated Measured Error (%)

Calculated Error (%)

VC (V) 0.36 14.11 12 97 15

VB (V) 1.04 1.75 1.91 41 9

VE (V) 0.31 0.99 1.21 69 22

IC (mA) 67.5 27.4 34.3 146 25

IB (mA) 1.7 0.16 0.27 962 69

IE (mA) 8.9 27.6 34.6 68 25

" Figure 6: Amplifier bias test in SPICE

" Figure 7: Resulting wave form, Vin = 500mV VPP @ 4kHz

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Common Collector The CC amplifier consisted of two darlington paired power transistors. Three TIP31As were used, and one TIP32A was used. This circuit was given to students. A darlington pair consists of two transistors that share a collector. The emitter of one transistor is connected to the base of the other. The overall device is still three terminals: a shared collector, a base, and an emitter. The advantage of a darlington pair is that the current gain is much higher. The resulting β for the two transistors is: βD = β1 + β1β2 + β2. This allows for a dramatically higher β value

than with a single transistor. It is because of this factor that the current gain can be so high without a large biasing current through the transistors.

Table 2: Vin = 500mV VPP @ 4kHz (RC fixed at 350Ω)

Discussion The DC power supply worked as expected. It outputted about 24V when the voltage was taken from the outputs of the two voltage regulators. The voltage regulation was necessary because the peak voltage output of the transformer was higher than desired. This is shown in Figure 4. The peak voltage output of one side of the power supply would be about 19V. This would result in a total of 38V. In order to get a 24V output voltage regulation was necessary. The voltage regulators also further aid in reducing ripple voltage. If the ripple voltage is small enough that the lowest voltage outputted from the filter capacitors is still over 12V then the output of the regulators will never drop below 12V. Thus an ideal DC power supply is created. There was a significant error in the biasing and gain of this circuit. The voltages at the terminals of the CE transistor make little sense. The voltages suggest that the transistor is in saturation mode. It was not designed to be in saturation nor would there be anywhere near sufficient amplification if it were actually in saturation. The currents calculated from these voltages made equally little sense. The collector current was found to be several times higher than the emitter current, and of course the collector current added to the base current did not equal the emitter current. There was likely an error in recording the data. The circuit would also have different values for different values of RC. The values were obtained with RC fixed at 350Ω. The differences between the calculated and simulated results were not as dramatic as the measured ones for the biasing. They were likely different because some assumptions were made in calculating resistor values. For instance, Equation 5 was an assumption. β may also have been slightly different in reality. The results for the gain of the circuit were very similar for measured and simulated. The gain was calculated to be significantly higher than both however. The gain may change significantly depending on what value the potentiometer at RC is equal to. The test frequency and amplitude used (500mV VPP @ 4kHz) were picked for the sake of comfort of those in Tompkins 301. The circuit may have different gain for different frequencies. The capacitor values chosen may be too resistive for higher frequencies. This would decrease the gain from each time the signal passes through a capacitor. Based on the gain found, the maximum power output of the speaker would be 4.4 W assuming a 1V input. This is well below the desired power of 10 W. However, as the potentiometer is adjusted the gain may increase significantly.

VPP (V) Av Power (W) Error (Av) (%)

Measured 2.96 5.92 0.27 7.5

Simulated 3.2 6.4 0.32 N/A

Calculated 4.5 9 0.63 41

Conclusion Transistors cascaded from CE to CC are ideal for large loads. Darlington paired transistors provide dramatically increased current gain over an ordinary CC circuit. A potentiometer used as RC in the CE stage is good for volume control over the output.