EE 420L Electronics II Laboratory. Laboratory Exercise #4 Output Stage

Transcription

1 EE 40L Electronics II Laboratory Laboratory Exercise #4 Output Stage Department of Electrical and Computer Engineering University of Nevada, at Las Vegas Objective: The purpose of this lab is to understand design of output stage. Equipment Usage For this lab the following equipment will be used: Power supply Multi-meter Breadboard Connection wires Oscilloscope Function Generator N3904, N3906 and UA741 Background: Power amplifiers are generally characterized by percentage of time output transistor conducts current. In Class A operation, output transistor entire cycle of the input. In Class B operation, transistor conducts for ½ of the input cycle. Finally, Class B amplifier conducts slightly more than ½ of the input cycle. Figure 4-1 illustrates operations of Class A and Class B amplifier. Figure 4-1 Class A Amplifier: Class-A amplifiers are the simplest in design, and can be the most distortion-free of all amplifier classes. In class-a, the output devices are biased on all the time with a current large enough to produce maximum output swing. Class A amplifier are highly inefficient. Maximum theoretical efficiency of the class A amplifiers is as follows:

2 η = 1 I 4 V I CC CQ CQ = 5% Practical efficiency is much smaller than theoretical efficiency as input swing is usually limited to reduce distortion. Class A amplifiers are usually employed in low to moderate power range such as pre-amp stages. Class B Amplifier: Class-B amplifiers have superior efficiency. Theoretical maximum efficiency of the class B amplifier is as follows: π V η = 4 V p CC = 78% Practical efficiency is much smaller than theoretical efficiency as input swing is usually limited to avoid transistor saturation. Class B amplifiers usually have efficiency in the V range of 50% when peak output voltage is limited to. While Class B amplifier can achieve moderate efficiency, it creates distortion at the zerocrossing point of the waveform, making it unsuitable for precision amplifier applications. Class AB Amplifier: Class AB amplifiers eliminates cross over distortion of Class B amplifier by keeping the output transistor at the edge of conduction at zero input. As a result, output transistor can quickly turn on when input changes. Heat Sink: Heat sink must be adequate not exceed safe operating limit of the transistor. As an example, power dissipated in the transistor Pt = 6 watts, T jamx = 150 degree C and ambient T A = 5 degree C. 6θ θ JA JA + 5 < 150 o < 1 C/W But, with θ JC = 1.9 C/W θ JA θ = θ CA JC < 19 + θ o CA C/W = θ CA A heat-sink rated at less than 0 C/W are required. π CC

3 Design Example 1 (Class A Amplifier): Design a Class A amplifier that delivers 100 mw to an 8 ohm speaker. A simple DC coupled class A amplifier circuit is shown in Figure 4-. Power supply = 5V. RL 8 Out R1 150K IN CB VB Q1 MPSW45 Vin 1kHz 100uF R 100k Figure 4- P 1 L = I AC * RL = 100mW I = AC 158 ma Vout = V CC I AC *R L = 5 158e-3*8 = 3.8 V V B 1.4 V, I(R 1 ) > *I BQ Selected component values are shown on the Figure 4-. Simulated AC signal is 19 db and gain is almost constant up to 0 KHZ. Circuit simulation shows that, THD = 8% for input voltage of 00mV peak to peak input signal. Efficiency: Total DC power supplied by the source = V CC *I CC = 5 V * 153 ma = 765 mw Peak load current for input voltage of 100 mv, I L = 118 ma -> P L = ½*.118*.118*8 = 56 mw Efficiency = 7.4% Note that Class- A amplifier is at best 5% efficient. Transistor Power Handling Requirement: P T (max) = V CE (max)*i CE (max) = V CC / * V CC /(*R L ) = 780 mw

4 Design Example (Class B Amplifier): Design a Class B amplifier that delivers 50 mw to an 8 ohm speaker. A simple AC coupled class B amplifier circuit is shown in Figure 4-3. Power supply = =/- 1 V. Rb1 3k Q1 N3904 IN Vin 1kHz Rs 50 CB 10uF VB Rb 3k Q N3906 Out RL 8 Figure 4-3 V P 1 outpeak 1 L = = I loadpeak * RL = 50mW Iloadpeak = 11 ma, Voutpeak = 894 mv R L Therefore, required peak base emitter voltage, base current and base voltages for NPN transistor are as follows: Vbe = V T ln(ic/is) =.06*ln(11 ma/1.4e-14) =.77 V Vb(max) = Voutmax + Vbe = = V Ibmax = Icmax / β = 11 ma/150 =.4 ma Rb1(max) = (-Vbmax)/Ibmax = ( )/.4 ma = 4.6 kω Letting I Rb1 = 1.5 * Ibmax RB1 = 3 KΩ Since no DC current flow thru the transistor, when input is zero, we must set R B1 = R B since = - Selected component values are shown on the Figure 4-3. Circuit simulation shows that, THD = 4% for input voltage of 4 V peak to peak input signal. Design Example 3 (Class AB Amplifier): Design a Class AB amplifier that delivers 50 mw to an 8 ohm speaker. A simple AC coupled class AB amplifier circuit is shown in Figure 4-4. Power supply = =/- 1 V. Quiescent current flow at the output transistor for zero input is 4 ma.

5 Rb1 k Q1 N3904 IN Vin 1kHz Rs 50 CB 10uF VB D1 1N4148 D 1N4148 Out Q N3906 RL 8 Rb k Figure 4-4 V P 1 outpeak 1 L = = I loadpeak * RL = 50mW Iloadpeak = 11 ma, Voutpeak = 894 mv RL Therefore, required peak base emitter voltage, base current and base voltages for NPN transistor are as follows: Vbe(max) = V T ln(ic/is) =.06*ln(11 ma/1.4e-14) =.77 V Vb(max) = Voutmax + Vbe = = V Ibmax = Icmax / β = 11 ma/150 =.4 ma Rb1(max) = (-Vbmax)/Ibmax = ( )/.4 ma = 4.6 kω However, we Rb1 and Rb must supply the current required to establish the diode voltage necessary for quiescent current flow at the output transistor. For 4 ma of quiescent current, Vbe1 = V T ln(ic/is) =.06*ln(4 ma/1.4e-14) =.69 V Vbe = V T ln(ic/is) =.06*ln(4 ma/4e-14) =.66 V VBB = Vbe1 + Vbe = 1.35 V Therefore, Vd =.67 V Required diode current Id = Isd*e (Vbe/VT) = 3e-14*e (.67/.06) = 4.6 ma Letting I Rb1 = 5 ma RB1 =.3 KΩ Selected component values are shown on the Figure 4-4. Circuit simulation shows that, THD = 1.8% for input voltage of V peak to peak input signal.

6 Design Example 4 (Class AB Amplifier with Feedback): Design a Class AB amplifier that delivers 50 mw to an 8 ohm speaker. A simple AC coupled class AB amplifier circuit with feedback is shown in Figure 4-5. Power supply = =/- 1 V. Quiescent current flow at the output transistor for zero input is 4 ma. Rb1 k Q1 N3904 IN Rs 50 Vin 1kHz K Rp Inv R R K 5 6 U1 UA741 R3 50 VB D1 1N4148 D 1N4148 Rb k Q N3906 Out RL 8 10K Figure 4-5 Design follows the steps shown analysis 3 and op-amp in the feedback is added. As shown on the Figure 4-5, peak output voltage will rise to 1. V since V+ = V- = 1V. Circuit simulation shows that, THD =.05% for input voltage of V peak to peak input signal. Final schematic using only transistor is shown on Figure 4-6: Figure 4-6

7 Prelab: Analysis 1: Design and simulate a class A power amplifier that can deliver a 100 mw to an 8 Ω load from an peak input of 10 mv with a source impedance of 10 KΩ. Include required buffer and pre-amplifier stages as required. Set the low frequency cutoff at 0 Hz and gain should be constant up to 0 khz. Calculate efficiency and THD of your amplifier. Perform DC analysis to show voltage at each nodes and AC analysis to show the gain of the amplifier. Perform harmonic analysis to show THD of your amplifier. Use TF function to find DC input and output impedance. Analysis : Design and simulate a class B power amplifier that can deliver a 100 mw to an 8 Ω load from an peak input of 10 mv with a source impedance of 10 KΩ. Include required buffer and pre-amplifier stages as required. Set the low frequency cutoff at 0 Hz and gain should be constant up to 0 khz. Calculate efficiency and THD of your amplifier. Perform DC analysis to show voltage at each nodes and AC analysis to show the gain of the amplifier. Perform harmonic analysis to show THD of your amplifier. Use TF function to find DC input and output impedance. Analysis 3: Design and simulate a class AB power amplifier that can deliver a 100 mw to an 8 Ω load from an peak input of 10 mv with a source impedance of 10 KΩ. Set the output quiescent current to 4 ma. Include required buffer and pre-amplifier stages as required. Set the low frequency cutoff at 0 Hz and gain should be constant up to 0 khz. Calculate efficiency and THD of your amplifier. Perform DC analysis to show voltage at each nodes and AC analysis to show the gain of the amplifier. Perform harmonic analysis to show THD of your amplifier. Use TF function to find DC input and output impedance. Analysis 4: Design and simulate a class AB power amplifier with feedback that can deliver a 100 mw to an 8 Ω load from an peak input of 10 mv with a source impedance of 10 KΩ. Set the output quiescent current to 4 ma. Include required buffer and preamplifier stages as required. Set the low frequency cutoff at 0 Hz and gain should be constant up to 0 khz. Calculate efficiency and THD of your amplifier. Perform DC analysis to show voltage at each nodes and AC analysis to show the gain of the amplifier. Perform harmonic analysis to show THD of your amplifier. Use TF function to find DC input and output impedance. Pre-Lab Deliverables: 1) Submit your completed analysis, schematics and simulation results. ) Report must include the following: a) Simulated and calculated values of voltage gain, input impedance and output impedance.

8 Lab Experiments: Experiment 1: Construct amplifier designed in prelab analysis 1. Measure the small signal voltage gain at DC. Now, increase the frequency of the input signal until gain drops by -3 db. Demonstrate your result to TA. At rated output power, measure input voltage, output voltage, Rin and Rout. Experiment : Construct amplifier designed in prelab analysis 4. Measure the small signal voltage gain at DC. Now, increase the frequency of the input signal until gain drops by -3 db. Demonstrate your result to TA. At rated output power, measure input voltage, output voltage, Rin and Rout. Post-Lab Deliverables: 1) Submit your completed analysis, measured data and the lesson learned from performing this lab.