Lab. No. 5 1 3/7/2007 Cite as: Ron Roscoe, course materials for 6.101 Introductory Analog Electronics Laboratory, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. DEPARTMENT OF ELECTRICAL ENGINEERING AND COMPUTER SCIENCE MASSACHUSETTS INSTITUTE OF TECHNOLOGY CAMBRIDGE, MASSACHUSETTS 02139 Spring Term 2007 6.101 Introductory Analog Electronics Laboratory Laboratory No. 5 READING ASSIGNMENT Horowitz and Hill: The Art of Electronics, pp. 91-94, pp. 98-101, pp.286-288 Neamen 2nd Edition: pp. 499-502, pp. 484 [8.3.2] – 491 top Neamen 3rd Edition: pp. 593-596, pp. 578 [8.3.2] – 586 top Objective: Build a small audio power amp and play loud music! Study the differential amplifier, the heart of most op-amps. Play with the ubiquitous 555-timer chip. NOTE: Your lab write-up should clearly show your circuit configurations, your element values and your calculations in addition to measurement results. The grader will not spend time trying to sift through messy presentations to find out what you did. NOTE: THIS LAB REQUIRES A CHECKOFF FOR EXPERIMENT ONE. Sign up for your checkoff time on the list posted on the lab door. Experiment 1: SMALL POWER AMPLIFIER WITH LOCAL AND OVERALL FEEDBACK NOTE: Do not use the variable power supply terminals on your kit for this experiment. The output current from these terminals is limited to 500 mA, and this is too low for this amplifier when driving a loudspeaker. Use the +/− 12 volt fixed terminals next to the power switch. These terminals can deliver 1 ampere before they start to current-limit. Even better, you can use the new adjustable power supplies on the lab benches. There are nine of them. First, prove your circuit using the kit variable +/− 15 volt power supplies, then, hook it up to the adjustable supplies [set for +/- 15 volts], hook up a speaker, and enjoy! NOTE: These adjustable supplies have very large filter capacitors, and they take a while to discharge after the power switch is turned off, if your circuit doesn’t draw much current. You may want to monitor the supply voltages with your DMM, and you can also place a “bleeder” resistor across the power supplies’ terminals to increase the speed of the capacitor discharge [time constant]. Be careful to choose a resistor with a safe power rating! In this experiment you will build and examine the performance of the dc-coupled amplifier whose schematic outline is given in Figure 1. This amplifier is intended to have a gain of 20 dB with less distortion than that found in the amplifier of experiment 5 of Lab 4. • Note that the function of resistors RB1, RB2 and the 1N914 diodes is to bias the output transistors slightly on to eliminate crossover distortion. [Q 1.1 How does this work?] [Use 4.7 kΩ 1/4-watt resistors for RB1 and RB2, at least to start.] In order to ensure that the output transistors Q1 and Q2 are not damaged make sure that the maximum power that they will have to dissipate is limited to 1.0 watt. Note that this calculation will involve both the DC power dissipated due to static quiescent current, plus each transistor sees one-half of the output sine wave. Q 1.2 What is the smallest value of load resistance RL that can be connected to the output of the amplifier toensure that this power dissipation limit is not exceeded? • Note also that the requirement for relatively high input impedance may conflict with the requirement for low DC offset voltage. Q 1.3 Explain this conflict in your write-up. You may choose to use either the inverting or non-inverting configuration for the op-amp. You may choose to use any op-amp available in your kit or at the instrument room window or in the lab drawers. Q 1.4 What is the function of the emitter resistors RE? Q 1.5 Explain why the output impedance of the amplifier is not at least 5.6 Ω. [These resistors are in your parts kit and are the larger 1/2-watt size.] 1. Design your amplifier to meet the following specifications: • Low frequency cutoff [-3dB point] 5 Hz. [To take advantage of extended low frequency response from CD’s] ≤C+vin_RF2N22192N2905D2 1N914D1 IN914RE=5.6Ω1/2 wattRB1+12 VRL-12 V+12 V+vout_RB2-12 VRE=5.6Ω1/2 watt+1223476-12-+?opamp[FromPreamplifier]1N40011N4001 Figure 1: Amplifier circuit for experiment 1. Lab. No. 5 2 3/7/2007 Cite as: Ron Roscoe, course materials for 6.101 Introductory Analog Electronics Laboratory, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].• Input impedance (as seen by preamplifier output) ≥ 20 kΩ. [To prevent loading down of preamps with relatively high source impedance] • Output stage offset voltage < 50 mV. [To keep DC from causing the speaker coil to heat and the cone to be offset from its center position between the magnet pole pieces.] • High frequency cutoff [-3dB] point = 25 kHz minimum. [To take advantage of extended high frequency response from DVD audio discs running at 48 kHz sampling rate; also some high-end sound cards.] • Mid-frequency [1000 Hz] voltage gain of +20 dB. • Output stage quiescent bias current between 1 and 10 mA [class AB or B operation]. [Measure the DC voltage drop across one or both emitter resistors.] Since we gave you the biasing resistor initial values, there is not much to design. However, you should measure the bias current in the output stages to make sure it is less than 10 mA. Output device βF variations will affect the value of bias current, and it will be hard to achieve a stable bias current if the βF’s of both output devices aren’t about the same. If your output device bias current is greater than 10 mA with no signal input, turn off your +/- 12-volt supplies immediately. Increase the value of both RB1 and RB2 to the same next highest standard value and recheck the output bias. Keep increasing the value of this pair of resistors until the bias current drops to the limits given above. Q 1.6 How does changing the value of these two resistors control the output stage bias current? [Note: It is also possible to increase these two resistors to the point where not enough DC bias current is supplied to the base of one of the output transistors, especially if it has low βF. It helps if your output devices have similar βF’s. From an AC point of view, large values of