Name: Date:

1 Spring 2014 6.101 Quiz

DEPARTMENT OF ELECTRICAL ENGINEERING AND COMPUTER SCIENCE

MASSACHUSETTS INSTITUTE OF TECHNOLOGY CAMBRIDGE, MASSACHUSETTS 02139

Spring 2015 Quiz

6.101 Introductory Analog Electronics Laboratory

NOTE: SHOW ALL CALCULATIONS FOR ALL ANSWERS BUT THE MOST OBVIOUS! [This will allow for partial credit for wrong answers.]

Please look through the whole quiz before beginning. There are lots of questions, but most of them are very easy. It’s always good test-taking procedure to look over the whole quiz before deciding where to start, and so you can plan your time. To receive full credit values must also indicate units eg; volts, mv, ma, uF, pF, sec, ms, ohm, mohm, mHz, etc 1 sheet double sided handwritten notes only. If needed, you may use these values for constants

VT (thermal voltage at 25 deg C) = kT/q = 26mv

pi = 3.14, sqrt(2) = 1.41

Problem 1 /18 Problem 2 /12 Problem 3 /7 Problem 4 /12 Problem 5 /4 Problem 6 /8 Problem 7 /12 Problem 8 /7 Problem 9 /20

Total

2 Spring 2015 6.101

? pF RPara240 Hto radio first stage

EM radiation: 0.530 - 1.710 MHz

Problem 1 [18 points ] Most radios in the late 1930’s, and the 1940’s through 1960’s used loop antennas to receive the AM signal. These antennae were about 1/3 the size of the transmitting antenna we used in Lab 1, and were usually wound flat on the inside back panel of the radio. These coils respond primarily to the M part of the EM radiation, and thus a current is induced in the coil. To get the largest input voltage to the radio, we like to resonate the coil with a tunable variable capacitor, which is actually inside the radio. 1a. What should be the range [in pF] that the tuning capacitor must cover to tune the whole AM band [0.530-1.710MHz]? From: pF To: pF 1b. If we manipulate the bandwidth using this circuit, we can help to reduce interference from adjacent signals, just as we did in Lab 1 with tuning the ferrite loopstick. What total value of parallel resistance [Rpara] do we need to keep the bandwidth to 10 kHz at 1 MHz? [Rpara includes resistance at the input to the 1st stage.] Rpara = 1c. What will the bandwidth be at the extremes of the AM band using the above resistance? 530 kHz 1.71 MHz 1d. What is the Q of this tank circuit at 1 MHz? Q =

3 Spring 2015 6.101

Problem2 [12 points]

2a. In the figure above, assume that each identical diode has a VF of 0.6 volts. If the current source is adjusted to produce a DC bias current through the diodes of 1.0 mA, then what is the small-signal [incremental] diode resistance for each diode? [The temperature is 25o C.] Ω 2b. What will be the AC component of the output voltage of this circuit? 2c. What will be the DC component of the output voltage of this circuit? 2c. Suppose the bias current is reduced to 500 µA. What is the AC output voltage?

100 VOUT

VIN = 100 mV,1000 Hz

D1 D2 D3 D4D5

DCcurrentsource

4 Spring 2015 6.101

Problem 3 [7 points] The following schematic represents a basic Zener diode regulator. It’s driven by a simple diode rectifier with capacitor filter, which have been replaced by an average DC voltage source in series with an AC ripple generator generating a sawtooth waveform. Assuming that we want to leave a minimum DC current of 1 mA through the Zener to keep it in reverse breakdown, what is the maximum DC current that can pass through the load?

R=100 ILOAD

VZ =12.0 V

1.0 V p-p

19.0 V Average

+

VLOAD

-

RL+

5 Spring 2015 6.101

Question 4 [12 points] The schematic above depicts an output stage using high-current transistors to drive 2 amperes DC into an 8Ω resistive load. The βF of the output devices is 100.

4a. What is the voltage across the 8Ω resistive load? VOUT =

4b. What is the voltage VCE across the npn transistor? VCE =

4c. What is the power dissipation in the npn transistor? PQ =

4d. Assuming that the saturation voltage of the npn transistor is 0.4V, what is the maximum current that can be flowing through the transistor to the load assuming an appropriate Vin?

IMAX =

6 Spring 2015 6.101

Problem 5 [4 points] Design a current source one using a 2N5462 PJFET. The output characteristic for for

the 2N5462 is shown below. Supply voltage is +15V. Assume that for the

PJFET that VDS = about –6 volts. What is the resistor value need to an

IOUT= 400uA?

J FET Current Source R = _______________

Find this value!

+15 V

IOUT = 400 A

2N5462

G

D

S

[Load goes here.]

7 Spring 2015 6.101

Problem 6 [8 points]

Based on the diagram above, design a circuit by adding resistors and wires to the op-amp so that the

output can be switched from a gain of +1 to a gain of –1. Assume that the op-amp has zero ac and

dc input current. Each input should have an input impedance of 100 kΩ. Draw the circuit below. Be

sure to label the positions for “Non-Inv” and “Inv”.

-

++15

LF356

3

2

4

7

6

vout-15

To output ofprevious stage.

Non-Inv

Inv

8 Spring 2015 6.101

Problem 7 [12 points]

Specifications for the LF356 are on the next page. [Frequency is on a log scale.] This is a JFET input op-amp. 7a What is the color code for the 47k resistor? __________________ 7b. Choose a value for R1 that will give a DC gain of 40 dB. __________________ 7c. At some high frequency, the gain will fall to 0dB What is the approximate value of this frequency? __________________ 7d. What is the gain at approximately 50 kHz? __________________

2

3

4

7

-

+15

LF356 6

vin

+

_

-1575k

R1

v-

47k

7.5k

71 nF

+

vout

_

+

9 Spring 2015 6.101

10 Spring 2015 6.101

Problem 8 [7 points] The Sallen-Key filter is a second order filter configured as a low pass filter (LPF). It requires an op-amp. In order to reduce cost, you replace the Sallen-Key filter with just a resistor and capacitor.

Compare the performance difference between the Sallen-Key filter and the resistor capacitor combination by plotting the magnitude (dB) vs frequency. Assume both filters have cutoff frequencies at 100hz. At 1hz, the magnitude is 20db

RV1

V2C

11 Spring 2015 6.101

Problem 9 [20 points] A student is building a simple digital clock. For input, the student is using a half wave rectified 60 Hz AC input as the clock. Unfortunately the rectified signal has 200mv noise pulses everywhere along the AC waveform. When fed directly into a comparator, the output contains more than 60 clock pulses per second as a result of the noise. The 60 Hz waveform crosses the reference voltage multiple times because of the 200mv noise. The student implements a circuit with a Schmitt trigger (“Your design”) using a LM6132 with the thresholds greater than 200mv to fix the problem. A LM7805 voltage regulator IC is used to provide a 5V output to power the digital clock circuit (modeled as a resistor) as well as to supply +5V for the LM6132 in “Your design”.

9a. Assuming the 1N4001 has a 0.7V drop, what is the peak voltage (2 decimal places) across the capacitor? [3 points] _____________________ 9b. Assuming a 10ma current going into pin 1 of the 7805, what is the ripple voltage for V1? [3 points] ______________________

12 Spring 2015 6.101

9c Implement your Schmitt trigger design with a LM6132. You have available 1N914/1N4148, resistors of any value, and capacitors. The output V3, should be a 60hz square wave. The duty cycle is not important. For this application, assume the LM6132 output is truly rail to rail and the slew rate of the op-amp is acceptable. The relevant page from the LM6132 datasheet is attached. The +5V is used to power the LM6132. Clearly label your all your component values. [8 points]

9d You may assume the LM6132 has rail to rail output. What is your high threshold voltage? [3 points] ______________________ What is your low threshold voltage? [3 points] ______________________

13 Spring 2015 6.101