Electronic Circuits LaboratoryEE462GLab #2

Diodes, Transfer Characteristics, and Clipping Circuits

InstrumentationThis lab requires:

Function Generator and Oscilloscope (as in Lab 1)Tektronix’s PS 280 DC Triple output (one fixed 5V / 3A and 2 variable 0 to 30V / 2A) Power Supply

http://www.tek.com/site/ps/0,,40-10199-INTRO_EN,00.html

What advantages are there to a chassis ground over an earth ground for a power supply?

chassis ground

Instrumentation

The 2 variable outputs of the power supply will be used for batteries (DC series sources) in the experiments’ circuits:

10kΩ Vs Vout

(C)

V1

+

-

V2

Minus Terminal

Plus Terminal

LET CHASSIS GROUND FLOAT!

Instrumentation

This lab requires:Tektronix’s Curve Tracer 370B.For the data sheet you must save the displayed waveform to a floppy disk and paste (if jpeg) or load and plot (if CSV) using excel or Matlab.Make special note of the kneeof the curve. Make sure scale allows the knee to be clearly seen so forward threshold voltage can be estimated with good precision.

http://www.tek.com/site/ps/0,,76-10757-SPECS_EN,00.html

InstrumentationTektronix’s Curve Tracer 370B operation. For diode, no connection is made to the step generator.

Figure taken from Tektronix User Manual 370B Programmable Curve Tracer070-A838-50 http://www.tek.com/site/ps/0,,76-10757-SPECS_EN,00.html

InstrumentationExample displays for diode with difference collector settings

AC collector sweep Positive DC collector sweep0,0

0,0

Transfer CharacteristicsA relation describing the amplitude input-output relationship of a device.

A sufficient characterization in most engineering problems for instantaneous systems (present output does not depend on future or previous values). These systems are sometimes referred to as memoryless, and are typical of systems with no energy storage elements.

Sketch the V-I transfer characteristics of a resistor R

Transfer Characteristics

Diode modelA semiconductor device with direction dependent current flow. Volt-Ampere Characterization of a diode.

Diode

http://www.wodonga.tafe.edu.au/eemo/ne178/tut2_1.htm

Id

Anode Cathode

+ Vd -The band on the component usually denotes the cathode terminal

Forward BiasReverse Bias

Ideal approximation:Short Circuit

Ideal approximation:Open Circuit

Knee

VD

ID

VZ

Ideal Diode ModelDiode junction acts as a short circuit for forward bias (VD > 0 (anode positive)).

Diode junction acts as an open circuit for reverse bias (VD < 0 (anode negative)).

Near-Ideal Diode ModelAdd a 0.7 voltage source for the forward offset voltage in series ideal diode with same polarity as the forward bias.

Ideal Diode Model

Anode Cathode

+ Vd = 0 -Id

Diode On

Anode Cathode

+ Vd -Id 0.7 V

Anode Cathode

- Vd +Id = 0

Diode Off

Analysis of Diode CircuitMethods of Analysis:

Load-line analysisDiode-State AnalysisSPICE

Diode-State AnalysisDetermine the on-off state of each diode by guessing a combination of on-off states.For on-diodes assume a forward biased current direction (0 volts) and for off-diodes assume a negative anode voltage (0 amps) and then analyze circuit. If guessed voltage or current direction is inconsistent with thecircuit analysis, guess another state combination until all are consistent with circuit analysis.

Problem:Use near-ideal model to determine the state of diode D1 when V1=10V. (First guess diode is off).Find range of V1 values such that D1 is off.Assume output is current I3 and input is V1, analytically find the transfer characteristics of this circuit.Also Solve via SPICE and plot:

Example

V1

R1 1k

D1 R

2 1k

R3

1k

I3

In SPICEbuild circuit and insert diode (use default characteristics)Place ammeter at outputSet source to DC (value is not important)In “simulation set up” select (Dual) Parameter DC Sweep. Select V1 as parameter to sweep and indicate range and increment.Ask for graph output and run!

SPICE Set Up and Result

V1

R1 1k

D1 R

2 1k

R3

1k

VAm1

V1 +900.000m I(VAM1) +352.554n D(V1) 0.0

D(I(VAM1)) 0.0

la2ex1-DC Transfer-2 V1

(Amp)

0.0

+1.000m

-4.000 -2.000 0.0 +2.000 +4.000

Example Sketch output waveform if input is a 5Vrms sinusoid of frequency 1kHz:

Determine transfer characteristics from input through waveform output.Map input waveform through the transfer characteristics.Note that when diode is on:

kVI

3411

2.−

=

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

x 10-3

-8

-6

-4

-2

0

2

4

6

8Input Voltage

seconds

Vol

ts

1.4 Volts

7.07 Volts

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

x 10-3

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2x 10

-3 Output Current

seconds

Am

ps

Example with SPICEFind output waveform if input is a 5Vrms sinusoid of frequency 1kHz:

Build circuit in SPICESet source to sinusoid, but do not check AC box. Go to transient parameters tab and select appropriate amplitude (7.0711) and frequency (1000), the rest can be zero.Set up simulation parameters, select “Parameter sweep of transient,” select source with start and stop magnitudes to same value and step size to a non zero number (you don’t want it to sweep, and it will not use these number since AC box was not check in source set up).The set up transient parameters to appropriate start and stop time and increment.The run simulation. If too many waveforms appear, make start and stop time shorter.

Example with SPICEFind output waveform if input is a 5Vrms sinusoid of frequency 1kHz:

TIME -1.000 I(VAM1) -1.000 D(TIME) -1.000

D(I(VAM1)) -2.753

la2ex2--6 time (s)

(Amp)

0.0

+1.000m

+2.000m

0.0 +2.000m +4.000m +6.000m +8.000m +10.000m

Problem:Given Vs is a 5 Vrms 1kHz sinusoid, find Vout with and without a 5.1kΩload

Analysis:Use .7 V forward offset voltage and ideal diode approximation. Can derive transfer characteristics or reason out directly.

Lab Procedure:Describe function generator settings, oscilloscope probe placement, and settings. Record resulting waveforms for at least one period but not more than 5.

Test Circuit A

10kΩVs Vout

(A)

+

-

Test Circuit B

Problem:Given Vs is a 5 Vrms 1kHz sinusoid, find Vout with and without a 5.1kΩload

Analysis:Use .7 V forward offset voltage and ideal diode approximation. Can derive transfer characteristics or reason out directly.

Lab Procedure:Describe function generator settings, power supply attachment and grounding issues, oscilloscope probe placement, and settings. Record resulting waveforms for at least one period but not more than 5.

10kΩVs Vout

(B)

V1

+

-

Test Circuit CProblem:

Given Vs is a 5 Vrms 1kHz sinusoid, find Vout with and without a 5.1kΩload

Analysis:Use .7 V forward offset voltage and ideal diode approximation. Can derive transfer characteristics or reason out directly.

Lab Procedure:Describe function generator settings, power supply attachment and grounding issues, oscilloscope probe placement, and settings. Record resulting waveforms for at least one period but not more than 5.

10kΩVs Vout

(C)

V1

+

-

V2

Test Circuit DProblem:

Similar to Circuit B: Does order of diode and DC source change output?

Analysis:Does this affect the KVL or KCL equation?

Lab Procedure:Power supply is isolated from earth ground; however stray capacitance to ground exists that practically affects the output. How will a capacitive path to ground at the 2 different points in Circuits B and D affect the output?

10kΩVs Vout

(D)

V1

+

-

Power:Recall instantaneous power is a product of the voltage and current waveforms. So for source power find Vs and Iswaveforms for one period and multiply point per point:

Recall average power is the integral of the instantaneous power divided by the period. For sinusoid waveforms it reduces to:

Is power absorbed by an ideal diode?Analysis:

A Thévenin equivalent circuit around the nonlinear component in the circuit may be helpful in explaining changes due to load resistances.

Other Notes

)()()( titvtp ss=

)cos( ivrmsrmsav IVP θθ −=

Final NotesData Sheet

Include a of V-I diode transfer characteristic.All general comments from previous labs apply.

Procedure DescriptionAddress all questions asked in the lab assignment sheet and lecture that pertain to the procedure of making a measurement and getting the requested quantity.Grounding issues are discussed in this section if they complicate probes placement.Discuss how the forward offset voltage is determined from the measurements made of the V-I diode transfer characteristics.All general comments from previous labs apply.

Final NotesDiscussion of Results

Respond to all bold face comments and questions in the lab assignment in this section.Explain why the waveforms appear as they do from diode theory and measurement issues. Make comparisons between measured values when appropriate (i.e. if something changes in the circuit, how does this affect the output!)

ConclusionBriefly sum up results and indicate what was learned through doing this experiment. Respond directly to the lab objectives.

GeneralNumber all figures, tables, and equations sequentially (learn how to use equation editors), and avoid first person voice.Describe all figures and table in text. What are the important features the reader should see?