lecture 3: diodes and transistors - mit … 3: diodes and transistors instructor: hong ma sept. 17,...
TRANSCRIPT
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2.996/6.971 Biomedical Devices Design Laboratory
Lecture 3: Diodes and Transistors
Instructor: Hong MaSept. 17, 2007
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Diode Behavior• Forward bias
– Exponential behavior
• Reverse bias– Breakdown– Controlled breakdown Zeners
0.7 V0V
-VZ
Breakdown
VZ = Zener knee voltage
V
I
Compressedscale
( ) 1t
VV
SI V I e⎛ ⎞
= −⎜ ⎟⎜ ⎟⎝ ⎠
tkTVQ
=
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Types of Diode
• Silicon diode (0.7V turn-on)• Schottky diode (0.3V turn-on)• LED (Light-Emitting Diode) (0.7-5V)• Photodiode• Zener• Transient Voltage Suppressor
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Silicon Diode• 0.7V turn-on• Important specs:
– Maximum forward current– Reverse leakage current– Reverse breakdown
voltage• Typical parts:
Part # IF, max IR VR, max Cost
1N914 200mA 25nA at 20V 100
50V
~$0.007
1N4001 1A 5µA at 50V ~$0.02
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Schottky Diode
• Metal-semiconductor junction• ~0.3V turn-on• Often used in power applications• Fast switching – no reverse recovery time• Limitation: reverse leakage current is higher
– New SiC Schottky diodes have lower reverse leakage
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Reverse Recovery Time Test Jig
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Reverse Recovery Test Results
• Device tested: 2N4004 diode
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Light Emitting Diode (LED)• Turn-on voltage from 0.7V to 5V• ~5 years ago: blue and white LEDs• Recently: high power LEDs for lighting• Need to limit current
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LEDs in Parallel
VS = 3.3V
R
VS = 3.3V
R R R
• IS is strongly dependent on temp.
• Resistance decreases with increasing temperature
• “Power Hogging”
( ) 1t
VV
SI V I e⎛ ⎞
= −⎜ ⎟⎜ ⎟⎝ ⎠
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Photodiode• Photons generate electron-hole pairs• Apply reverse bias voltage to increase sensitivity• Key specifications:
– Sensitivity (short-circuit current for a given light level)– Spectral response– Reverse breakdown voltage– Dark current
VBIAS
R
PD LOAD
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Zeners
• Utilize reverse breakdown mechanism• Sharper transition than forward biased diode• Knee Voltages range from 1.8V to 200V to kV• Reverse leakage current is higher• Applications
– Limiter– Voltage reference
0.7 V0V
-VZ
Breakdown
VZ = Zener knee voltage
V
I
Compressedscale
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Transient Voltage Suppressor
• TVS or TransOrb• Place in parallel with power supply• Absorbs over-voltage• Unipolar or bipolar• Typical specs:
– Absorb 1000W for 1ms– Breakdown voltage (VBR)– Standoff voltage (~0.9VBR)– Vsupply <= Vstandoff
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Diode Application: Preventing Inductive Kickback
• Instantaneous current switching produces very large voltages!
I
• From Maxwell’s equations:dIV Ldt
=−
in
S S
in
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Voltage References
• With forward biased diode• With Zener• Temperature compensated reference – bandgap reference
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Peak Detector
• AKA: Envelop detector
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Rectifier
Half-wave Full-wave
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Diode Clamper
• Zener has bad leakage• Don’t forget about failure mode
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Diode Tx-Rx Switch
• Mylar balloon used both as a speaker and a microphone• D3 and D4 limit the voltage at the input of U2
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Transistors
(as switches)
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BJT
• Three-terminal device: base, emitter, collector• Two types: NPN and PNP• IC=βIB, β ≈100
Typical parts: 2N3904 (NPN), 2N3906 (PNP)
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BJT as a Switch• Need a resistor to limit base current• Many IC’s leave RL unconnected – open collector output• Emitter follower: output tracks input with 0.7V offset
Common-Emitter Emitter follower
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Problems with BJTs
• Negative temperature coefficient• Parallel BJTs: Power hogging• Large BJTs: secondary breakdown
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MOSFET
• Four-terminal device: gate, source, drain, and body• N-type and P-type• Negative temperature coefficient can be parallelized• Bidirectional - so long as body-drain diode remain reverse biased
Gate
Source
Drain
Body Gate
Source
Drain
Gate
Source
Drain
BodyGate
Source
Drain
N-channel MOSFET
P-channel MOSFET
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MOSFET as Switches
Important Specs• Gate capacitance (CG)
– Hundreds of pF
• On resistance (RDS(on))– RDS N-ch < RDS P-ch– Use N-channel whenever
possible
• Threshold voltage (VTH)– As low as 1.8V
• Drain-source breakdown voltage (VDSS)
RLOAD
VS
VIN RLOAD
VS
VIN
Low-side Switch High-side Switch
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Gate Drivers• Efficiency dependent on transition time• Low-side driver – low impedance drive• High-side driver – charge pump to create gate voltage
above the source voltage
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H-Bridges
Key Issue• Shoot through
current
LMD18200• High, low gate
drivers• Current sensing• Current limiting• Thermal
shutdown
Reprinted with permission of the National Semiconductor Corporation.
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CMOS Analog Switches
Key Issues for Analog Circuits• Signal range• Switch on-resistance• Resistance matching
VSSignal in Signal out
Control
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Logic Gates
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Logic Family Conversion TableTOFROM TTL
HCTACT
HCAC
HC, AC
@3.3VNMOS
LSI
4000B,74C@5V
4000B,74C
@10VTTL OK OK A OK OK A B
HCT, ACT OK OK OK NO OK OK BHC, AC OK OK OK NO OK OK B
HC, AC @3.3V OK OK NO OK OK B BNMOS, LSI OK OK A OK OK A B4000B, 74C
@5V OKa OK OK NO OK OK B
4000B, 74C@10V C C C C C C OK
(a) with limited fanout.A - pullup to +5V, or use HCT as interface.B - use i)OC pullup to +10V, or ii)40109, 14504, or LTC1045 level translator.C - use 74C901/2, 4049/50, 14504, or LTC1045 level translator.