Transistor Digital Circuits
Transistor – voltage-controlled current source
n-type p-type
•n -channelMOSFET
•npn BJT
•p-channel MOSFET
• pnp BJT
Non-linear controlled sources: square - MOSFET
exponential – BJT
Transistor - operating principle and regionsa brief remembrance
Treshold voltage
Using Tn transistor
Output quantities: IO, VO
transfer characteristics:
IO(VCT), VO(VCT)
IT = IO
Transfer Characteristics, Tn
• VCT<VThn, Tn – off, IO=IT=0
• VCT>VThn, Tn – on, IO=IT>0
VPS=RIO+VO ; VO= VPS- RIO
VCT , IO , VO
VO,min=0 R
VI PS
Oex
• Two extreme regions, passive regions:
- cutoff (off) IO=0; VO>0; ideal switch in off state
- extreme conduction (exc) IO=IOex; VO=0; ideal switch in
the on state
If VCT < VThn or VCT > VCTex switching transistor
• An intermediary region, active forward region aF
VCT(VThn; VCTex) – permanent conduction (amplifier)
Tn
Symbols of the transistors
G - gate or grid D - drainS - source
n-type p-type
n-channel enhancement-type MOSFET
p-channel enhancement-type MOSFET
B – baseC – collector E – emitter
npn BJT pnp BJT
Controlled switch model
vCT >VCTex; T- (exc); iOT >0; vOT0
vCT <VThn; T- (off); iOT = 0; vOT=VPS
vCT >VThp ; T- (off); iOT =0; vOT=VPS
vCT <VCTex ; T- (exc) ; iOT >0 ; vOT0
The controlled switches are complementary
TpTn
Switching Transistor
MOSFET DIGITAL CIRCUITS
➢ Ideal controlled switch model
➢ Implementation with MOSFET
Logic inverter
0V logic 0
VPS logic 1
vA Tn vy
VCoL = 0 (off) VPS
VCoH = VPS (on) 0
logic 0 - 0V
logic 1 - VPS
Tn and R Tp and R
Logic inverter
Critical analysis of the logic inverter
With Tn and R
Disadvantage elimination:
R as small as possible, ideal R→0;
Critical analysis of the inverter
withTn and R
Solution: replacement of R with a controlled switch
Disadvantage elimination:
R as small as possible, ideal R→0; R as large as possible, ideal R→∞
Two possible solutions:✓ complementary switches
✓ same input
TTLCMOS
✓ complementary inputs
✓ same switch
MOS transitors BJT transitors and R
CMOS Logic Inverter
vGSn=vI
vGSp=vI-VDD
Logic function NAND, NOR
a) What is the truth table?b) What are the states of thetransistors?c) How can we eliminate the disadvantages due to the presence of R ?
Problems
Logic function NAND, NOR
CMOS NAND
a) What is the truth table?b) What are the states of alltransistors?
Problem
CMOS NAND
CMOS NOR CMOS AND
20
Transfer characteristicof the CMOS inverter
Ideal (input) Real
Noise margins
maxmax
minmin
OLILL
IHOHH
VVNM
VVNM
Voltage levels and
noise margins for
CMOS logic family
supplied at +5V
V1V5.0V5.1 LNM
V1V5.3V5.4 HNM
BJT DIGITAL CIRCUITS
➢ RTL technology
➢ TTL technology
Switching BJT, digitalcircuit
Resistor to limit the base current
B
Co
B
BECoB
R
v
R
Vvi
7.0
Cex
Bsat
ii
C
Al
C
CEsatAlCex
R
V
R
vVi
2.0
T – (off), if vCo< 0.6V
T – (on), if iB > iBsat
Problem
V5;150
;k2;k100
Al
CB
V
RR
a) What is the state of T if vCo=0V?
What is the vCE value?
b) What is the state of T if vCo=5V?
What is the vCE value?
c) What is the vCo range to keep T off?
What about to keep it in saturation?
d) If 0V => logic 0 and 5V => logic 1,
what is the logic function of the
circuit if ]V}[5;0{Cov
Bipolar Digital Circuits
• Inverter RTL Technology
• NOR
a) What is the schematic for the 2-input NAND gate in a RTL technology?b) What is the truth table?c) What are the states of the transistors?
Problem
TTL: Transistor-Transistor Logic
• Logic Inverter
• from technological reasons in TTL integrated circuits the use of only npn type transistors is preferred
• identical transistors but complementary control
vI1, vI2 ?
Standard TTL gate OPTIONAL
Voltage levels and noise margins for TTL family
Supply only with 5V
TTL sub-families with improved performances
Schottky transistor Schottky diode: a metal to semiconductor junction, in conduction 0,5V
vBE=0,8V; vBC=0,5V; vCE=0,3V
• Schottky transistor doesn't enter saturation mode
• increases the commutation speed
Simplified structure of a 2-input NAND gate from the Low-power Schottky family
Analog Switch (AS)
The AS is an electronic circuit that allows or blocks an input signal to pass to the output of the circuit according to a control signal.
vCo - two levels: VCoL<VTh and VCoH>VCTex
vCo=VCoL ; Tn-(off) ; vO=0
vCo=VCoH ; Tn- (exc) ; vO= vI
no power consumption
VThpVThn
Bidirectional AS with two complementary
controlled switches
vCo=VCoL ; AS - (off) ;
vCo=VCoH ; AS - (on) ;
C=0; AS - off; vO=0
C=1; AS - on; vO=vI
IC 4066 –4 transmission gates; supplied at 10V, ron=150Ω
DG400 (Siliconics) ron=20Ω
VCoH=VDD; VCoL=VSS
vI (VSS; VDD)
AS
AS – CMOS implementationbidirectional transffer
Application: three-channel MUX
What is the output voltage in each of the three situations? Justify the answer.
Application: three-channel MUX
