# Voltage Transfer Characteristic for TTL

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Voltage Transfer Characteristic for TTL
VCC = 5 V Summary of transfer characteristic When the input is low, The current iR goes out the E of Q1 So Q2 and Q3 get no base current and are off So the output is high When the input increases, Some of iR gets directed into the B of Q2, so Q2 gets some base current and comes on in active mode C current of Q2 increases, so IR drop across R1 increases and output voltage drops (B to C) As Q2 comes on, it provides base current to Q3 and Q3 come on in active mode (C). When the input increases further, More of iR gets directed into the C of Q1, so Q2 gets more base current and moves into the saturation mode (C to D) Q2 provides more base current to Q3 and then Q3 moves into saturation mode (D). Further increases in the input direct all of iR into the C of Q1, so Q2 gets even more base current and moves deeper into the saturation mode (D to E). Similarly, Q3 moves deeper into saturation. R3 = 0.13 K iR + vi _ Q1 in saturation, Q2 and Q3 off vo Q2 comes on A B Q3 comes on, Q2 heads towards saturation VOH =3.7 V II I C III 2.7 V Q3 reaches saturation, Q2 already in saturation Q1 comes out of saturation D VOL= 0.1 V IV E 0.5V 1.2V 1.4V V VIL VIH vi Bipolar Digital Pt. 4

Voltage Transfer Characteristic for TTL
VCC = 5 V Noise Margins Noise Margin for low state NML = VIL -VOL VOL = low output voltage for typical high input voltage = 0.1 V VIL= maximum input voltage recognized as a low input = 0.5 V NML = VIL-VOL =0.5 V V = 0.4 V Noise Margin for high state NMH = VOH -VIH VOH = high output voltage for typical low input voltage = 3.7 V VIH= minimum input voltage recognized as a high input = 1.4 V NMH = VOH -VIH = 3.7 V - 1.4V = 2.3 V Noise margins are very unequal for this technology. iR + vi vo _ vo A B VOH =3.7 V II I C III IV 2.7 V NMH NML D E VOL= 0.1 V 0.5V 1.2V 1.4V V VIL VIH vi Bipolar Digital Pt. 4

Comparison of Simplified TTL and TTL
VCC = 5V VCC = 5V RC = 1.6K R=4K + vo Noise Margin (Low state) NML = VOL - VIL = 0.6V V = 0.5 V Noise Margin (High state) NMH = VOH - VIH = 5 V V = 4.3 V Noise Margin (Low state) NML = VOL - VIL = 0.5V V = 0.4 V Noise Margin (High state) NMH = VOH - VIH = 3.7 V – 1.4 V = 2.3 V vo vo A B A B 3.7 V II VCC = 5V I C I II III III IV 2.7 V NML 0.2V 0.1V C NMH D D NMH NML VOL= 0.1 V E vi 0.5V 1.2V 1.4V V VIL VIH vi 0.6 V 0.7V V VOL= 0.1 V Bipolar Digital Pt. 4

Voltage Transfer Characteristic for TTL
VCC = 5 V What is the function of Q4? Q4 is weakly on but producing little current when the output is low. This helps to minimize power dissipation since Q3 is on and in saturation so ready to conduct current. Q4 is weakly on when the output is high. This is because the following gate has a reverse biased E junction for Q1 and so draws almost no current. The reason Q4 is included in the circuit is to provide a large current to ensure a fast transition time when the output is going from low to high so tPLH is small. At all other times we want Q4 off (or only weakly on) to minimize power dissipation. A simple resistor in place of Q4 gives a very long rise time ~ 100’s nsec, as we saw for the RTL inverter, so the use of Q4 and the diode D is an improvement. iR + vi vo _ vo A B VOH =3.7 V II I C III 2.7 V D IV E VOL= 0.1 V 0.5V 1.2V 1.4V V VIL VIH vi Bipolar Digital Pt. 4

Transistor - Transistor Logic (TTL)
Fan – Out Capability What is the ability (fan-out) of the TTL logic to drive simultaneously a number of subsequent inverters? Fan-out = NMax = maximum number of subsequent inverters that can be simultaneously driven (connected to the output). For the output high, i.e. vo = 3.7 V, the output is connected to a reverse biased E junction for Q1 for each subsequent inverter, so current load is very small. However, for output low, i.e. vo = 0.1 V E junction of each Q1 forward biased, so So this adds to the collector current of Q3 so iC3 = N iE1 = N (1.0 mA) Fan-out limit = maximum value of N In saturation, iC3 < β iB3 In active, iC3 = β iB3 So limit is when Q3 comes out of saturation into active mode and iC3 = β iB3. VCC = 5 V R3 = 0.13 K iRi = 1 mA p n iC3 iE1 =1 mA + + VCE3 + _ vo _ sat Recall, we found when the output was low Fan-out limt Bipolar Digital Pt. 4

TTL Propagation Delay vo Output going high + vo C t tPLH
iC4 iB4 VOH= 3.7V iR Goes on 1.9V iE4 iCap Goes off Goes low + vo Goes high C VCE,sat = 0.1 V t Goes off tPLH Output going high Input goes low Transistors Q2 and Q3 turn off (cutoff) due to low input to gate. Large charging current flows through Q4 to charge up C. Q4 called pull-up transistor Charging time is small ~ 1 nsec tPLH is the time it takes the output to rise from VOL = VCE,sat = 0.1 V to 1/2(VOH + VOL) = ½( ) V = 1.9 V Bipolar Digital Pt. 4

TTL Propagation Delay Output going high vo t vo tPLH C
Charging current for capacitor is emitter current of Q4. At outset, vo= VCE,sat = 0.1 V, so iB4 and VCE4 are initially large Charging current iCap ≈ iE4 is large since for  = 10 As vo rises, iB4 decreases, so iE4 = iCap decreases, but Q4 stays in active mode. At vo = 1.9V So iE4 = iC has decreased to So capacitor charging current is not constant and calculation of tPLH is more difficult. iC4 iB4 VOH= 3.7V 1.9V iE4 iCap VOH= 0.1V t vo tPLH C iC4 Goes high R S P vCE4 Bipolar Digital Pt. 4

TTL Propagation Delay Output going high vo t vo C tPLH
Approximating the charging current for capacitor as a constant (average value), We can calculate the propagation delay tPLH using iC4 iB4 VOH= 3.7V iE4 iCap VOH= 0.2V t vo C tPLH iC4 Goes high R S So Q4 provides a large charging current to reduce the rise time for the output going high. P vCE4 Bipolar Digital Pt. 4

TTL Propagation Delay vo Output going low + vo C t tPHL iR1 iR
VOH = 3.7V Goes off iB2 Goes on iCap 1.9V + iB3 vo Goes high iC3 Goes low C VCE,sat = 0.1 V t Goes on tPHL Output going low Input goes high Transistors Q2 and Q3 turn on (first in active then saturation) as iR is redirected from the input into the base of Q2 Q4 is turned off as VB4 = VCC – iR1 R1 decreases since iR1≈ iC2. Large discharge current flows through Q3 Q3 called pull-down transistor Discharge time is small ~ 1 nsec tPHL is time it takes the output to fall from VOH = 3.7 V to 1/2(VOH + VOL) = ½( ) V = 1.9 V Bipolar Digital Pt. 4

TTL Propagation Delay vo Output going low + t vo tPHL iR VOH = 3.7V
VCC=5V vo iR VOH = 3.7V off iB2 iE4=0 1.9V iCap on + VCE,sat = 0.1 V Goes high t iC3 vo tPHL on iB3 iC3 S R What current to use for the transistor? P vCE3 Bipolar Digital Pt. 4

Power Dissipation for Transistor - Transistor Logic (TTL)
For input high and output low. Q2 and Q3 are in SATURATION. Since Q2 is in saturation mode, iC2 < β iB2 but VCE2 = 0.2 V and Q4 is weakly on, iC4 ≈ 0. Power dissipation VCC = 5 V iR1 iC4=0 R3 = 0.13 K iR VB1 VC2 p io n + sat vi = 3.7V low _ sat Bipolar Digital Pt. 4

Power Dissipation for Transistor - Transistor Logic (TTL)
For input low and output high. Q2 and Q3 are off, iC2 ≈ 0, iC3 ≈ 0. Q4 is weakly on, iC4 ≈ 0. iR1 ≈ 0. Q1 is on so VBE1 = 0.7 V Power dissipation Average Power Dissipation Power Delay Product VCC = 5 V iR1=0 iC4=0 R3 = 0.13 K iR VB1 p io≈ 0 n + off Low =0.2V high _ off Bipolar Digital Pt. 4

TTL vs. Simplified TTL vo vo vi vi Bipolar Digital Pt. 4
* Logic levels and noise margins Noise Margin for Low State NML = VIL – VO = 0.6 V V = 0.5V Noise Margin for High State NMH = VOH - VIH = 5 V V = 4.3 V Unequal noise margins for high and low states. Propagation delays Output going low Output going high Propagation delay Power – Delay Product * Logic levels and noise margins Noise Margin for Low State NML = VIL – VO = 0.5 V V = 0.4 V Noise Margin for High State NMH = VOH - VIH = 3.7 V – 1.4 V = 2.3 V Unequal noise margins for high and low states. Propagation delays Output going low Output going high Propagation delay Power – Delay Product Bipolar Digital Pt. 4

TTL Summary Advantages: Fast switching times, ~ 1 nsec.
Low power-delay product (~ 10 pJ) Good fan-out capability Adequate noise margins Disadvantages: Static power dissipation, higher than CMOS Complexity – four transistors Time delay due to saturating transistors. Small noise margin for low state, e.g V. Bipolar Digital Pt. 4

Comparison of Digital Logic Families
vo vi J. Millman and A. Grabel, Microelectronics, McGraw Hill, p. 261 (1987). Bipolar Digital Pt. 4

TTL Gates Bipolar Digital Pt. 4

Schottky TTL Gates Schottky diode clamp prevents transistors from going deep into saturation. Reduces transistor switching time. Reduces propagation delay, e.g. from ~ few nsec to < 1 nsec. Power-delay product is not reduced due to lower resistances used. Low power version of Schottky TTL has DP ~ 4 pJ. Bipolar Digital Pt. 4

Comparison of Digital Logic Families
Power delay product = a constant Bipolar Digital Pt. 4

Comparison of Digital Logic Families
Bipolar Digital Pt. 4

Emitter-Coupled Logic (ECL)
Sub nsec propagation delay (fastest of bipolar technologies). 40 mW/gate power dissipation (high). Power delay product = 30 pJ. Noise margins nearly equal, ~ 0.15 V High fan-out capability. Bipolar Digital Pt. 4

BiCMOS Basic Inverter Advanced Inverter Two input NAND Gate
Bipolar Digital Pt. 4

Comparison of Digital Logic Families
Bipolar Digital Pt. 4