9.2 DTL AND TTL LOGIC CIRCUITS 427
EXAMPLE 9.1.1
For the circuit of Figure 9.1.2, given thatVCC=5V,RC=1k, β=100, and the high range is
4 to 5 V, chooseRBsuch that any high input will saturate the transistor with the base overdriven
by a factor of at least 5. AssumeVsatto be 0.2 V.
Solution
SinceiC∼=βiB+ICEO, one must have (neglectingICEO)IBsat>ICsat/β. But
ICsat=
VCC−Vsat
RC
Hence,
IBsat>
VCC−Vsat
βRC
With a factor of 5 included for the desired overdrive, one has
iB= 5
VCC−Vsat
βRC
= 5
5 − 0. 2
100 ( 1000 )
= 240 μA
Whenviis in the high range, the emitter–base junction is forward-biased. The base current that
flows is given approximately by
iB=
vi− 0. 7
RB
or RB=
vi− 0. 7
iB
Settingvi=4 V, which is the lowest value in the high range,
RB=
4 − 0. 7
2. 4 × 10 −^4
=13,750
One chooses the closest standard resistance smaller than 13,750, since makingRBsmaller
increases the overdrive and hence improves the margin of safety.
9.2 DTL and TTL Logic Circuits
Bipolar transistors were the first solid-state switching devices commonly used to implement digital
logic circuits in the 1950s and 1960s. These circuits used diodes at the input of the gate for logic
operation followed by a transistor (BJT) output device for signal inversion. One of the bipolar logic
families that emerged was called DTL (diode–transistor logic). The TTL (transistor–transistor
logic) soon replaced DTL and then became the principal bipolar technology for the next two
decades. It is still often used today. In TTL circuits, the diodes used in DTL at the gate input are
replaced with a multiemitter transistor for increased performance. Primarily by reducing the size
of the transistors and other components, speed and performance improvements have been made
in TTL circuits.
Both DTL and TTL are calledsaturatinglogic families, because the BJTs in the circuit
are biased into the saturated region to achieve the effect of a closed switch. The inherent slow
switching speed is a major difficulty with saturating logic because saturated BJTs store significant
charge and switch rather slowly. Schottky TTL is a nonsaturating logic family that was later
developed to achieve higher speed performance by preventing the transistors from saturating.
Another bipolar nonsaturating logic family is ECL (emitter-coupled logic), which has BJTs that
remain biased in the active region. These circuits consume more power, are less dense, but are
extremely fast.
