direct coupled transistor logic

1. Direct Coupled
Transistor Logic

2. Resistor Transistor Logic
●Resistor–transistor logic (RTL) is a class of digital circuits built
using resistors as the input network and bipolar junction transistors (BJTs) as
switching devices.
● RTL is the earliest class of transistorized digital logic circuit used; other
classes include diode transistor logic (DTL) and transistor-transistor
logic (TTL).
●Special type of RTL gate known as the direct-coupled transistor logic (DCTL)
gate is one wherein the bases of the transistors are connected directly to
inputs without any base resistors.

3. Implementation of RTL
●RTL inverter: A bipolar transistor switch is the simplest
RTL gate (inverter or NOT gate) implementing logical
negation.
●One-transistor RTL NOR gate: With two or more base
resistors instead of one, the inverter becomes a two-input
RTL NOR gate.
●Multi-transistor RTL NOR gate: The limitations of the
one-transistor RTL NOR gate are overcome by the multi-
transistor RTL implementation.

4. What is DCTL?
Direct-coupled transistor logic (DCTL) is similar to resistor-
transistor logic (RTL) but the input transistor bases are
connected directly to the collector outputs without any
base resistors.
It uses no biasing and loading resistors. They are capable of
microwatt power dissipation.
Consequently, DCTL gates have fewer components, are
more economical, and are simpler to fabricate
onto integrated circuits than RTL gates.
Direct Coupled Transistor Logic (DCTL) is also called as
Integrated injection logic (I2C) circuit.

6. Circuit operation of Integrated Injection Logic Circuit
I2L NOR logic circuit is
shown in figure. Here,
transistor Q1 and Q2 act as
current sources to the
bases of Q3 and Q4
respectively.
01
If A input goes low, the
current to the base of Q2
will be shorted to ground
which will result in Q2
being turned OFF. The
input B controls Q4 in a
similar way.
02
If A is high, base current
flowing to Q2 will turn it
ON, making output C low.
Same would be the case
when B is high.
03
It is obvious that output
would be high only when
both inputs A and B are
low.
04
The output would be low
when either A or B or both
are high i.e. NOR logic
function
05

7. DCTL- NOR Gate
In DCTL-two input NOR gate there
are two transistors Q1, and Q2,
which share a common load
resistor R.
If any one or both inputs A and B
are high at level "1", then base
current will be supplied to one or
both the transistors, causing then
to conduct and thus collector
current will flow through load
resistor R.

8. This makes the output voltage at Y to go
low. Whereas when both A and B are low,
both transistors Q1 and Q2, remain in
cut-off state and thus the output at Y
approaches that of the supply voltage V
at high level.

9. DCTL- NAND Gate
In the DCTL- two input
NAND gate,both the
transistors,Q1 and Q2, are
connected in series. Only
when both inputs A and B
are high, both transistors,
Q1 and Q2, will conduct, as
they are in series. This
causes output voltage at Y
to go low.

10. DCTL-
NAND Gate
When any one or both
inputs A and B are low,
both transistors Q1 and
Q2, cannot conduct.
Therefore the output
voltage at Y remains
high.

11. DCTL Inverter

12. Merits
ONLY ONE LOW VOLTAGE
SUPPLY (1.5V) NEEDED.
IT MAY USE TRANSISTORS WITH
LOW BREAKDOWN VOLTAGES
LOW POWER DISSIPATION.

13. Limitations
The reverse saturation currents of all the fan-
out transistors add up in the resistor Rc
which is common to all the collector circuits
of all the transistors.
With VCC>>VCE,sat and VCC>>VBE,sat, direct
coupling results in the base current almost
equal to the collector current. For a
transistor driven deep into saturation region,
there results very large stored base, charge,
resulting in reduced speed of switching.
Voltage levels are rather low, output voltage
steps being only about 0.6 volt for Si
transistors. Hence, the noise spikes may
cause false operation.
Base of all the fan out transistors are
connected together. But the input
characteristics of these transistors are never
identical. To meet these requirement, DCTL
circuits use FETs in preference to BJTs.

14. Real life used cases
Because, of its high speed and less power
dissipation, it us used in large computers.
Such circuits are also used where high
packing density is of prime consideration as
in digital wrist watches.
I2L chips are capable of microwatt power dissipation
yet can provide high currents when necessary to drive
LED displays.

15. Thank You!!