150
Digital Electronics
Figure 5.33 ECL input/output characteristics.
We will also note that voltage changes in ECL are small, largely governed by V BE of the various
conducting transistors. In fact, the magnitude of the currents flowing through various conducting
transistors is of greater relevance to the operation of the ECL circuits. It is for this reason that emitter
coupled logic is also sometimes called current mode logic (CML).
5.4.3 Salient Features of ECL
There are many features possessed by MECL family devices other than their high speed characteristics
that make them attractive for many high-performance applications. The major ones are as follows:
1. ECL family devices produce the true and complementary output of the intended function
simultaneously at the outputs without the use of any external inverters. This in turn reduces package
count, reduces power requirements and also minimizes problems arising out of time delays that
would be caused by external inverters.
2. The ECL gate structure inherently has high input impedance and low output impedance, which is
very conducive to achieving large fan-out and drive capability.
3. ECL devices with open emitter outputs allow them to have transmission line drive capability. The
outputs match any line impedance. Also, the absence of any pull-down resistors saves power.
4. ECL devices produce a near-constant current drain on the power supply, which simplifies power
supply design.
5. On account of the differential amplifier design, ECL devices offer a wide performance flexibility,
which allows ECL circuits to be used both as linear and as digital circuits.
6. Termination of unused inputs is easy. Resistors of approximately 50 k allow unused inputs to
remain unconnected.
Digital Electronics
Figure 5.33 ECL input/output characteristics.
We will also note that voltage changes in ECL are small, largely governed by V BE of the various
conducting transistors. In fact, the magnitude of the currents flowing through various conducting
transistors is of greater relevance to the operation of the ECL circuits. It is for this reason that emitter
coupled logic is also sometimes called current mode logic (CML).
5.4.3 Salient Features of ECL
There are many features possessed by MECL family devices other than their high speed characteristics
that make them attractive for many high-performance applications. The major ones are as follows:
1. ECL family devices produce the true and complementary output of the intended function
simultaneously at the outputs without the use of any external inverters. This in turn reduces package
count, reduces power requirements and also minimizes problems arising out of time delays that
would be caused by external inverters.
2. The ECL gate structure inherently has high input impedance and low output impedance, which is
very conducive to achieving large fan-out and drive capability.
3. ECL devices with open emitter outputs allow them to have transmission line drive capability. The
outputs match any line impedance. Also, the absence of any pull-down resistors saves power.
4. ECL devices produce a near-constant current drain on the power supply, which simplifies power
supply design.
5. On account of the differential amplifier design, ECL devices offer a wide performance flexibility,
which allows ECL circuits to be used both as linear and as digital circuits.
6. Termination of unused inputs is easy. Resistors of approximately 50 k allow unused inputs to
remain unconnected.