Question

The DTL propagation delay is relatively ___________

(a) Large

(b) Small

(c) Moderate

(d) Negligible

The question was asked in a job interview.

My question comes from Diode-Transistor Logic(DTL) topic in portion Logic Families of Digital Circuits

Answer 1

The correct answer is (a) Large

The explanation is: Propagation delay refers to the time taken by the output to change it’s state when the input is altered. When the transistor goes into saturation from all inputs being high charge is stored in the base region. When it comes out of saturation (one input goes low) this charge has to be removed and will dominate the propagation time which results as a large propagation delay. Thus, it has small clock frequency.