# Digital Circuits Questions and Answers – Diode-Transistor Logic(DTL)

his set of Digital Electronics/Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Diode-Transistor Logic(DTL)”.

1. Diode–transistor logic (DTL) is the direct ancestor of _____________
a) Register-transistor logic
b) Transistor–transistor logic
c) High threshold logic
d) Emitter Coupled Logic

Explanation: Diode–transistor logic (DTL) is a class of digital circuits that is the direct ancestor of transistor–transistor logic. To overcome the shortcomings of DTL, TTL came into existence.

2. In DTL logic gating function is performed by ___________
a) Diode
b) Transistor
c) Inductor
d) Capacitor

Explanation: Diode serves as the input network and the switching operation is performed by the transistor.

3. In DTL amplifying function is performed by ___________
a) Diode
b) Transistor
c) Inductor
d) Capacitor

Explanation: The amplifying and switching function is performed by a transistor and the diode acts an input network in DTL.

4. How many stages a DTL consist of?
a) 2
b) 3
c) 4
d) 5

Explanation: The DTL circuit shown in the picture consists of three stages: an input diode logic stage, an intermediate level shifting stage and an output common-emitter amplifier stage.

5. The full form of CTDL is ___________
a) Complemented transistor diode logic
b) Complemented transistor direct logic
c) Complementary transistor diode logic
d) Complementary transistor direct logic

Explanation: The full form of CTDL is Complemented transistor diode logic.
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6. The DTL propagation delay is relatively ___________
a) Large
b) Small
c) Moderate
d) Negligible

Explanation: 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.

7. The way to speed up DTL is to add an across intermediate resister is ___________
a) Small “speed-up” capacitor
b) Large “speed-up” capacitor
c) Small “speed-up” transistor
d) Large ” speed-up” transistor

Explanation: One way to speed up DTL is to add a small “speed-up” capacitor across intermediate resister. The capacitor helps to turn off the transistor by removing the stored base charge; the capacitor also helps to turn on the transistor by increasing the initial base drive.

8. The process to avoid saturating the switching transistor is performed by ___________
a) Baker clamp
b) James R. Biard
c) Chris Brown
d) Totem-Pole

Explanation: Another way to speed up DTL other than adding a small “speed-up” capacitor across intermediate resister is to avoid saturating the switching transistor which can be done with a Baker clamp. The name Baker clamp is given at the name of Richard H. Baker, who described it in his 1956 technical report “Maximum Efficiency Switching Circuits”.

9. A major advantage of DTL over the earlier resistor–transistor logic is the ___________
a) Increased fan out
b) Increased fan in
c) Decreased fan out
d) Decreased fan in

Explanation: A major advantage over the earlier resistor–transistor logic is the increased fan in. Fan-in is the measure of the maximum number of inputs that a single gate output can accept.

10. To increase fan-out of the gate in DTL ___________
a) An additional capacitor may be used
b) An additional resister may be used
c) An additional transistor and diode may be used
d) Only an additional diode may be used

Explanation: To increase fan-out of the gate in DTL, an additional transistor and diode may be used. Here, the fan out means the number of maximum input that a single gate output can feed.

11. A disadvantage of DTL is ___________
a) The input transistor to the resister
b) The input resister to the transistor
c) The increased fan-in
d) The increased fan-out

Explanation: A disadvantage of DTL is the input resistor to the transistor and its presence tends to slow the circuit down. Hence limiting the speed at which the transistor is able to switch states. Thus, the propagation delay increases.

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