Element 8 Question 211

Question:

 
[EL8-211] The nominal output high of the comparator shown in Diagram EL8J is 4.5 volts. Choose the most correct statement which describes the trip points.
 
  a) Upper trip point is 4.5 Volts. Lower trip point is approximately 0 Volts.
  b) Upper trip point is 2.5 Volts. Lower trip point is approximately 2.0 Volts.
  c) Upper trip point is 900 microVolts. Lower trip point is approximately 0 Volts.
  d) Upper trip point is 750 microVolts. Lower trip point is approximately 0 Volts. [Correct Answer]

Challengers excerpt:

A211 corresponding to Diagram EL8J in the FCC pool. The answer of "750 microvolts upper trip point and 0 volts lower trip point" appears incorrect. In fact, a commercial study guide indicates the correct answer as "Upper trip point is +1.285 volts. Lower trip point is -1.285 volts" How does the FCC respond to problems identified in their question pool?

Response:

In any comparator, the – input must exceed the + input for the comparator to start the output swing from + rail to the – rail. Since the output is designated as +4.5 volts, then the + supply must be the only supply with the (-V) connected to ground.

Using straight ohms law, the two resistors form a voltage divider, dividing the 4.5 volts, making the input to the + side of the comparator approximately positive 1.285 volts. The – input must be greater than positive 1.285 volts for the output to swing towards the – rail.

If the diagram indicated both plus and minus power source connections, then the challengers comments would be correct.

Simulations by TINA

Three simulations were done. None of the simulations would get anywhere within 10% of any of the answers.
This question begs for clarification.

The Voltage Generator (VG) was set as follows:

  • triangle wave
  • 1.5 Volts peak [both positive and negative]
  • 0 V offset
Circuit Diagram Presentations  


Oscilloscope

 

1.5 V peak input

 

0.0 V input

     
The above illustrates a single power source
and is the example used in the question.
     
Oscilloscope Presentation with Vcc and Vee

Oscilloscope

 

1.5 V peak input

 

0.0 V input

     
The above illustrates a dual power supply source
and is congruent with the challenger's excerpt.
   
     


Oscilloscope

 

1.5 V peak input

 

0.0 V input

     
The above illustrates a single power source.    

Recommendations:

We could replace EL8-211 with three questions, illustrating the parameters illustrated in the simulations.

Replace EL8-211 with this question.

 
EL8-211]  The nominal output high of the comparator shown in Diagram EL8J is 4.5 volts.  Choose the most correct statement describing the trip points.
 
 a) Upper trip point is approximately 1.3 Volts and the lower trip point is approximately negative 1.3 Volts. [Correct Answer]
 b) Upper trip point is approximately 1.3 Volts and the lower trip point is approximately 0 Volts.
 c) Upper trip point is approximately 1.3 Volts and the lower trip point is approximately 0.75 Volts.
 d) Upper trip point is negative 1.3 Volts and the lower trip point is approximately 1.3 Volts.