Finding Output Voltage: Maths & Graphs

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Discussion Overview

The discussion revolves around determining the output voltage (Vo) in a circuit involving multiple diodes based on varying input voltages (Vi). Participants explore both mathematical and graphical approaches to analyze the circuit behavior under different conditions, including specific input values and the states of the diodes.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that for Vi < 9.3 V, all diodes are ON and Vo = Vi, but questions arise when Vi exceeds this threshold, leading to contradictions in diode states.
  • Another participant challenges the assumption that a diode can be conducting if the required forward bias voltage cannot be achieved.
  • A participant proposes that for Vi = 5V, all diodes can be ON, leading to a calculated output voltage of Vo = 5V, and seeks confirmation on this output.
  • When the input voltage is adjusted to Vi = 4V, a similar calculation is presented, resulting in Vo = 4V, with a suggestion to verify currents through the diodes.
  • One participant expresses a desire to determine which diodes are ON or OFF without calculating currents, prompting discussions on alternative methods to analyze the circuit.
  • Another participant describes a method of imagining diodes not conducting to find the standing potential at certain points in the circuit, which could indicate when diodes might forward bias.
  • A participant notes the complexity of analyzing nonlinear circuits and suggests following the conditions for conduction to understand diode states better.

Areas of Agreement / Disagreement

Participants express differing views on the conditions under which diodes conduct and the resulting output voltage. There is no consensus on the correct approach to determine diode states or the output voltage for varying input conditions, indicating ongoing debate and exploration of the topic.

Contextual Notes

Participants highlight the need to consider the currents through the diodes and resistors to fully understand the circuit behavior, indicating that assumptions about diode states may lead to incorrect conclusions if not verified.

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Homework Statement


I need to find the output voltage mathematically or graphically


Homework Equations





The Attempt at a Solution


My approach :
as long as the Vi is < 9.3 V , the diode D1 will be ON because in that case VA will be < 10 V and current can flow through 10k resistor. And if this is true then D2 will also be 0N and Vo = Vi.
Now second interval i.e Vi > 9.3 V, same reasoning for D3 i.e D3 will be ON as long as Vi > -9.3 V & D2 is ON & D4 is also ON. But in this case if we track from input to the output through D3 & D4 , then i get Vo = Vi and this is making D1 ON which i assumed to be OFF. Summarizing my results :

0<Vi<9.3 all diodes are ON & Vo = Vi
Vi>9.3 all diodes are ON & Vo=Vi... but this is not possible because this makes VA > 10 which shows current can't flow from +10V battery. So in short i know i am doing but the question is where ?
 

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erece said:
Vi>9.3 all diodes are ON & Vo=Vi... but this is not possible because this makes VA > 10 which shows current can't flow from +10V battery. So in short i know i am doing but the question is where ?
If, for a diode to be forward biased, you need a voltage of a level which can't exist, then your assumption of that diode conducting is wrong.
 
can you tell me what will be the output voltage Vo for Vi = 5V ?
According to me, Vi=5V so all diodes can be ON & in that case VA = Vi+0.7 = 5.7V and Vo = 5.7-0.7 = 5V, this will make Vc = 4.3 V again possible. So is this the correct output for Vi=5V ?
 
EDIT: I've changed to Vi=4V for your example.
erece said:
can you tell me what will be the output voltage Vo for Vi = 4V[/color] ?
According to me, Vi=4V[/color] so all diodes can be ON & in that case VA = Vi+0.7 = 4.7V[/color] and Vo = 4.7-0.7 = 4V, this will make Vc = 3.3 V again possible. So is this the correct output for Vi=4V ?
✔ [/size][/color]
For Vi=4V that sounds right, but you need to check the currents through the diodes to be sure — do this by determining the currents through all 3 resistors since you know the [expected] voltages across them.

If you go on increasing Vi much beyond 4V, the currents through D1 and D4 will be decreasing and at some point these currents will become zero. At that stage, Vo is no longer directly determined by Vi.

Evaluate the currents for Vi of 5V and you'll see what I'm talking about.
 
Last edited:
i got it nascent
is there a way to find that which diodes will be ON and which will be OFF for a particular input voltage without calculating the currents ?
 
erece said:

The Attempt at a Solution


My approach :
as long as the Vi is < 9.3 V , the diode D1 will be ON because in that case VA will be < 10 V and current can flow through 10k resistor. And if this is true then D2 will also be 0N and Vo = Vi.


That's not what I got..

If Vi=-10V ...

Point A will be at Vi+0.7 = -9.3V D2 will be off.

Current will flow from ground, through the 10K near the output, through D4 and the 10K to -10V. Vo will be about -9.3/2 = -4.65V
 
erece said:
i got it nascent
is there a way to find that which diodes will be ON and which will be OFF for a particular input voltage without calculating the currents ?
If you imagine D1 and D4 not conducting, you can calculate the standing potential at point A (and B). This defines the changeover point which, if Vi falls further, will forward bias D1.
 
NascentOxygen said:
If you imagine D1 and D4 not conducting, you can calculate the standing potential at point A (and B). This defines the changeover point which, if Vi falls further, will forward bias D1.

this is the solution given in the textbook. Still i have a doubt that how do we come to know that we have to observe the state of D1 & D4 ?
 
It's always harder to see what is happening in nonlinear circuits.

What I do is follow the shorts in the diodes (pretend they conduct) and see if some conditions for conduction arise.

For this circuit, you will see if Va > Vc (plus some diode drops), all the diodes conduct. So Va=Vc (plus some diode drops) is one transition point that will see one or more diodes shut off. If the diodes are on then there is a relationship between Vi, Vo, Va and Vc.

(This circuit is a variation of the diode-bridge modulator / diode-bridge switch. One use is in AM radio modulation).
 
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