DC biasing using voltage divider

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Discussion Overview

The discussion revolves around the use of a voltage divider for DC biasing a signal that varies from -5V to 5V, with the goal of shifting this range to 0V to 2.5V. The conversation includes theoretical considerations and practical implications of using voltage dividers in this context.

Discussion Character

  • Debate/contested
  • Homework-related
  • Technical explanation

Main Points Raised

  • One participant questions the feasibility of using a voltage divider to change the mean value of a signal, asserting that it can only alter amplitude.
  • Another participant suggests that achieving a 1.25V output from a 0V input using only resistors is implausible without additional components like an op-amp or a capacitive coupling.
  • A different viewpoint proposes a method involving a three-resistor network to achieve the desired biasing, suggesting specific resistor values and encouraging calculations to explore the output behavior.
  • A later reply emphasizes the importance of considering source and input impedances in practical applications, while noting that for the homework context, certain assumptions about impedances can simplify the analysis.

Areas of Agreement / Disagreement

Participants express disagreement regarding the capability of a voltage divider to achieve the desired biasing. While some believe it is not possible without additional circuitry, others propose a method that may allow for the desired outcome.

Contextual Notes

Participants highlight the need to consider source and input impedances in real-world applications, which may affect the validity of the proposed solutions. Assumptions about negligible output impedance and high input impedance are noted as relevant for the specific homework problem.

david90
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how do I use a voltage divider to biasing a signal that goes from -5v to 5v so that it goes from 0 to 2.5?
 
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It's not possible. A voltage-divider can only change a signal's amplitude, but cannot change its mean value.

You're hoping to put a signal of 0V (the midpoint of -5 to 5V) and get out a signal of 1.25V. That'd be quite a feat for nothing but a couple of resistors.

You'll need to use an op-amp, or at least capacitively-couple your input to a bias network (perhaps made out of a voltage divider).

- Warren
 
chroot said:
It's not possible. A voltage-divider can only change a signal's amplitude, but cannot change its mean value.

You're hoping to put a signal of 0V (the midpoint of -5 to 5V) and get out a signal of 1.25V. That'd be quite a feat for nothing but a couple of resistors.

You'll need to use an op-amp, or at least capacitively-couple your input to a bias network (perhaps made out of a voltage divider).

- Warren
No, I think it can be done in this case. But since this sounds like a homework problem, I'll start by only giving a couple of hints to David.

Make a 3-resistor network with two resistors in series from 10V to -10V. Couple your input voltage into the middle of these two resistors using another resistor. Make the top resistor 10K, the bottom resistor 20K, and the input resistor 10K. Now vary the input voltage between -5V and 5V, and calculate what happens at the output (the intersection of the 3 resistors). Do you see a way to get the voltage offset and scaling that is asked for in the problem? Keep in mind that you can set different voltages on the top and bottom of the voltage divider if that helps. Just write some simultaneous equations...
 
Last edited:
BTW, in a real world application, you would need to consider the source impedance and the input impedance of whatever circuit came next after the voltage biasing network. In the case of this homework problem, you can probably assume that the output impedance of the source voltage is negligibly low, and the input impedance of whatever is using the output voltage is very high.
 

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