An electronic scale system is designed such that if a mass is placed on the scal

Question

An electronic scale system is designed such that if a mass is placed on the scale LEDs will light up to indicate that it is under mass (between 0.750kg and 0.900kg, 1 LED on), the correct mass (between 0.900kg and 1.10kg, 2 LEDs) or over mass (over 1.10kg, 3 LEDs).

The voltage gain of the non-inverting amplifier used to amplify the output of the Wheatstone bridge is given by the formula:

1) What gain should be used if the output of the buffer amplifier should not exceed 5V when a mass 10% larger than the maximum correct weight is applied?

2) If you have access to the standard E24 resistors what values should you choose for RB1 and RB2?

3) What is the voltage at the output of the buffer amplifier for a mass on the limit of being under (low)

4) What is the voltage at the output of the buffer amplifier for a mass on the lower limit of being the correct (mid)

5) What is the voltage at the output of the buffer amplifier for a mass on the limit of being over (high)

Explanation / Answer

A buffer amplifier (sometimes simply called a buffer) is one that provides electrical impedance transformation from one circuit to another, with the aim of the signal source being unaffected by ("buffered from") whatever currents (or voltages, for a current buffer) that the load may produce. Two main types of buffer exist: the voltage buffer and the current buffer.

voltage buffer amplifier is used to transfer a voltage from a first circuit, having a high output impedance level, to a second circuit with a low input impedance level. The interposed buffer amplifier prevents the second circuit from loading the first circuit unacceptably and interfering with its desired operation. In the ideal voltage buffer in the diagram, the input resistance is infinite, the output resistance zero (output impedance of an ideal voltage source is zero). Other properties of the ideal buffer are: perfect linearity, regardless of signal amplitudes; and instant output response, regardless of the speed of the input signal.

If the voltage is transferred unchanged (the voltage gain Av is 1), the amplifier is a unity gain buffer; also known as a voltage follower because the output voltage follows or tracks the input voltage. Although the voltage gain of a voltage buffer amplifier may be (approximately) unity, it usually provides considerable current gain and thus power gain. However, it is commonplace to say that it has a gain of 1 (or the equivalent 0 dB), referring to the voltage gain.

As an example, consider a Thévenin source (voltage VA, series resistance RA) driving a resistor load RL. Because of voltage division (also referred to as "loading") the voltage across the load is only VA RL / ( RL + RA ). However, if the Thévenin source drives a unity gain buffer such as that in Figure 1 (top, with unity gain), the voltage input to the amplifier is VA, and with no voltage division because the amplifier input resistance is infinite. At the output the dependent voltage source delivers voltage Av VA = VA to the load, again without voltage division because the output resistance of the buffer is zero. A Thévenin equivalent circuit of the combined original Thévenin source and the buffer is an ideal voltage source VA with zero Thévenin resistance.

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