Differential Amplifier Calculator

Unipolar to Bipolar Converter Example

If you need to design a differential amplifier, here is a handy calculator. All you need to define are the input range, the output range and a choice of voltage reference.

The differential amplifier was explained in different articles on this website. Solving the Differential Amplifier – Part 1, Part 2 and Part 3 shows a numerical example and how to design such an amplifier. Also, the common mode voltage level and the common mode output error were explained in the series of articles The Differential Amplifier Common-Mode Error – Part 1 and Part 2.

Enter the input range, Vin1 to Vin2, the output range, Vout1 to Vout2 and a reference voltage Vref. You need to choose two resistors, R2 and R3. The calculator will compute R1 and R4.

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Design a Bipolar to Unipolar Converter with a 3-input Summing Amplifier

Since the publication of Design a Bipolar to Unipolar Converter to Drive an ADC, several readers contacted me with requests to help in solving their particular converter. The common problem they had was the fact that the components’ calculation resulted in a negative value for at least one resistor.

To provide a solution, first we need to understand the root cause of the problem. Let’s take one of the circuits I received and analyze it.

The reader wrote that he would like to drive an ADC with the input range of 0 to 2.5V from a signal with the range of –5V to +5V, connected at V1 (see Figure 1).

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Summing Amplifier Calculator

Bipolar to Unipolar Converter Example

The calculator solves the summing amplifier resistors based on the input and output voltage range requirements. It is a great tool to design a bipolar to unipolar converter, as an example and other circuits.

Enter the input range, Vin1 to Vin2, the output range, Vout1 to Vout2 and a reference voltage Vref which helps in adjusting the common-mode level of the amplifier. Since the 2-input summing amplifier has 4 resistors, you need to choose two resistors, R1 and R3, and calculate R2 and R4. For more details about this calculator read How to Design a Summing Amplifier Calculator.

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How to Design a Summing Amplifier Calculator

Several articles in this website describe the Summing Amplifier.  In one of these articles, Solving the Summing Amplifier, I showed a numeric method to design a non-inverting summing amplifier based on its input and output voltage range requirements.

This article shows how to design a summing amplifier calculator and the mathematical relations it uses.  You can find the calculator here:

JavaScript Summing Amplifier Calculator

Type the input voltage range, output range, a reference voltage and a choice of two resistors.  The calculator gives you the answer for the remaining two resistors.  The default values are for a bipolar to unipolar converter, which is explained in Design a Bipolar to Unipolar Converter to Drive an ADC.

What are the underlying equations?

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The Virtual Ground

In my articles I talked about the op amp virtual ground and sometimes I wrote a brief explanation of this concept. In this article I will show you why an op amp input can be considered at a zero potential, without being galvanically connected to ground. Let’s take a simple circuit, the inverting amplifier.

inverting_amplifier_1Figure 1

In MasteringElectronicsDesign.com : How to Derive the Inverting Amplifier Transfer Function I showed the proof of its formula by using the virtual ground. The inverting input is at a zero potential, therefore virtual ground, which is a direct consequence of the feedback provided by R2 and the op amp high gain. Let’s see why.

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Bipolar to Unipolar Converters Based on a Summing Amplifier Configuration

In a previous article, Design a Bipolar to Unipolar Converter to Drive an ADC, I presented a method for designing a bipolar to unipolar converter using a summing amplifier. In this article I am going to show more examples of bipolar to unipolar converters which are based on a summing amplifier configuration. You can adapt them to your needs if you use the method I described in the previous article.

Input -1V to +1V, Output 0V to +5V, Reference voltage +5V

bipolar_to_unipolar_-1v_1v_to_0v_5v_1

Figure 1

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The Non-Inverting Amplifier Output Resistance

It is customary to consider the output resistance of the non-inverting amplifier as being zero, but why is that? An Op Amp’s own output resistance is in the range of tens of ohms. Still, when we connect the Op Amp in a feedback configuration, the output resistance decreases dramatically. Why?

To answer these questions, let’s calculate the output resistance of the non-inverting amplifier.

It is widely accepted that the output resistance of a device can be calculated using a theoretical test voltage source connected at the device output. The input, or inputs, are connected to ground. Nevertheless, instead of using this method, let’s try a different one: The small signal variation method.

Figure 1 shows the non-inverting amplifier, which drives a load, RL. This circuit has an equivalent Thevenin source as in Figure 2.

non_inverting_amplifier

Figure 1

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