Unipolar to bipolar converters are useful when we have to have a unipolar component to do a certain job in a mixed signal design environment. For example, Digital to Analog Converters (DACs) may have the output voltage range 0 to 2.5 V, or 0 to 5 V, while the design asks for a range of –5 V to +5 V. To comply with this requirement, we have to design a unipolar to bipolar converter which will be inserted between the DAC output and the following bipolar stage. It looks like the circuit in Figure 1. How did I design it?
Figure 1
The unipolar to bipolar converter design starts with writing down the requirements:
If Vin = 0 V, then Vout = –5 V.
If Vin = +5 V, then Vout = +5 V.
It is always a good idea to write down the specifications on your page top. You will see this behavior in all my articles. That way you have the design specifications in front of your eyes at all times while you pencil down your calculations. It also helps you to better “see” what is required, so that you do not stray off the course with some other calculations, while all you need is to reach your goal: a certain output voltage range for a given input range.
This circuit can be solved in two ways: A solution by reasoning on the design requirements and a math method. Let’s start by reasoning on the design requirements.
First, the output range doubles versus the input range. The input has a span of 5 V while the output has a span of 10 V. The immediate conclusion is that the converter gain has to be 2.
Second, if we multiply the input by a gain of 2, the output will swing between 0 and +10 V. However, our output range has to be –5 V to +5 V, so we will need to introduce an output offset of –5 V. If our voltage reference is +5 V for a DAC output of 0 to +5 V, it is clear that we need to subtract this voltage from the converter output. What Op Amp configuration performs subtraction? A differential amplifier.
Any linear circuit has a transfer function defined by gain and offset as in the following equation.
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(1) |
Since we know the gain and offset, we can write down the transfer function of the unipolar to bipolar converter.
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(2) |
The differential amplifier is shown in Figure 2,
Figure 2
and its transfer function is as follows.
 |
(3) |
For the proof of this transfer function read How to Derive the Differential Amplifier Transfer Function.
Let’s compare equations (2) and (3). In equation (3), V1 becomes Vin. Also, we have a voltage +5 V reference in our system. Since we need to subtract 5 V from the circuit output we will make V2 = +5 V. If V2 is 5 V, then R4/R3 = 1. We can choose R3 = R4 = 10 kOhm.
One of the V1 factors in equation (3) is 1+R4/R3 = 2. Therefore, R2/(R1+R2) has to be one, so R1 = 0 and R2 can be anything, including no resistor. The final schematic of the unipolar to bipolar converter is the circuit in Figure 1.
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The math solution is a system of two equations with two unknowns. Starting with the design specifications we wrote earlier, the transfer function of the differential amplifier is written for both extremes of the output range:
 |
(4) |
Solving is simple. From the first equation, R4/R3 = 1. Then, from the second equation R2/(R1+R2) = 1, the same result as before.
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Categories: Analog Design, Differential Amplifier, Electronic Circuits Examples, Mixed-Signal Design
if the range of my input is 0-230 V. what should be the reference voltage??
You choose a reference voltage based on the input and output range. You gave me the input range, but I need the output range as well to give you advice. 230V? Is it DC? Sounds a lot for an Op Amp that usually works at maximum 36V. My point is, make sure that the voltage level that reaches the Op Amp is within it’s operating limits.
Hello I like the solution but I gess: If I have a signial to swing from 0V to 5V and need to convert it to -2.5 to +2.5V, I limit the output using a supply of +/-3V? or limit the out with 2 zener face to face? I cant figure it, could you help me?
Regards
You can do it both ways. First, you need to check what is the maximum allowed voltage in the input of the following stage. If you choose to limit the signal with the power supply, take into consideration what is the maximum output trip of the op amp. An op amp output cannot go to the power supply level, unless it is a rail-to-rail op amp. Then set your power supplies, both positive and negative accordingly.
If you choose to limit the output with zener diodes, the limiting value will be the zener voltage plus 0.6V, which is the forward voltage of the other zener diode. When limiting, the current through the diodes will be limited by op amp, usually at a current between 10 to 15 mA, depending on its short circuit specs. Therefore, you need to check what is the zener voltage at that current to use the correct part. You can decrease the current by inserting a small resistor between the op amp output and the diodes. You can use this resistor only if the following stage input impedance is high enough so that the limiting resistor does not count.