Current Differencing Opamp (CDA)

Current Differencing Amplifier The current differencing amplifier (CDA), also known as the Norton amplifier, is very similar to the opamp in principle, but is different in its operation. These differences lead to certain design advantages like a single power supply operation.

Along with being able to operate from single supplies, the CDA is able to operate over a wide voltage range. They can also operate with a single supply as low as 5 volts. Finally, the CDA is available at low cost. These factors make them ideal for automotive and industrial applications.

When schematics lack the power connections to CDA devices, it is assumed to be connected to a single power supply (opamps are assumed to be connected to bipolar power supplies).

Unlike the opamp, the CDA is sensitive to the differences in input currents. This page will outline the various amplifier configurations and the design procedures for them.

Testing LM3900s

Sometimes LM3900 can be purchased in cheap lots. But once you have the parts in your hand it is a good idea to test them, especially if you intend to use them without a socket. The page LM3900 Testing shows you how you can perform a simple test on them, with a minimum of parts.

Introduction to the Norton Amp


The Norton amplifier could exist without the non-inverting input. In fact, the non-inverting input need not be used for anything.

In the figure, you can see that Q2 forms the current mirror that acts as the non-inverting function. As Q2 is forward biased, it steals current away from the inverting input (at the base of Q1).

Dynamic Resistance:

The value of the dynamic resistance is a factor in the design of various CDA circuits to be described later. The dynamic resistance of Q2 is calculated (at room temperature) with:

r = \frac {26} {I_b}


I_b is the input base current, in milliamperes.

Norton Output Impedance

Norton equivalent output The output of the CDA differs somewhat from a normal opamp. The equivalent circuit shown at right shows that the output voltage appears like a voltage source that generates voltage of E_{in} times the amplifier's configured voltage gain A_v. Between that and the output pin, is an equivalent output resistance r_o.

In opamps, the value of r_o is low, usually between 50 to 100 ohms (recall that ideal opamps have zero output impedance). The CDA however has a much higher value for r_o. For the LM2900, LM3900, MC3401 and MC3301, r_o = 8k ohms.

This factor is important when considering the source resistance of the prior CDA stage. Since the output impedance of a CDA is typically 8k ohms, the input impedance of the following stage should be at least ten times this, whether CDA or opamp. A design input impedance of 100k ohms is often used to allow for this.

Circuit Designs

cda_opamp.txt · Last modified: 2012/12/15 05:05 by ve3wwg
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