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Which problems with the input impedance of the differential amplifier are easily ignored
There’s always something going wrong with circuits, isn’t there? Let’s dive into the details and figure out why!
A monolithic differential amplifier is an integrated circuit that combines an operational amplifier (op-amp) with at least four precision resistors within the same package. These are invaluable tools for analog designers looking to convert differential signals into single-ended ones while rejecting common-mode signals. Take the INA134, for instance, which is commonly used as a line receiver for differential audio interfaces, as shown in Figure 1.
Most engineers think these basic building blocks are straightforward to work with, but I’ve noticed one detail often gets overlooked: the two inputs of a differential amplifier have different effective input impedances. By “effective input impedance,†I mean the combined input impedance influenced by both the internal resistor values and the way the op-amp operates.
Figure 2 illustrates a typical INA134 configuration, complete with labeled input voltages and currents at the internal op-amp input nodes.
For each input, we can calculate the effective input impedance using Equation 1:
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Let’s start with the non-inverting input, which is relatively simpler. Looking at Figure 2, R3 and R4 are in series. Assuming no current flows into or out of the op-amp input, we can use Equation 2 to calculate the effective input impedance:
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Now, let’s turn our attention to the inverting input. Remember the ideal op-amp rule: the two inputs should always be at the same potential (Equation 3):
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You’ll notice that R3 and R4 create a voltage divider at the non-inverting input. We can calculate the voltage at the non-inverting input (VP) using Equation 4:
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Why does this matter? Well, this voltage influences the effective input impedance of the inverting input. The current through R1 (IIN(N)) equals the voltage across R1 divided by its resistance (Equation 5):
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Substituting this into Equation 1 gives us Equation 6, a general formula for calculating the inverting input impedance:
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Looking again at Equation 1 and substituting Equation 5, we arrive at Equation 6, a general formula for calculating the inverting input impedance:
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Notice that the effective input impedance of the inverting input depends on the ratio of the voltages at the two inputs. To understand the implications, consider an audio line receiver application using the INA134, where the input voltages are equal in magnitude but opposite in polarity (Equation 8):
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From Equation 2, the effective input impedance at the non-inverting input is straightforward:
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However, the inverse relationship between the two input voltages significantly affects the effective input impedance of the inverting input (Equation 9):
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The effective input impedance of the inverting input ends up being one-third that of the non-inverting input. This means that when selecting an input coupling capacitor and designing the filter circuit, you must account for the lower impedance of the inverting input. Additionally, any amplifier driving the differential amplifier's input needs to handle the lower impedance effectively.
Even the simplest circuits can trip you up in the lab if you overlook the basics. So never dismiss the obvious details!
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It’s fascinating how even the smallest oversight can lead to big problems. Engineers often focus so much on complex designs that they forget about the fundamentals. It’s a good reminder that sometimes, the best solutions come from revisiting the basics.
This experience has taught me to always double-check my assumptions, especially when dealing with something as critical as circuit design. It’s not just about getting the math right—it’s about understanding how every component interacts with others.
In conclusion, whether you’re working with audio interfaces or other applications, remember that the two inputs of a differential amplifier behave differently. Paying attention to these nuances can save you hours of troubleshooting and frustration down the road.
So next time you encounter a pesky circuit issue, take a step back and review the fundamentals. You might be surprised at what you uncover!