Use a series feedback circuit to set the output impedance savings to produce 3dB of output power loss

2023-01-14 11:08:01

Series terminations are often used on op amps to match the impedance of the load. But this practical method produces 3dB of output power loss on the termination resistor (Figure 1). The newer op amps operate at 3V and 5V, limiting the output swing, which means that the series matching resistor method should be avoided. Another option is to use a series feedback circuit to set the output impedance. More than 40 years ago, John Wittman, a senior research engineer at GTE Lenkurt Electronics, introduced the technology.

In Figure 1, using a series termination resistor equal to the load wastes 3dB of power, halving the output swing.

With this technique, the output impedance can be increased by 6dB of mutual feedback, resulting in a return loss better than 30dB. What needs to be added is a series current sense resistor, another op amp, and a current limiting resistor (Figure 2). This example shows a high side sensor and an unbalanced load. The forward amplifier is designed to double the gain required for no-load. In this example, the open circuit gain is 2.7 and the input impedance is 1Î©. The input current is 1A and the input signal is 1V.

In Figure 2, this method uses a high-side current-sense resistor and a second amplifier to set the output impedance to match the load so that almost all of the output swing can be achieved.

To match the amplifier's 1Î© load, the series feedback circuit must divert half of the input current from the op amp's negative input. The original 1A input current flows through RF?, reducing to 0.5A, which means that the output voltage is half of the open circuit voltage. The output impedance is now 1Î© and the series feedback is 6dB, so the output impedance can match the load and still get the almost full voltage swing of the amplifier, which will no longer waste half of the output power at the series termination. The current sense resistor value used in this example is 3% of the output load, so the power consumption will be 3%. By careful design, the power consumption can be reduced to less than 1%.

In the telecommunication line, in order to obtain the longitudinal balance, the impedance of the two wires to the ground should be equal. Longitudinal balance prevents crosstalk and 60Hz induced noise. This is also important at the higher frequencies of DSL (Digital Subscriber Line) services. Telecommunications companies typically use transformers to provide a longitudinal balance of 80dB to 120dB. The transformer also isolates the instantaneous flow caused by lightning. The application of this technology can be coupled by transformer coupling with low side current (Figure 3). The design process remains the same, but as long as two resistors provide 6dB of feedback.

The formal equation can be used to formalize the circuit analysis. For the circuit in Figure 2, since the negative input of the op amp is virtual ground, the relationship between input voltage and current can be obtained: IIN = (VIN - V -) / 1 Î© = VIN / 1 Î©. Since the negative input of the op amp is high impedance, the current at this end must be applied to 0A, which yields another equation.

Summarize the current on V-, but the junction current should refer to the sense resistor, including 0.3Î© resistor and 0.03Î© sense resistor: 0=VIN/1Î©+VOUT/2.7Î©+0.37VOUT/ RLOAD. The circuit function can be vectored. And matrix form representation: (I) = (ADMITTANCE) & TImes; (V). It is also possible to expand the appropriate current state: