Electronic Design

Reduce AC-Couupling Capacitance In Transmission Systems

Communication systems often require large ac output coupling to remove dc voltage on the transmission line and to isolate ground connections between transmit and receive systems. Generally, a feedback network is used to minimize the output capacitance. Yet once the feedback's tradeoffs are understood, the circuit can be reorganized and improved. The final feedback network presented here yields a more compact solution with equivalent performance.

The traditional feedback network uses three coupling capacitors (Fig. 1). CIN allows the amplifier's input to be biased mid-supply, C2 holds the amplifier's dc gain to unity, and COUT eliminates dc power from the load. Since the output pole depends on the value of the line impedance (75Ω) ) instead of kilohms, COUT is the largest of the three and the main target for improvement.

Capacitive size can be traded off for additional load on the driver. Simplistically, if the capacitor is three times smaller, the driver must drive three times harder to deliver an equivalent signal to the load through the voltage divider. The driver's output capabilities limit the additional output.

The feedback network allows COUT to be reduced to 47 μF from 220 μF by adding CSAG (22 μF) and a few resistors. The area consumed by this feedback network is relatively small, despite the number of components. R1, R2, and R3 can have small footprints and be placed close to the inverting input. The size and proximity to the driver input also reduce the parasitic capacitance.

Figure 2 plots the gain versus frequency for CSAG (the feedback capacitance) as it is varied from 1 to 220 μF. The lower cutoff is extended proportionally to the value of CSAG as a result of peaking. When the feedback capacitance is 1 μF, that extension isn't enough to pass the vertical sync information at 60 Hz. For all plotted values greater than 1 μF, the lower cutoff is sufficient. Larger values can be used, but they will increase settling time.

In the improved feedback circuit, the series combination of CSAG and R3 was swapped, R2 moved from one side of CSAG to the other, and CCOMP was added (Fig. 3). Moving R2 sets the dc gain at unity, which places the output and inputs at the center of their operating region.

CCOMP is a small, layout-dependent capacitor added to compensate for the parasitic capacitance on the inverting input node. Figure 4 plots this circuit's frequency response with and without CCOMP.

The most appealing aspect of the circuit in Figure 3 is the dual function of CSAG. It not only supplies the feedback to scale the output capacitance, it also provides the unity-gain dc characteristic. Though CCOMP has been added in this solution, its 3-pF value is four orders of magnitude smaller than the original C2, which was eliminated.

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