No.40
Amateur Radio

It certainly is impossible to load power into a purely capacitive load. Some applications with grounded cathode tubes or MOSFETs, though, require RF voltage across their capacitive input. This article shows how to achieve broadband matching to such capacitive loads.

The only way to get to a good VSWR is to feed the power into a resistor. Let's assume a 100 pF input of a tube amplifier which would be shunted by a 50 ohm resistor. As shown in Figure 1, the circuit is not good enough at frequencies above approximately 10 MHz. With a lower capacitance of about 45 pF we could get to a VSWR better than 1.5 up to 28 MHz.

To realize larger bandwidths, LC compensation is necessary. The easiest method is to compensate the capacitance with a parallel inductance. This, of course, cannot result in broadband matching. The circuit shown in Figure 2, however, has a remarkably large bandwidth of 6 MHz to 33 MHz for a VSWR <1.5 as shown in Figure 3. To switch to two or three different parallel inductances with the transceiver's frequency information could result in a good VSWR from 1.8 to 29 MHz.

The better method to achieve large bandwidths is to use low-pass LC compensation. The simple LC compensation circuit shown in Figure 4 improves the bandwidth considerably. The series-L value is the optimum value. As can be seen in Figure 5, the circuit is still not good enough, so let's investigate some more complex circuits.

The LC configuration can easily be completed to a Pi low-pass filter. The CLC circuit shown in Figure 6 has a very good impedance up to 30 MHz. Figure 7 shows the graph. Some of you might not be familiar with the return loss figures. Figure 1 can be used to find out what return loss corresponds to VSWR values of 1.2 or 1.5.

An LCL circuit shows even better results (see Figures 8 and 9). All the circuits shown have been run through the optimizer to get to the best possible result.

But how about matching larger capacitances? And how about getting to 50 MHz? You can use a lower shunt resistance such as 12.5 ohms and use a 1:4 broadband transformer to get to 50 ohms again. Six dB of voltage swing is lost, however. This sort of transformation will be necessary with MOSFET amplifiers with their large input capacitances.

There still is another way that should be mentioned, however. Consider cascading two CLC low-pass filters in the way shown in Figure 10. This results in twice the capacitance in the middle of the circuit. The same voltage is present there as at the 50 ohm termination, so you can achieve the same bandwidth at twice the capacitance (or get 50% more bandwidth with the same capacitance!).

A practical circuit might look like Figure 11, and the frequency response with a 200 pF load is shown in Figure 12.

**The information provided in this application note is intended for general design guidance only. The user assumes all responsibility for correct and safe usage of this information. Svetlana Electron Devices does not guarantee the usefulness or marketability of products based on this material.

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