A parallel LC filter has impedance that's theoretically infinite at its resonant frequency. You can exploit this characteristic to decouple a voltage generator from its load. Doing so lets the generator drive any load.
The resonant frequency of a LC filter can be obtained by equating the denominator of its impedance, shown in Figure 1, to zero.
Usually the resonant frequency is centered at the working frequency, but why?
Consider, for example, the simple circuit of Figure 2. A driver with output impedance Rs (typically 50Ω) has to feed a small load, let’s say 50Ω.
The maximum voltage that the driver can provide to the load is 6 V, half of its nominal value. This loss of power supplied to the load is caused by the current in the load, which creates a voltage drop of 6 V that represents a loss. To solve this problem we use a buffer with high input impedance and low output impedance, as shown in Figure 3.
A high-impedance circuit won't create an additional voltage drop. The voltage applied to the load is equal to the input voltage, so there is no power loss. An LC parallel resonant circuit has high input impedance, as the buffer in Figure 3 shows. You can use the circuit to decouple the driver from the load.
A few months ago, I measured the small-signal parameters of a military-grade power switch. The measurements had to match the requirements of the guideline MIL-STD-750. One of the methods described in this standard is for small-signal measurements. This method contains an LC resonant block to drive the switch by decoupling the driver signal generator from the switch. (See Figure 4.)
To perform my task, initially I had to test an electrical circuit assembled according to the diagram in Figure 4. I expected that the LC filter would supply the input voltage to the DUT. The reality, however, was different. I have found in the laboratory some waveforms very much more attenuated than at the entrance. So, I simulated the circuit with LTspice.
The problem was that at DC, the inductor behaved like a short circuit by altering the working point of the power switch, in this case the power BJT, but the same happened with a power MOSFET.
I tried to insert a decoupling capacitor, but this solution affected the frequency response of the system around the resonant frequency, which had been designed to 1 MHz, the frequency at which I wanted to measure the parameters of small signal of the device according to MIL-STD-750.
I solved the problem by modifying the circuit. I assembled a first prototype yarn and then a PCB using a bias resistor bridge. The result of the simulation is shown in Figure 6.
Using the circuit in Figure 6, I measured in the laboratory parameters of small-signal device and fixed the decoupling problem. Have you ever used the LC resonant filters for driving loads? Have you had any problems related to the operating point of the circuit because of the short in DC of the inductor?