This feels like an ignorant question, but here it goes.
I'm trying to understand the antenna circuit from the 430 MHz reference design for TI's CC1125 transceiver IC. The antenna circuit is at the bottom right of this schematic. Here's a link to the datasheet for the device.
I tried to simulate the circuit's behavior in receive mode (TRX floating, in transmit mode it's grounded to protect the receiver). I made the assumption that the PA would be effectively a short to ground in transmit mode. Here's the schematic I'm using in LTSpice. The source has a 50 ohm series resistance and is set to 1 V AC.
The output voltage is the voltage across L5. Here are the results of the AC sweep:
I have a secondary request before my main question. I would appreciate help understanding the function of the circuit. I understand the general operation of the balun in the upper left, but I don't understand the purpose of L5. It shorts the receive pins together at DC, but it seems like it would seriously mess with the tuned balun operation. I assume that the lowpass filter is meant to remove harmonics. What is the purpose of the series LC elements connecting the lowpass filter to the balun? What is the purpose of the parallel LC structure connecting to the Tx pin?
This is my main question:
How is it possible that any voltage in the simulated circuit is larger than the input?
Shouldn't the output voltage have a hard limit at 0 dB (or even -6 dB with a series resistance in the source)?
Best Answer
Voltage gain is not the same as power gain, and this is exactly what happens at resonance.
Your circuit has an ac source that could potentially continuously add power to the circuit. But it has no lossy elements (resistors or resistive parastics), so there is no where for power to to be removed from the circuit. In fact, of course, this circuit has only reactive impedance so no real power is ever actually delivered by the source.
In fact you can have the voltage at an intermediate point in the circuit go to infinity even with a much simpler circuit:
simulate this circuit – Schematic created using CircuitLab
At resonance, the input impedance of the series LC combination goes to zero. So the current delivered by the source goes to infinity. But the impedance across C1 is not zero, so therefore the voltage at the intermediate node (where L1 and C1 are connected) goes to \$-\infty\$ at resonance.
Of course in a real circuit there would be some parasitic loss somewhere and neither could the voltage source deliver infinite current, so the actual voltages would all remain finite. But in the ideal circuit approximation, you expect the possibility of infinite voltages (or currents) at resonant frequencies.