RF Power Amplifier Pi-Network Output CircuitAuthor: R.J.Edwards G4FGQ © 15th December 2000
This program can be used to design the Pi-network output circuit of pentodes, tetrodes, triodes, FET and bipolar transistor amplifiers. It is only necessary to know RF power output and peak voltage swing at the anode, collector or drain. The peak RF voltage should be a high proportion of the DC supply voltage for high efficiency in the amplifying device itself. Anode (or collector or drain) loss is least if maximum current flows at the instants when anode-to-cathode voltage is at its minimum value. This means the anode load impedance should be as near as possible to a pure resistance.
A bipolar transistor RF-amplifier with a Pi-network output circuit
The internal resistance of the tube or transistor need not be known. It is not involved in the design of the 'matching network whose primary purpose is to transform the external load resistance to a value which conforms to the power and voltage ratings of the amplifying device.
A secondary purpose of the matching network is to filter-out internally generated harmonics of the fundamental frequency due to the non-linear class B or C amplifying-device bias conditions necessary for high efficiency. Harmonic filtering action improves with increased in network operating Q which is set indirectly by choosing the phase-shift-angle between the network's input and output terminals.
The Pi-network consists of capacitor C1 at the input end, a second capacitor C2 at the output end, and coil L1 in between. For a single band, C1 and C2 may have fixed values, but are usually variable. The coil may be band-switched or one value for each band. Turret coil switches allow a neat component layout and higher operating efficiencies.
There is an RF choke from the amplifying device's output terminal to the DC supply and a DC blocking capacitor between the device and the network's input terminal. An RF-choke usually is connected across C2 as safety precaution.
There is a conflict of requirements between maximizing harmonic rejection and maximizing Pi-network working bandwidth (bandwidth being that frequency range over which it is not necessary to re-tune the network). Another design consideration is the physical size of variable capacitors. As the network Q is increased so does capacitor size, but bandwidth decreases and tank-circuit circulating-current and coil-loss both increase.
The actual phase shift of a Pi-network in this application is not of great importance unless multiple transmitters are operated in parallel or are used to feed different parts of a common antenna array. A phase shift between 120 and 150 degrees usually provides convenient values of L and C with adequate harmonic rejection. Network Q values between 8 and 14 are typical for tube amplifiers. Lower network Q values may necessitate use of an additional low-pass filter, e.g., a Pi-L network. High-current, low-voltage power amplifiers such as bipolar transistors are inherently low-Q wide-band devices.
Note: higher working Q = lower efficiency
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