Grid Driven Tetrodes |
This page explains how to calculate resistors for a grid-driven class AB1 amplifier. These are all class AB1 amplifiers, with grids never going positive.
C1 is a low RF impedance capacitor, roughly 2 ohms reactance or less on the lowest frequency. C2 is a very low RF impedance capacitor, roughly Rg/25 at the lowest frequency. Must have a very low impedance all the way to VHF! L1 is a small RF choke with a few hundred ohms impedance at the lowest frequency. It might be a good idea to place a small resistance across L1. Rg is the grid swamping resistor Rs is the series resistor. This resistor might not always be used. If Rg is higher than 50 ohms, you will have to use a step up transformer or L network to match the grid. If RG is 50 ohms or higher, Rs is omitted.
Alternate circuit. Not generally as stable as the first circuit because the choke is added. L1 should be shunted with a 2k-5k resistor. C1 must present a low impedance all the way to VHF.
(Please give me credit for this if you use it.) The following formulas apply to class AB1 amplifiers. Where: Eb = bias voltage Pd = drive power in maximum PEP Ipk = peak current in grid resistor Eb² / 2Pd = Rg For cases where grid resistance is less than 50 ohms, use the following: The first step finds the peak resistor current in a 50 ohm load..... (√ of Pd/50) * 1.41421356 = Ipk The next step finds the grid resistor value.... Eb/Ipk = Rg The final step finds the series resistance.... 50-Rg = Rs
For cases where grid resistance calculates to be over 50 ohms, use the following:
L and C are an L network. Otherwise you might be able to use a broadband transformer.
Where: Eb = bias voltage Pd = drive power in maximum PEP Ipk = peak current in grid resistor Eb² / 2Pd = Rg
Examples:
next case:
Another case:
Also see Neutralizing Amplifiers |