WDØDAN's
A High Speed Switching Circuit for linear amplifiers

To view a scheme of the circuit, click here.

A very brief description of the circuit is that it will actually cut the pickup time of a relay in half. It also provides for a solid state key interface to your radio. It is a circuit I designed to interface my Henry amplifier to my HF radio but it should work with most any amp.

Lets see how this circuit works. When this circuit is in a stand-by state, transistors Q1, Q2, Q6, Q7 & Q8 are in the "off state" or not turned "on". Likewise, Q3, Q4 & Q5 are in the "on state" or turned "on". If you look at the current path through Q4 & Q5, you can see that the 100 ufd capacitor will charge to almost 28 VDC with current being limited by the 100 ohm resistor.

Now lets key the circuit and see what happens.........

 We will assume a "ground" from the transmitter to key the amplifier through this circuit (Note 1 & 2). When a ground is applied to the base of Q1, it turns the transistor on. Raising the voltage with respect to ground on its collector to approximately 12 VDC. This is clamped by D1 and keeps the base key voltage to approximately 12.6 VDC when floating or not keyed. When Q1 changes from the "off state" to the "on state", all the other transistors likewise change their state to the opposite condition they were in prior to switching. Q1, Q2, Q6, Q7 & Q8 are now in the "on state" while Q3, Q4 & Q5 have just switched to the "off state". Now, with Q4 & Q5 turned off, and Q6 & Q7 turned on, the 100 ufd capacitor now has a discharge path through the amplifier switching relay. What is special about what just happened is that the 100 ufd capacitor's polarity has just been switched and it is in series with the 28 VDC supply. The relay sees a voltage level of approximately 50 VDC. A little less than the two added together as the transistors are just at saturation and are dropping a little of the voltage. As the 100 ufd capacitor discharges and drops to the zero point (Note 3), diode D2 starts to conduct to keep the amplifier relay energized at 28 VDC and prevent a reverse voltage charging of the capacitor. It will reverse charge by about .7 VDC but in the 3 years I have used this, it has not caused any problem.

When the "key" is released, the circuit drops back to its previous state and re-charges the 100 ufd capacitor and it is ready to switch again in about 50 milliseconds.

Notes

  1. The circuit will accept either a "ground" or a +12 VDC to key the amplifier.

  2. The .1 ufd capacitors are speed-up caps to help speed up the switching of the transistors.

  3. The 100 ufd capacitor may need adjustment for timing depending on the resistance of the relay being operated. Especially if the circuit is changed to operate at a different voltage level. For most applications it should be OK.