Amplifier for 5 MHz

Ale

I need to generate a 5 MHz preferably sinusoidal signal. I wanted to use a MOSFET (IRF7104 for example) and a small transformer (isolated output). The MOSFET would be driven with a 5 MHz square wave and then a small low pass filter. While this works, it produces very little power. I'd need to supply some mA at the output to a capacitive load (~500 pF). I think that those MOSFETs are a bit slow but it works. Has someone an idea of an alternative output stage I can use ? (I can also get a 5 MHz sinusoid, albeit very low power as input for this stage).
I tried to find a MOSFET driver suitable for this frequency, but they top at 1 MHz... any ideas ?
Thanks !

Phil Pilgrim (PhiPi)

You might be better off with a square-wave generator that can produce the necessary power and following it with a parallel-resonant LC tank to filter out the harmonics. A cap and a pair of windings on a ferrite toroid should give you both the filtering and the isolation you need. Also, try Googling QRP transmitter schematic for some other circuit ideas.

-Phil

K2

There is absolutely no question that the average power MOSFET is adequate for generating RF power @ 5 MHz. In fact a humble 2N7000 (in a T0-92 case) can produce well over a Watt of RF power when biased as a Class C amplifier. With an IRF510, 20 Watts of RF output is possible. For what you're doing, you may need nothing more than all six gates of a CMOS inverter IC, like a 74HC04.

As Phil suggests, a very nice sinusoid is obtained by passing the transistor or IC output through a 5th-order LC filter made from common and inexpensive components. If you're interested in this, I'd be happy to recommend appropriate capacitor and inductor values.

Leon

A simple 5 MHz Colpitts oscillator and buffer followed by a Class A amplifier using a BD139 will give over 5W at 5 MHz.

Phil Pilgrim (PhiPi)

Here's an example of the kind of circuit Leon is talking about:

http://www.al7fs.us/AL7FS2.html

-Phil