Harmonics sprogs after amplification of 40dB

I have filtered a 25MHz signal its level is -40dBm no second order sprog, 3rd order is at -35dBc then amplified it with a simple transistor amplifier ( feed back resistor from collector to base, with supply connected to resistor above collector) and the 2nd harmonic sprogs have risen to within about -12dB of fundamental, 3rd order are at -30 dBm as expected ( gain of 40) 2nd order -12dbC. How can I reduce this massive aamount of unwanted signal amplification, can any one help.

Cheers
Bikerbiker

Make the transistor work in a linear class (e.g Class A), and avoid reaching its saturation point, decreasing the input level.

If you don't want to see 2nd harmonic, you can use differential amplifier.In the differential amplifiers, second harmonics theorytically zero..( practically there will be some of course..)
For third harmonic, you may use more linear amplifier or fedbacked amplifier to supress the harmonics.