frequency of response of UHF GaAs mesfet

Iam usign a GaAs FET (SGM2006). The datasheet says it can be used as UHF LNA.
Iam using it as an voltage amplifier at 1 MHz. I get a frequency response characterstics with voltaage gain of 15 (vout/vin) at 20 kHz and at 1 Mhz voltage gain=2.

The amplifier config is fixed bias common source with source grounded. gate 2 shorted to source.

why am i getting a band pass characteristics with 20 khz center freq?

Could be many reasons. Upload a schematic diagram with component values so we all can take a look.

Give us more information, I am sure we can help and learn together

i am attaching amplifier circuit diagram. the dc bias is vds=2.2V id=0.51 mA
vg1s=-1.3 V. Gate2 is shorted to source.