Hi,

I have a FM transmitter chip (higher grade one) that I am feeding into two MMIC amps. When hooking up the transmitter to a perfect load, it works well, but if I hook up an amplifier to the output, it causes these MMIC amps to oscillate. The output power of the transmitter from the second MMIC is 100mW roughly.

I already tried adding additional capacitance from the output of the second amp to ground and that improved things a little (the output increased about 20mW), but the oscillations still occur, but are less powerful. I experimented with a variable cap and determined the best value, so there is nothing to change here.

The MMICs are wired with one 100pf cap on the input and output of each and the power feed circuits are a 100nH fed into a 100pf to ground and then a 10R to the power supply.

Could anyone offer any advice about what I can change to prevent the oscillations?

Thanks!

Your 100pF to ground at each supply choke is too small - try 1nF.

MMICs, depending on the model are generally quite stable so it could be your drive which is oscillating.

1. Like Albert says, try extra decoupling and filtering on the supply lines first. You can use multiple capacitors in parallel there, for example 1nF, 10nF and 100nF. Also an electrolytic here (10-47uF) wouldn't hurt.

2. If it's still oscillating it could be the drive. Try adding an attenuator in between the transmitter and the MMIC (you will get less power but at least it'll be clean). Upto a few dB should do.