The Q value of a first-order infinite impulse response (IIR) microwave photonic filter based on a semiconductor optical amplifier (SOA) in a loop is investigated theoretically and experimentally. The maximum Q value measured is around 200 by setting a narrow bandwidth optical filter in the active loop and changing the input optical power of the loop and the pump current of the SOA. Analysis shows that increasing the signal to noise ratio and the gain of the signal light through the loop contributes to achieving higher Q value. The influences of the input optical power, the pump current of the SOA and the bandwidth of the optical filter in the loop on the Q value are calculated, based on the amplified spontaneous emission (ASE) noise of the SOA being investigated. Experimental results are in good agreement with the calculated results.