  We present an impedance engineered Josephson parametric amplifier capable of
providing bandwidth beyond the traditional gain-bandwidth product. We achieve
this by introducing a positive linear slope in the imaginary component of the
input impedance seen by the Josephson oscillator using a $\lambda/2$
transformer. Our theoretical model predicts an extremely flat gain profile with
a bandwidth enhancement proportional to the square root of amplitude gain. We
experimentally demonstrate a nearly flat 20 dB gain over a 640 MHz band, along
with a mean 1-dB compression point of -110 dBm and near quantum-limited noise.
The results are in good agreement with our theoretical model.
