There are two types of amplifier circuits for flat panel sensors: a passive pixel type and an active pixel type.

The passive pixel type has an amplifier for each column of the photodiode array, where the amplifier is connected to each photodiode via address switch. The amplifiers are formed on one side of the two-dimensional photosensitive area as an amplifier array. The passive pixel type allows a high fill factor and high radiation durability. However, the input capacitance caused by the data line limits the reduction in amplifier thermal noise.

The active pixel type structure eliminates the foregoing problem in the passive pixel type. The active pixel type has an amplifier for each pixel, and the accumulated charges are converted into voltage there. This structure lowers the noise level, which is about one figure less than the passive pixel type. Because of its low noise and high S/N features, the active pixel type flat panel sensor acquires high definition images from low energy X-rays.