In a conventional semiconductor device, in order to derive an output signal from a semiconductor chip and supply to an external circuit on condition that the impedances of both the circuits are well matched, an impedance matching circuit becomes necessary. In most cases, the impedance matching circuit is consisted of several stages of source follower amplifiers fabricated on the on-chip, and in some cases in the external circuit, too. That is to say, the signal is once amplified in the on-chip amplifier and again in the external amplifier. Accordingly, the total length of wiring increases in both the amplifiers, and the capacitive load of the last stage of the on-chip amplifier becomes large. A typical semiconductor device with the aforementioned structure is a solid state image sensor. The solid state image sensor comprises a semiconductor chip, on which a CCD (Coupled Charge Device) and an output amplifier are mounted, a package, which seal up the semiconductor chip, and a buffer amplifier, which is attached to the package and has a role of impedance matching with an external circuit. In order to obtain a fine picture, it is indispensable that the size of a picture element is as small as possible, and the output and buffer amplifiers have wide frequency ranges. Then, it is extremely desirable to decrease the captive load caused by wiring in the output amplifier.